30.11.2014 Views

Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>European</strong><br />

<strong>Summer</strong> <strong>School</strong> <strong>of</strong> <strong>Photovoltaics</strong><br />

4 – 7 July 2012<br />

KRAKÓW<br />

Poland<br />

„Dissemination <strong>of</strong> <strong>the</strong> Achievements <strong>of</strong> Polish and Global <strong>Photovoltaics</strong><br />

in <strong>the</strong> Process <strong>of</strong> Education on University Level – 2 nd Edition ” POKL.04.02.00-00-006/09.


Instytut Metalurgii i Inżynierii Materiałowej Polskiej Akademii Nauk<br />

w Krakowie realizuje projekt „Upowszechnianie osiągnięć polskiej oraz<br />

światowej Instytut Metalurgii fotowoltaiki i Inynierii w Materiaowej procesie Polskiej kształcenia Akademii na Nauk poziomie w Krakowie wyższym realizuje<br />

projekt „Upowszechnianie osigni polskiej oraz wiatowej fotowoltaiki w procesie ksztacenia<br />

– II Edycja” POKL.04.02.00-00-006/09 w ramach Programu Operacyjnego Kapitał<br />

Operacyjnego Ludzki – Kapita Priorytet Ludzki IV ––Szkolnictwo Priorytet IV –Szkolnictwo Wyższe i Nauka Wysze – i Nauka Działanie – Dziaanie 4.2 „Roz-<br />

4.2<br />

na poziomie wyszym – II Edycja” POKL.04.02.00-00-006/09 w ramach Programu<br />

wój „Rozwój kwalifikacji kwalifikacji kadr systemu kadr systemu B+R i wzrost B+R świadomości i wzrost wiadomoci roli nauki roli w rozwoju nauki<br />

gospodarczym”. w rozwoju gospodarczym”. Zgodnie Zgodnie z harmonogramem z projektu w w dniach od 4 od do 47 do lipca 72012<br />

lipca<br />

2012 roku odbya roku odbędzie si w Krakowie się Europejska w Krakowie Letnia Szkoa Fotowoltaiki (ELSF).<br />

W Europejskiej Letniej Szkole Fotowoltaiki uczestniczyo 30 studentów i modych<br />

Europejska Letnia Szkoła Fotowoltaiki (ELSF).<br />

pracowników nauki, polskich uczelni technicznych, prowadzcych badania naukowe<br />

w dziedzinie fotowoltaiki. Wykadowcami byli pr<strong>of</strong>esorowie z czoowych europejskich<br />

W placówek Europejskiej naukowo-badawczych Letniej Szkole w dziedzinie Fotowoltaiki fotowoltaiki: Institut uczestniczyło d’Électronique 30 du studentów<br />

et des i młodych Systèmes 23 pracowników Rue du Loess – nauki, BP 2067037 polskich Strasbourg uczelni Cedex technicznych, 02 – Francja, oraz pro-<br />

Solide<br />

wadzących Fraunh<strong>of</strong>er-Institut badania für Solare naukowe Energiesysteme w dziedzinie ISE Heidenh<strong>of</strong>strasse fotowoltaiki. 2, 79110 Wykładowcami<br />

Freiburg –<br />

byli Niemcy. pr<strong>of</strong>esorowie z czołowych europejskich placówek naukowo-badawczych<br />

w dziedzinie fotowoltaiki: Institut d’Électronique du Solide et des<br />

Tematyka wykadów i prezentacji podczas Europejska Letnia Szkoa Fotowoltaiki dotyczya<br />

Systčmes ogniw sonecznych 23 Rue na du bazie Loess krzemu – BP krystalicznego 2067037 Strasbourg i mikrokrystalicznego Cedex 02 i – ich Francja, waciwoci, oraz<br />

Fraunh<strong>of</strong>er-Institut charakterystyki i technologii für procesów, Solare ogniw Energiesysteme sonecznych cienkowarstwowych ISE Heidenh<strong>of</strong>strasse na bazie CdTe, 2,<br />

79110 CIS i Freiburg krzemu amorficznego, – Niemcy. ogniw sonecznych na bazie zwizków organicznych, ogniw<br />

wysokosprawnych i wielozczowych na bazie GaAs, pomiarów i charakterystyki parametrów<br />

Tematyka materiaowych wykładów i parametrów i prezentacji pracy podczas ogniw Europejskaiej sonecznych, Letniej oraz Szkoły moduów Fotowoltaiki,<br />

dotyczyła ogniw słonecznych na bazie krzemu krystalicznego i mikro-<br />

i systemów fotowoltaicznych.<br />

krystalicznego Ponisze wydanie i w ich ramach właściwości, czasopisma ELEKTRONIKA charakterystyki zawiera i technologii zbór nadesanych procesów, artykuów. ogniw<br />

słonecznych cienkowarstwowych na bazie CdTe, CIS i krzemu amorficznego,<br />

ogniw słonecznych na bazie związków organicznych, ogniw wysokosprawnych<br />

i wielozłączowych na bazie Projekt GaAs, Nr POKL.04.02.00-00-006/09<br />

pomiarów i charakterystyki parametrów materiałowych<br />

i parametrów pracy ogniw słonecznych oraz modułów i systemów<br />

Publikacja wspófinansowana ze rodków Unii Europejskiej<br />

fotowoltaicznych.<br />

w ramach Europejskiego Funduszu Spoecznego.<br />

Poniższe wydanie w ramach czasopisma ELEKTRONIKA zawiera<br />

zbiór nadesłanych artykułów.<br />

Projekt Nr POKL.04.02.00-00-006/09<br />

Publikacja współfinansowana ze środków Unii Europejskiej<br />

w ramach Europejskiego Funduszu Społecznego.


Spis treści<br />

str.<br />

1. Analysis <strong>of</strong> selected factors reducing <strong>the</strong> efficiency <strong>of</strong> silicon solar cells 80<br />

2. Low-ohmic contacts on <strong>the</strong> basis <strong>of</strong> silver nanopowder dedicated to photovoltaic cells 82<br />

3. Alternative solutions for texturization <strong>of</strong> silicon solar cells 85<br />

4. Laser texturing and microtreatment <strong>of</strong> silicon for photovoltaics 89<br />

5. Recycling <strong>of</strong> silicon solar cells 92<br />

6. Evaluation <strong>of</strong> surface morphology <strong>of</strong> texturized Si wafers for solar cells applications 94<br />

7. Printed transparent electrodes with graphene nanoplatelets 97<br />

8. Photoelectrical properties <strong>of</strong> photovoltaic structures based on CdTe/ZnO 100<br />

9. Multifunctional coatings for solar cells application 102<br />

10. Antireflection coating with plasmonic metal nanoparticles for photovoltaic applications 105<br />

11. Technological issues and optimization processes <strong>of</strong> junctions in a multijunction pv<br />

cell based on ingap/ingaas/ge materials 107<br />

12. GaAsN as a photovoltaic material – photoelectrical characterization 109<br />

13. AP-MOVPE technology <strong>of</strong> AIIIBV-N heterostructures for photovoltaic applications 111<br />

14. Rare earth activated YAM materials as solar spectrum converters for photovoltaics 113<br />

15. Photocatalytic degradation <strong>of</strong> <strong>the</strong> organic compounds enhanced by chemical oxidants 115<br />

16. Organic photovoltaics – chosen aspects 118<br />

17. Surface morphology and optical properties <strong>of</strong> polymer thin films 120<br />

18. Photovoltaic bulk heterojunctions with interpenetrating network based on<br />

semiconducting polymers 122<br />

19. Experimental Validation <strong>of</strong> New Classes <strong>of</strong> Dyes for Dye-Sensitised Solar Cells 126<br />

20. Azaheterocyclic materials for organic photovoltaic cells 129<br />

21. The influence <strong>of</strong> inclination and azimuth angle <strong>of</strong> PV modules on <strong>the</strong> energetic gain 131<br />

22. Research achievements <strong>of</strong> <strong>the</strong> DSOD Lodz in <strong>the</strong> field <strong>of</strong> photovoltaics 134<br />

Elektronika 6/2012 79


Analysis <strong>of</strong> selected factors reducing <strong>the</strong> efficiency<br />

<strong>of</strong> silicon solar cells<br />

Agnieszka Stańco, AGH University <strong>of</strong> Science and Technology, Department <strong>of</strong> Electronics, Kraków<br />

Nowadays <strong>the</strong> dominated method <strong>of</strong> receiving energy from natural<br />

sources is photovoltaic (PV). Electric power generation from<br />

sun is realized by solar cells closed in modules. Presently used<br />

materials for <strong>the</strong> cells include mono – crystalline or multi – crystalline<br />

silicon, amorphous silicon and o<strong>the</strong>r compounds based<br />

on exemplary cadmium telluride, copper indium gallium selenide<br />

or even polymers and organic combinations. The main factor describing<br />

PV cells is its efficiency, which is determined by electrical<br />

parameters. The possibility <strong>of</strong> long term using solar systems<br />

requires to fulfill o<strong>the</strong>r conditions which have to limit <strong>the</strong> impact<br />

<strong>of</strong> technology, temperature and lapse <strong>of</strong> time on correct working<br />

<strong>of</strong> cells [1]. The producers state decreasing 90% <strong>of</strong> power solar<br />

systems in about 12 years and 80% in 20 years [2]. The author <strong>of</strong><br />

this work call attention that it is very important for cell to be laminated<br />

after manufacturing processes in order to secure <strong>the</strong>m from<br />

air and humidity. In addition, ambient conditions have also great<br />

influence for working <strong>of</strong> solar cells. It is reported that increasing<br />

temperature in conjunction with solar irradiation also decreases<br />

efficiency <strong>of</strong> PV systems [3].<br />

Experimental<br />

The eight year-old mono and multi-crystalline silicon solar cells<br />

(Cz-Si and mc-Si) <strong>of</strong> area 100 cm 2 were investigated. Morphology<br />

<strong>of</strong> cells surface was studied by using <strong>of</strong> Scanning Microscopy<br />

(NOVA NANO SEM 200, FEI Company). The measurements <strong>of</strong><br />

reflectance R(λ) were carried out by two beam spectrophotometer<br />

UV-VIS-NIR Lambda 19 Perkin Elmer with integration sphere.<br />

Electrical parameters such as: short circuit current I sc<br />

, open<br />

circuit voltage U oc<br />

, fill factor FF and efficiency η were measured<br />

by computer controlled global spectrum sun simulator I-V Curve<br />

Tracer For Solar Cells Qualification. The simulation <strong>of</strong> temperature<br />

impact on efficiency were realized by PC1D Version 5.4<br />

application.<br />

Results and Discussion<br />

SEM studies reveal map <strong>of</strong> elements presence for surface mc-Si<br />

with tagged electrical contact, where it is shown amplified amount<br />

<strong>of</strong> silver (Ag) – Fig. 1.<br />

We can see <strong>the</strong> presence <strong>of</strong> oxygen and titanium on <strong>the</strong> whole<br />

surface <strong>of</strong> solar cell. It is normally because <strong>the</strong> silicon during<br />

manufacturing process was coated by antireflective (ARC) layer<br />

(TiO 2<br />

). On <strong>the</strong> o<strong>the</strong>r hand <strong>the</strong> presence <strong>of</strong> oxygen on electrical<br />

Fig. 2. SEM image <strong>of</strong> Ag contact structure<br />

contacts is not desirable. The structure <strong>of</strong> contact is porous, which<br />

favors <strong>the</strong> oxidizing process – Fig. 2. Finally, it leads to significant<br />

decrease <strong>of</strong> current parameters and <strong>the</strong>n efficiency.<br />

The optical investigation were carried out on multicristalline<br />

(ARC – a-Si:N:H) and moncrystalline (ARC – TiO 2<br />

) cells with thickness<br />

300 µm (eight – year old) and 210 µm (three – year old) and<br />

confronted. The measurements <strong>of</strong> reflectance R(λ) allowed to calculate<br />

effective reflectance R(λ) eff,<br />

where N ph<br />

(λ) constitute stream <strong>of</strong><br />

solar spectrum fotons AM1,5 in range <strong>of</strong> length <strong>of</strong> light (λ) [4].<br />

The range <strong>of</strong> integration (400…1100 nm) results from having <strong>the</strong><br />

greatest solar cell photosensitive, and <strong>the</strong>n quantum efficiency.<br />

The parameter R(λ) eff<br />

indicates to what degree a part <strong>of</strong> short<br />

circuit current is lost as a result <strong>of</strong> light reflectance. The best value<br />

<strong>of</strong> effective reflectance (5,1%) has Cz-Si with thickness 210 µm<br />

– Fig. 4. It results from manufacturing process <strong>of</strong> mono-crystalline<br />

solar cells, where first <strong>the</strong>y are texturized and <strong>the</strong>n coated with<br />

antireflective layer. Texturized surface allows for better radiation<br />

absorption in infrared.<br />

80<br />

Fig. 1. SEM image <strong>of</strong> mc-Si solar cell with<br />

electrical contact – a) and map <strong>of</strong> elements<br />

distribution<br />

Fig. 3. Spectral reflectance dependence for silicon solar cells with<br />

thickness 300 µm<br />

Elektronika 6/2012


Fig. 4. Spectral reflectance dependence for silicon solar cells with<br />

thickness 210 µm<br />

As is shown on Fig. 3 and 4, <strong>the</strong> worst distribution <strong>of</strong> reflectance<br />

in range <strong>of</strong> wavelength has mc-Si with 300 µm thickness.<br />

Mono-crystalline cells regardless <strong>of</strong> its age and thickness have<br />

better optical parameters than mc-Si ones.<br />

The measurement <strong>of</strong> I-V characteristic is <strong>the</strong> most important<br />

method <strong>of</strong> evaluation <strong>of</strong> changes electrical parameters toge<strong>the</strong>r<br />

with lapse <strong>of</strong> time. Change <strong>of</strong> parameters eight year out, such as:<br />

short circuit current I sc<br />

, open circuit voltage U oc<br />

, fill factor FF and<br />

efficiency η are presented for chosen solar cells – Table.<br />

Change <strong>of</strong> electrical parameters for chosen mc-Si and Cz-Si silicon solar<br />

cells.<br />

mc-Si<br />

solar cells<br />

Decrease η<br />

[%]<br />

Decrease I SC<br />

[%]<br />

Decrease U OC<br />

[%]<br />

Decrease FF<br />

[%]<br />

mc-Si_1 19 4,7 5,4 10<br />

mc-Si_2 19 3,8 5,2 10<br />

mc-Si_3 11 1,9 4,8 2<br />

mc-Si_4 19 5,9 5,3 9<br />

Cz-Si<br />

solar cells<br />

Decrease η<br />

[%]<br />

Decrease I SC<br />

[%]<br />

Decrease U OC<br />

[%]<br />

Decrease FF<br />

[%]<br />

Cz-Si_5 17 5,1 9,1 2<br />

Cz-Si_6 16 4,5 9,3 2<br />

Cz-Si_7 20 6,2 10 3<br />

Cz-Si_8 19 7 10 2<br />

Cz-Si_9 25 5,8 15 5<br />

Cz-Si_10 16 6 8,9 2<br />

Fig. 5. The I-V characteristics <strong>of</strong> chosen solar cells (mc-Si and Cz-Si)<br />

measured eight years ago and at present<br />

The comparison between I-V characteristic for chosen multicrystalline<br />

(mc-Si_4) and mono – crystalline solar cell (Cz-Si_9) is<br />

shown on Fig. 5. The average decrease <strong>of</strong> efficiency for mc-Si is<br />

17% and Cz-Si – 19%. The producers <strong>of</strong> solar cells claim average<br />

fall <strong>of</strong> productivity about 10% after ten years. It is worth to pay attention,<br />

that that value refers to cells placed in solar modules and<br />

being protect against external circumstances influence.<br />

The average fall <strong>of</strong> short circuit current is about 4% and 6%.<br />

Density <strong>of</strong> short circuit current depends on current flow when <strong>the</strong><br />

voltage is zero and also on solar cell surface. Both groups <strong>of</strong> cells<br />

have <strong>the</strong> same construction. Change <strong>of</strong> materials parameters has<br />

a great impact on current. Open circuit voltage reduces about 5%<br />

(mc-Si) and 10% (Cz-Si). It can be seen that mono – crystalline<br />

cells are more sensitive to lapse <strong>of</strong> time. The potential difference<br />

between p and n regions, which define open circuit voltage<br />

depends on number <strong>of</strong> generated carriers in p area (holes) and<br />

n area (electrons). Reducing <strong>of</strong> this voltage vindicates material<br />

changes in p and n – type regions. Fall <strong>of</strong> I sc<br />

and U oc<br />

represents<br />

diminish <strong>of</strong> investigated cells efficiency.<br />

Temperature has a great impact on working PV systems. Behavior<br />

<strong>of</strong> electric parameters in conjunction with changes <strong>of</strong> tempe-<br />

Fig. 6. The efficiency η, open circuit voltage U oc<br />

, short circuit current<br />

I sc<br />

depending on temperature<br />

Elektronika 6/2012 81


ature were simulated for solar cell with standard area (100 cm 2 )<br />

and thickness – 210 µm – Fig. 6.<br />

The mechanism <strong>of</strong> current flow in internal structure <strong>of</strong> cell is<br />

linked with temperature. Increase <strong>of</strong> temperature leads to linear<br />

fall <strong>of</strong> open circuit voltage and almost linear growth <strong>of</strong> short circuit<br />

current. Heating reduces <strong>the</strong> size <strong>of</strong> energy gap. The increase in<br />

<strong>the</strong> short-circuit current is caused by rising <strong>the</strong> absorbed photon<br />

current [5]. The heating has a destructive effect on <strong>the</strong> opencircuit<br />

voltage. Finally, <strong>the</strong> efficiency <strong>of</strong> solar cell is reduced<br />

accordingly. It is caused by reducing <strong>of</strong> conductivity. The rise <strong>of</strong><br />

temperature is also linked with high irradiation in areas with poor<br />

ventilation. It is very desirable to employ a good air circulation<br />

among PV systems.<br />

Summary<br />

The work referred to research on <strong>the</strong> factors which may contribute<br />

to decrease <strong>the</strong> efficiency <strong>of</strong> solar cells. Progressive degradation<br />

process for multi- and mono -crystalline solar cells was observed.<br />

Optical measurements showed that effective reflectance is worse<br />

for mc-Si than Cz-Si cells although <strong>the</strong>y were coated antireflective<br />

layer. It is caused generally by <strong>the</strong>ir heterogeneous structure. The<br />

process <strong>of</strong> texturization in case <strong>of</strong> Cz-Si improves <strong>the</strong>ir optical parameters.<br />

On <strong>the</strong> o<strong>the</strong>r hand it contributes to faster oxidation <strong>the</strong>ir<br />

surfaces. The process <strong>of</strong> obsolescence was finally corroborated by<br />

electrical measurements. The simulation <strong>of</strong> impact <strong>of</strong> heating showed,<br />

that its high values cause decrease <strong>of</strong> solar cells efficiency.<br />

References<br />

[1] Swatowska B., A. Stańco, P. Panek: The causes <strong>of</strong> silicon solar cells<br />

degradation. 35th International Microelectronics and Packaging IMAPS<br />

– IEEE CPMT 21–24 September, Gdańsk–Sobieszewo, Poland, 2011.<br />

[2] http://www.globalmarket.com/<br />

[3] Swatowska B., T. Stapiński, J. Chojnacki: Parameters <strong>of</strong> silicon solar<br />

cells in changeable external conditions. 23 International Conference <strong>of</strong><br />

IMAPS – CPMT IEEE 21–24 September, Pszczyna, Poland, 2009.<br />

[4] Strehlke S., S. Bastide, J. Guillet, C. Lévy – Clément: Design <strong>of</strong> porous<br />

silicon antireflection coatings for silicon solar cells. Materials<br />

Science and Engineering B69 – 70 (2000) 81–86.<br />

[5] Würfel P.: Physics <strong>of</strong> Solar Cells – From Principles to New Concepts.<br />

WILEY – VCH Verlag GmbH & Co. KGaA, Weinheim, Germany 2005.<br />

Low-ohmic contacts on <strong>the</strong> basis <strong>of</strong> silver nanopowder<br />

dedicated to photovoltaic cells<br />

Joanna Kalbarczyk, Marian Teodorczyk, Konrad Krzyżak, Grzegorz Gawlik,<br />

Mateusz Jarosz, Anna Młożniak<br />

Institute <strong>of</strong> Electronic Materials Technology, Warszawa<br />

Obtaining satisfactory parameters <strong>of</strong> solar cell is largely dependent<br />

on mastering <strong>the</strong> technique <strong>of</strong> producing contacts metal<br />

– semiconductor. The m-s contact can be strictly mechanical<br />

connection (e.g. trough contact electrode with semiconductor) as<br />

well as inseparable, integral connection. The m-s contacts may<br />

be regarded as ideal if <strong>the</strong>y generate no additional resistance for<br />

<strong>the</strong> current, do not react chemically with semiconductor material,<br />

do not change characterictics while changing illumination,<br />

temperature or value <strong>of</strong> applied electric filed and do not exhibit<br />

rectifying effects. Such contact can be described with Ohm’s law<br />

– its current-voltage characteristic is linear. It is obvious that ideal<br />

contacts do not exist, but <strong>the</strong> aim <strong>of</strong> <strong>the</strong> technology is to approach<br />

as close as possible to <strong>the</strong> imaginary ideal contact.<br />

To eliminate serious mistakes while creating ohmic contacts it<br />

is necessary to know <strong>the</strong> type <strong>of</strong> conduction. The material chosen<br />

for contacts should create impurity centers that comply with semiconductor’s<br />

type <strong>of</strong> conduction – it should contain donor impurities<br />

in case <strong>of</strong> contact with n – type semiconductor and acceptor<br />

impurities in case <strong>of</strong> p-type semiconductor.<br />

Except specified electrical properties <strong>the</strong>re are many o<strong>the</strong>r<br />

physicochemical properties (such as good <strong>the</strong>rmal conductivity,<br />

compatibility <strong>of</strong> metal and semiconductor coefficient <strong>of</strong> heat expansion,<br />

proper mechanical strength in different temperatures)<br />

that should characterize a useful m-s contact [1].<br />

In <strong>the</strong> following <strong>the</strong>sis, two methods <strong>of</strong> creating contacts will<br />

be presented.<br />

The first method is <strong>the</strong> cathode sputtering. The silicon base is<br />

subjected to high vacuum ion bombardment in inert atmosphere,<br />

<strong>the</strong>n silver layer is deposited. The method provides obtaining layer<br />

<strong>of</strong> high degree <strong>of</strong> purity.<br />

The second method <strong>of</strong> making contacts is applying <strong>the</strong> paste<br />

with silver nanopowder. The method is quite new and is an attempt<br />

<strong>of</strong> confrontation already well-known and bringing satisfactory<br />

effects in silver contact preparation procedure (sputtering)<br />

and a new method which uses up-to-date nanomaterial that may<br />

exhibit interesting properties for <strong>the</strong> technique <strong>of</strong> solar cells contact<br />

preparation [2, 4].<br />

82<br />

Tabl. 1. Properties <strong>of</strong> nanosilver layers in Si substrates [4]<br />

Sample<br />

Polished side,<br />

firing in 300 0 C<br />

Mat side, firing<br />

in 300 0 C<br />

Polished side,<br />

firing in 600 0 C<br />

Mat side, firing<br />

in 600 0 C<br />

Resistance/□<br />

[mΩ/□]<br />

Layer<br />

thickness [µm]<br />

Resistivity<br />

[Ω∙m×10 -8 ]<br />

Up to now silver pastes used in screen printing technique have<br />

been composed <strong>of</strong> silver powder (conductive phase), where grain<br />

size varied between 1 and 3 µm, and glaze (auxiliary phase)<br />

which has helped in <strong>the</strong> sintering process. The characteristic temperature<br />

<strong>of</strong> sintering silver layers is 850 0 C. If <strong>the</strong> composition <strong>of</strong><br />

glaze has been appropriate – <strong>the</strong> temperature <strong>of</strong> sintering might<br />

have been reduced to about 650 0 C. The temperature <strong>of</strong> firing<br />

commercially used pastes (e.g. Du Pont) achieves about 750 0 C,<br />

which is temperature <strong>of</strong> silicon wafer when temperature setting is<br />

920 0 C [3–5].<br />

Silver nanopowder paste represents a new generation <strong>of</strong><br />

thick-layer materials dedicated to screen printing. The conductive<br />

phase is silver powder where grains size range from a few to over<br />

a dozen nanometers. Methyl polimethacrylate has been used as<br />

<strong>the</strong> matrix <strong>of</strong> nanocomposite [4, 5].<br />

Nanomaterials <strong>of</strong>ten manifest new specific properties – physicochemical,<br />

electrical, magnetic or catalytic. These new characteristics<br />

arise from <strong>the</strong> quantum effects and high share <strong>of</strong> surface<br />

atoms causing high reactivity. In consequence melting point<br />

<strong>of</strong> nanoparticles can be much lower than microcrystalline solid.<br />

Many o<strong>the</strong>r new phenomena may also occur in nanoscale [2-4].<br />

Preliminary research in <strong>the</strong> field <strong>of</strong> silver nanopowder paste<br />

contact preparation for silicon solar cells has brought to wellconducting<br />

layer forming through firing in much lower tempera-<br />

Adhesion<br />

12,2 2 2,44 +<br />

12,0 2 2,40 +<br />

14,35 2 2,87 +<br />

14,7 2 2,94 +<br />

Elektronika 6/2012


tures in comparison to so far used pastes. Obtained resistance<br />

results for paste firing in 300 and 600 0 C are similar. Resistivity<br />

– firing temperature relationship has shown that already from<br />

250 0 C layers exhibit very good electric conductivity which insignificantly<br />

fluctuates with <strong>the</strong> firing temperature escalation. Results<br />

<strong>of</strong> layer resistance after firing in different conditions have<br />

been shown in Table 1 [4].<br />

The main advantages <strong>of</strong> silver nanopastes is that <strong>the</strong>y do not<br />

contain <strong>the</strong> glaze phase, which impaired electric conduction <strong>of</strong><br />

<strong>the</strong> layer, and <strong>the</strong> possibility <strong>of</strong> firing in lower temperatures. Obtained<br />

results demonstrates that layers made from pastes with<br />

silver nanopowder can be used as ohmic contacts for silicon solar<br />

cells [3].<br />

Experiment<br />

The aim <strong>of</strong> <strong>the</strong> first stage <strong>of</strong> research was to determine electrical<br />

properties <strong>of</strong> contacts made with sputtering method. Such contacts<br />

have been already used to investigate electrical and optical<br />

properties <strong>of</strong> edge-illuminated silicon solar cells and enabled to<br />

acquire repeatable, satisfactory results. Because <strong>of</strong> that <strong>the</strong> sputtering-method-made<br />

contacts have been acknowledged as a good<br />

comparative base for new type <strong>of</strong> silver nanopowder contacts.<br />

Resistance has been measured with two point probe connected<br />

with multimeter. The silver layers have been applied trough<br />

a mask and three values <strong>of</strong> resistance have been measured. The<br />

scheme <strong>of</strong> <strong>the</strong> sample with applied contacts and measurement<br />

points have been shown on Fig. 1.<br />

a)<br />

b)<br />

c)<br />

Fig. 1. The scheme <strong>of</strong> measurement sample with “sputtered” contacts;<br />

R1, R2, R3 – values <strong>of</strong> resistance between marked silver layers<br />

The resistance measurement has to be made with comparative<br />

method, because obtained values do not define directly single<br />

contact resistance, but contain also resistances <strong>of</strong> particular meter<br />

circuit elements and foundation material.<br />

After sputtering process, <strong>the</strong> layer has been firing in RTP AG<br />

Heatpulse system (Rapid Thermal Processing) in nitrogen atmosphere.<br />

The parameters – temperature and time – <strong>of</strong> contacts’<br />

firing have been varied to optimize <strong>the</strong> process.<br />

Resistances have been measured both directly after sputtering<br />

process and after <strong>the</strong> firing in different conditions. It enabled<br />

to determine <strong>the</strong> optimum processing parameters <strong>of</strong> <strong>the</strong> contact<br />

preparing.<br />

Analogically experimental work concerning applying nanosilver<br />

pastes has been barely begun. The screen printer and suitable<br />

masks has been prepared for applying <strong>the</strong> paste on basic silicon<br />

wafers. Experimental research schedule has been established.<br />

Results <strong>of</strong> sputtering-method-made contacts<br />

The results for <strong>the</strong>se method have been elaborated for three kinds<br />

<strong>of</strong> base material: silicon wafer doped with boron (B), silicon wafer<br />

doped with phosphorus (P) and silicon wafer doped with antimony<br />

(Sb). The results are shown on graphs below. Resistances R1, R2<br />

and R3 are resistances between points as shown on Fig. 1.<br />

Fig. 2. Dependence <strong>of</strong> resistance on firing time for silicon doped with<br />

boron samples for defined measuring points: a) R1, b) R2, c) R3<br />

The results <strong>of</strong> resistance for boron-doped silicon samples have<br />

been presented only for measurements carried out after <strong>the</strong> firing<br />

process, because <strong>the</strong> values <strong>of</strong> resistance before that process<br />

varied between about 400 and 650 Ω. Putting such high values on<br />

<strong>the</strong> graphs would made <strong>the</strong>m illegible. The improvement <strong>of</strong> contact<br />

resistance on boron doped basis after firing is tremendous. The<br />

best results have been obtained for firing in 600 0 C.<br />

The results <strong>of</strong> P and Sb doped silicon samples have been<br />

shown both for measuring before and after <strong>the</strong> firing. The improvement<br />

for resistances after firing is clearly apparent. The optimum<br />

temperature <strong>of</strong> <strong>the</strong> process is 500 0 C for phosphorus doped<br />

samples and 600 0 C for antimony doped samples.<br />

Results <strong>of</strong> silver nanopowder paste contacts<br />

Preliminary results have been obtained for contacts applied on<br />

alundum basis with screen printing method through a mask shown<br />

on Fig. 5. After printing contacts have been dried in a band drier in<br />

120 0 C for 10 minutes. On that stage <strong>of</strong> research <strong>the</strong> contacts have<br />

been fired in Czylok chamber furnace in different temperatures for<br />

1 hour. The results <strong>of</strong> resistance measurement have been shown<br />

in Table 2. During fur<strong>the</strong>r research <strong>the</strong> firing process will be executed<br />

in RTP AG Heatpulse system.<br />

Elektronika 6/2012 83


a)<br />

a)<br />

b)<br />

b)<br />

c)<br />

c)<br />

Fig. 3. Dependence <strong>of</strong> resistance on firing time for silicon doped with<br />

phosphorus samples for defined measuring points: a) R1, b) R2, c) R3<br />

Fig. 4. Dependence <strong>of</strong> resistance on firing time for silicon doped with<br />

antimony samples for defined measuring points: a) R1, b) R2, c) R3<br />

Tabl. 2. Measurement results <strong>of</strong> silver nanopowder layers on alundum basis<br />

after firing in chamber furnace<br />

Firing<br />

temperature<br />

[ 0 C]<br />

Layer thickness<br />

after firing [µm]<br />

Path<br />

resistance<br />

[Ω]<br />

Resistance/□<br />

[mΩ/□]<br />

Resistivity<br />

[Ω∙m×10 -8 ]<br />

300 5 1,5 7,5 3,75<br />

300 5 1,5 7,5 3,75<br />

350 3 1,5 7,5 2,25<br />

Fig. 5. The scheme <strong>of</strong> a mask used for screen printing <strong>of</strong> silver<br />

nanopowder contacts on alundum basis<br />

350 3 1,4 7,0 2,10<br />

84<br />

Elektronika 6/2012


Summary<br />

The silver nanopowder pastes are relatively new kind <strong>of</strong> nanomaterials<br />

that exhibit interesting and useable properties. Research<br />

in field <strong>of</strong> preparing contacts for silicon solar cells with silver<br />

nanopowder will be continued to obtain rational results which enable<br />

to evaluate <strong>the</strong> new kind <strong>of</strong> contacts with respect to possible<br />

application in larger scale.<br />

References<br />

[1] Szaynok A., S. Kuźmiński: Podstawy fizyki powierzchni półprzewodników.<br />

Wydawnictwa Naukowo-Techniczne, Warszawa 2000<br />

[2] Kurzydłowski K., M. Lewandowska: Nanomateriały inżynierskie<br />

konstrukcyjne i funkcjonalne. Wydawnictwo Naukowe PWN, Warszawa<br />

2011<br />

[3] Panek P.: Fotowoltaika Polska 2011.<br />

[4] Młożniak A., P. Ungier, M. Jakubowska: Nowe kompozyty grubowarstwowe<br />

o obniżonej temperaturze spiekania przeznaczone na<br />

kontakty ogniwa słonecznego. Materiały elektroniczne T. 37 – 2009<br />

nr 4, 8–12.<br />

[5] Janeczek K., G. Kozioł, M. Jakubowska, A. Araźna, A. Młożniak, J.<br />

Borecki, K. Lipec: Polymer composites with nanoadditives for printed<br />

electronics. Elektronika, 2011-7, 43–46.<br />

Alternative solutions for texturization <strong>of</strong> silicon solar cells<br />

Karolina Drzymała, Politechnika Wrocławska, Wydział Elektroniki Mikrosystemów i Fotoniki<br />

Teksturyzacja ogniw krzemowych jest istotnym czynnikiem<br />

zwiększającym ich sprawność w wyniku zmniejszenia strat odbiciowych.<br />

Tekstura wprowadzona w latach 70. wytwarzana była<br />

poprzez trawienie chemiczne podłoży krzemowych o orientacji<br />

krystalograficznej (100). W patencie zgłoszonym w 1975 roku<br />

[1] zaprezentowano jako roztwór trawiący mieszaninę wodorotlenku<br />

potasu, glikolu etylenowego i wody. Parametry procesu to:<br />

temperatura 80°C, czas trawienia 1 godzina. W rezultacie na powierzchni<br />

krzemu wytworzone zostały charakterystyczne struktury<br />

w kształcie piramid, będące efektem anizotropowego trawienia<br />

i ujawniania się płaszczyzn {111} (rys. 1 i 2).<br />

Do dzisiaj z powodzeniem w przemyśle fotowoltaicznym wykorzystywane<br />

są roztwory teksturyzujące na bazie KOH. Jed-<br />

Rys. 1. Płaszczyzny {111}<br />

w krysztale krzemu<br />

Rys. 2. Piramida powstająca<br />

na powierzchni (100) w trakcie<br />

trawienia anizotropowego [2]<br />

nakże prowadzi się prace dążące do usprawnienia tego procesu<br />

przez np. skrócenie czasu trawienia, co prowadzi do oszczędności<br />

energii. Szuka się także nowych dodatków (surfaktantów) do<br />

podstawowego roztworu w celu uzyskiwania tekstury o niższym<br />

współczynniku odbicia niż osiągane do tej pory.<br />

Przegląd roztworów teksturujących<br />

Wodorotlenek potasu<br />

Wodorotlenek potasu jest podstawowym roztworem do teksturyzacji.<br />

Wyniki przedstawione w pracy [3] wskazują, że do<br />

teksturyzacji należy stosować roztwory o małym stężeniu KOH<br />

(rys. 3). Wodorotlenek potasu w czystej postaci jest obecnie<br />

rzadko stosowany, ze względu na małą jednorodność piramid<br />

na trawionej powierzchni, wynikającą z trudności w odprowadzaniu<br />

wodoru wydzielającego się podczas procesu. Łatwiejsze<br />

usuwanie wodoru z trawionej powierzchni można osiągnąć<br />

przez obniżenie napięcia powierzchniowego roztworu, dlatego<br />

często do roztworu KOH dodaje się związki powierzchniowo<br />

czynne (surfaktanty).<br />

Wodorotlenek potasu z dodatkiem alkoholu<br />

izopropylowego<br />

W pracy [4] zbadano wpływ dodatku alkoholu izopropylowego<br />

o różnych stężeniach molowych na morfologię otrzymywanej<br />

w procesie trawienia powierzchni. Roztwór taki jest obecnie najszerzej<br />

stosowany w technologicznych procesach wytwarzania<br />

ogniw słonecznych. Zaobserwowano, że wraz ze wzrostem stężenia<br />

dodanego alkoholu zmniejsza się rozwinięcie powierzchni<br />

tzn. staje się ona coraz gładsza. Zestawienie rezultatów trawienia<br />

przedstawiono na rys. 2.<br />

a) b)<br />

Rys. 3. Powierzchnia (100) Si trawiona w KOH o różnym stężeniu: a) 70°C, b) 90°C [3]<br />

Elektronika 6/2012 85


0,5 M IPA 1 M IPA 1,5 M IPA 2 M IPA<br />

Rys. 4. Morfologie powierzchni uzyskiwane w wyniku trawienia w roztworze 3 M KOH + IPA [4]<br />

Zestawienie parametrów procesów teksturowania krzemu i wyników teksturyzacji<br />

Nazwa<br />

roztworu<br />

Warunki procesu<br />

Zdjęcia SEM steksturowanej powierzchni<br />

Współczynnik<br />

odbicia<br />

Wodorotlenek potasu 2% KOH, 60°C [5]<br />

70%<br />

Wodorotlenek potasu<br />

+ izopropanol<br />

(KOH+IPA)<br />

KOH (45%):IPA:DIW<br />

1:6:55 (obj)<br />

ok.1% KOH+10% IPA,<br />

80°C, 30 min [6]<br />

10~11%<br />

Wodorotlenek potasu<br />

+ tert-butanol<br />

(KOH + TBA)<br />

KOH (45%):TBA:DIW<br />

1:6:55(obj)<br />

ok.1% KOH+10% IPA,<br />

80°C,30 min [6]<br />

12%<br />

Wodorotlenek<br />

tetra-metylo-amoniowy<br />

+ izopropanol (TMAH<br />

+ IPA)<br />

2% TMAH + 9% IPA,<br />

90°C, 30 min [7]<br />

13%<br />

Wodorotlenek<br />

tetra-etylo-amoniowy<br />

(TEAH)<br />

5 wt.% TEAH, 90°C,<br />

60 min [8]<br />

14,5%<br />

Z rysunków 3 i 4 wynika, że do teksturowania należy stosować<br />

roztwory o małym stężeniu KOH z niewielkim dodatkiem związku<br />

powierzchniowo-czynnego.<br />

Inne roztwory do teksturyzacji<br />

86<br />

Na podstawie danych literaturowych zebrano parametry procesu<br />

i wyniki trawienia w kilku wybranych roztworach stosowanych do<br />

teksturyzacji. W tabeli zestawiono roztwory trawiące, warunki procesu<br />

i rezultaty trawienia w postaci zdjęć SEM oraz współczynników<br />

odbicia (reflektancji).<br />

Wodorotlenek potasu z dodatkiem 1,2-penatanediolu<br />

Pomimo popularności, roztwory z dodatkiem alkoholu izopropylowego<br />

mają poważną wadę – niską temperaturę wrzenia alkoholu<br />

(82 o C). Fakt ten powoduje, że w trakcie trwania procesu trawienia<br />

znaczna część alkoholu może odparować, zmieniając stężenie<br />

Elektronika 6/2012


B<br />

A<br />

Rys. 5. Modele cząsteczek stosowanych surfaktantów: A – izopropanol,<br />

B – 1,2-pentanediol, C – 1,2-heksanediol<br />

C<br />

A B C<br />

Rys. 6. Wyniki teksturyzacji w roztworze 1 M KOH + 2% 1,2-pentanediolu<br />

[2]:<br />

A – temperatura 75 o C, czas 10 min, B – temperatura 80 o C, czas 15 min,<br />

C – temperatura 90 o C, czas 20 min<br />

Rys. 7. Zdjęcia mikroskopowe podłoży krzemowych trawionych w roztworze 3 M KOH z dodatkiem 1,2-penatanediolu o różnym stężeniu<br />

Rys. 8. Zdjęcia mikroskopowe podłoży krzemowych trawionych w różnych warunkach<br />

molowe roztworu. Dodatkowo, niemożliwe jest skrócenie czasu<br />

trwania procesu poprzez podwyższenie temperatury (prędkość<br />

trawienia zwiększa się wraz ze wzrostem wartości temperatury).<br />

W celu wyeliminowania tej wady, zaproponowano jako surfaktant<br />

diole, tzn. alkohole z dwiema grupami hydroksylowymi (rys. 3).<br />

Diole mają dużo wyższe temperatury wrzenia – w przypadku 1,2-<br />

pentanediolu jest to 206 o C, a 1,2-heksanediolu 223 o C.<br />

Wstępne badania, przeprowadzone w Zakładzie Mikroelektroniki<br />

i Nanotechnologii WEMiF PWr wskazują, ze możliwe jest<br />

teksturowanie z wykorzystaniem 1,2-pentanediolu (rys. 6).<br />

W ramach pracy własnej przebadano morfologię otrzymywanych<br />

powierzchni krzemowy podłoży o orientacji krystalograficznej<br />

(100) w zależności od zawartości 1,2-pentanediolu w roztworze<br />

3 M KOH. Badania morfologii powierzchni przeprowadzono wyko-<br />

Elektronika 6/2012 87


Rys. 9. Zdjęcia mikroskopowe podłoży krzemowych trawionych w roztworze 3 M KOH z dodatkiem 1,2-heksanediolu o różnym stężeniu<br />

rzystując mikroskop optyczny. Proces trawienia przeprowadzany<br />

był z wykorzystaniem mieszania mechanicznego, w temperaturze<br />

75 o C. Dla kilku próbek zbadano także wpływ innych parametrów<br />

procesu. Zdjęcia otrzymanych powierzchni w zależności od stężenia<br />

molowego surfaktantu przedstawiono na rys. 7.<br />

W trakcie doświadczenia zbadano także wpływ mieszania<br />

i wzrostu temperatury na morfologię otrzymywanych powierzchni.<br />

Rezultaty przedstawiono na rys. 8.<br />

Wodorotlenek potasu z dodatkiem 1,2-hexanediolu<br />

W następnym etapie prac zbadano zależność powstającej tekstury<br />

od zawartości w roztworze 1,2-hexanediolu. Jest to diol o większej<br />

ilości węgla w łańcuchu, a przez to o większej masie molowej<br />

i gęstości niż 1,2-penatanediol. Zdjęcia spod mikroskopu optycznego<br />

otrzymanych w rezultacie trawienia powierzchni zestawiono<br />

na rys. 9.<br />

Wnioski<br />

W roztworze 3 M wodorotlenku potasu z dodatkiem IPA uzyskiwanie<br />

rozwiniętych powierzchni możliwe jest tylko dla stężenia<br />

alkoholu mniejszego od 1 M. Przy wyższych stężeniach obserwowane<br />

jest powstawanie pojedynczych struktur piramidalnych na<br />

gładkiej powierzchni. W celu otrzymania tekstury charakteryzującej<br />

się niską reflektancją, dąży się do tego, aby steksturowanie<br />

było jednorodne i pokrywało całą powierzchnię (bez odstępów<br />

pomiędzy piramidami). Tekstury otrzymane w roztworach 0,5 M<br />

i 1 M IPA spełniają powyższy warunek.<br />

W roztworze 3 M wodorotlenku potasu z dodatkiem 1,2-pentanediolu<br />

obserwuje się powstawanie tekstury tylko dla stężeń diolu<br />

poniżej 0,2 M, przy czym najbardziej jednorodna powierzchnia powstaje<br />

w rozworze o stężeniu 0,1 M. Wpływ mieszania w procesie<br />

teksturyzacji jest niezauważalny, więc może ono zostać pominięte<br />

w procesie przemysłowym. Podwyższona do 90 o C temperatura<br />

wyraźnie zmienia otrzymywaną powierzchnię. Widoczne są pojedyncze<br />

duże hilloki wyróżniające się nad powierzchnią tekstury.<br />

W celu określenia, jak takie zmiany wpływają na poziom reflektancji<br />

wykonane zostaną pomiary spektralne tego współczynnika.<br />

Teksturyzacja w roztworze 3 M KOH z dodatkiem 1,2-heksanediolu<br />

możliwa jest tylko dla stężenia diolu 0,01…0,03 M. Najbardziej<br />

jednorodnie rozwiniętą powierzchnię otrzymuje się dla<br />

roztworu o zawartości molowej diola równej 0,02 M.<br />

Wynika stąd, że im większe są cząsteczki surfaktantu, tym<br />

mniejsze jego stężenie wymagane jest do otrzymania prawidłowej<br />

teksturyzacji.<br />

W celu oceny jakości teksturyzacji przeprowadzone zostaną<br />

spektralne pomiary współczynnika odbicia, a także pomiary mikroskopowe<br />

SEM, w celu dokładnego określenia kształtu i wielkości<br />

piramid tworzących teksturę.<br />

Wskazane byłyby również badania w roztworach o mniejszej<br />

koncentracji wodorotlenku potasu a także badania z wykorzystaniem<br />

innych surfaktantów.<br />

Literatura<br />

[1] Bailey W. L. i In.: Texture etching <strong>of</strong> silicon: method. United States<br />

Patent 4137123, 1979.<br />

[2] Banaszczyk K. A.: Teksturyzacja powierzchni podłoży krzemowych<br />

w zastosowaniu do baterii słonecznych. Praca magisterska, Wydział<br />

Elektroniki Mikrosystemów i Fotoniki, Politechnika Wrocławska<br />

2011.<br />

[3] Zubel I.: Kształtowanie struktur przestrzennych w krzemie metodą<br />

trawienia anizotropowego do zastosowań w mikroelektronice. Oficyna<br />

Wydawnicza Politechniki Wrocławskiej. Wrocław 2004.<br />

[4] Zubel I., Kramkowska M.: The effect <strong>of</strong> isopropyl alcohol concentration<br />

on <strong>the</strong> etching process <strong>of</strong> Si-substrates in KOH solutions. Sensors<br />

and Actuators A 171 (2011) 436–445.<br />

[5] Chen Jian Wu, Pal-Jen Wei, Jen Fin Lin: Reflectivity <strong>of</strong> an etched<br />

silicon surface with pyramids: II. Experimental results from different<br />

etching conditions. Journal <strong>of</strong> Micromechanics and Microengineering,<br />

Taiwan 2009, s. 1–5.<br />

[6] Park H., J.S. Lee, H.J. Lim, D. Kim: The Effect <strong>of</strong> Tertiary-Butyl Alcohol<br />

on <strong>the</strong> Texturing <strong>of</strong> Crystalline Silicon Colar Cells. Journal <strong>of</strong> <strong>the</strong><br />

Korean Physical Society, Vol. 55, No. 5, Listopad 2009.<br />

[7] Papet P., O. Nichiporuk, A. Fave, A. Kaminski, B. Bazer-Bachi,<br />

M. Lemiti: TMAH texturisation and etching <strong>of</strong> interdigitated backcontact<br />

solar cell. Materials Science-Poland, Vol. 24, No. 4, 2006,<br />

s. 1044–1049.<br />

[8] Chaoui R., Y. Si Ahmed: Teksturisation <strong>of</strong> silicon in TEAH. 19th<br />

<strong>European</strong> Photovoltaic Solar Energy Conference. Paryż 2004,<br />

s. 860–863.<br />

88<br />

Elektronika 6/2012


Laser texturing and microtreatment <strong>of</strong> silicon<br />

for photovoltaics<br />

LECH A. Dobrzański, ALEKSANDRA Drygała, MAŁGORZATA Musztyfaga<br />

Politechnika Śląska, Instytut Materiałów Inżynierskich i Biomedycznych, Gliwice<br />

Wyczerpujące się zasoby konwencjonalnych źródeł energii oraz<br />

wzrastające zanieczyszczenie środowiska naturalnego powodują,<br />

że od wielu lat prowadzi się badania nad pozyskiwaniem<br />

energii ze źródeł odnawialnych. Należą do nich także ogniwa słoneczne,<br />

ich zdecydowana większość wykonywana jest z krzemu.<br />

Obecnie, techniki wytwarzania krzemowych płytek i ogniw fotowoltaicznych,<br />

zdominowały znaczą część komercyjnego rynku<br />

fotowoltaicznego poprzez ich ogromny zapas. Prognozuje się<br />

nawet, że aktualny stan dominacji utrzyma się aż do roku 2020.<br />

Technologia laserowa w produkcji ogniw fotowoltaicznych staje<br />

się nieodzownym elementem współczesnej fotowoltaiki, stwarza<br />

możliwość precyzyjnej mikroobróbki materiału i powierzchni dzięki<br />

wykorzystaniu wiązki laserowej jako zautomatyzowanego narzędzia<br />

służącego do ich kształtowania. W pracy przedstawiono<br />

wykorzystanie technik laserowych do teksturowania powierzchni<br />

krzemu oraz selektywnego spiekania laserowego elektrod przednich<br />

na powierzchni krzemu [1–4].<br />

Zastosowanie laserów do wytwarzania<br />

krzemowych ogniw fotowoltaicznych<br />

Technologia laserowa znajduje szerokie zastosowanie w różnych<br />

obszarach przemysłu fotowoltaicznego. Na rys. 1 przedstawiono<br />

różne aspekty zastosowania laserów w inżynierii powierzchni<br />

w Zakładzie Technologii Procesów Materiałowych i Technik Komputerowych<br />

w Materiałoznawstwie Instytutu Materiałów Inżynierskich<br />

i Biomedycznych Politechniki Śląskiej.<br />

Główny kierunek prac badawczych z zakresu fotowoltaiki dotyczy<br />

problematyki technologii wytwarzania ogniw fotowoltaicznych<br />

z krzemu krystalicznego z adaptacją technologii laserowych do:<br />

● teksturowania powierzchni krzemu polikrystalicznego w celu<br />

zmniejszania współczynnika odbicia promieniowania słonecznego<br />

i zwiększenia sprawności fotoogniw,<br />

● selektywnego spiekania laserowego SLS (ang. Selective Laser<br />

Sintering) elektrody przedniej do jego monokrystalicznej<br />

powierzchni krzemowej, z użyciem lasera CO 2<br />

w celu poprawy<br />

jakości ogniwa przez minimalizację rezystancji połączenia<br />

elektrody przedniej z podłożem.<br />

Teksturowania powierzchni krzemu<br />

polikrystalicznego<br />

Usunięcie uszkodzonej warstwy powstałej w wyniku<br />

technologicznego cięcia bloku krzemowego<br />

Laserowe teksturowanie powierzchni i usunięcie uszkodzonej<br />

warstwy powstałej w wyniku obróbki laserowej<br />

Wysokotemperaturowe formowanie złącza p -n<br />

Usuwanie złącz z krawędzi bocznych i mycie szkliwa<br />

Jednym z zasadniczych etapów wytwarzania ogniwa fotowoltaicznego<br />

jest teksturowanie jego powierzchni. Większość chemicznych<br />

metod teksturowania powierzchni, opartych na trawieniu<br />

w wodnych roztworach wodorotlenków, stosowanych dla krzemu<br />

monokrystalicznego jest nieefektywna w przypadku krzemu polikrystalicznego<br />

z uwagi na chaotyczny rozkład orientacji ziarn.<br />

W związku z tym poszukuje się innych, alternatywnych metod<br />

teksturowania [1– 4].<br />

Metodyka badań<br />

Badania wykonano na płytkach z krzemu polikrystalicznego<br />

domieszkowanego borem firmy Bayer. Laserowe teksturowanie<br />

powierzchni krzemu zrealizowano przy użyciu systemu laserowego<br />

Allprint DN 50A firmy Alltec, w którym źródłem promieniowania<br />

jest laser ze stałym ośrodkiem czynnym – kryształem<br />

granatu itrowo-aluminiowego domieszkowanego jonami<br />

neodymu (Nd:YAG). Wykonano laserową teksturę odpowiadającą<br />

równoległym rowkom z odstępami między nimi 0,05 mm.<br />

Badanie topografii powierzchni krzemu po obróbce laserowej<br />

wykonano w skaningowym mikroskopie elektronowym ZEISS<br />

SUPRA 25. Badania właściwości elektrycznych ogniw fotowoltaicznych<br />

wytworzonych z płytek teksturowanych, laserowo<br />

wykonano na skomputeryzowanym stanowisku SOLAR‐LAB<br />

do pomiaru charakterystyk prądowo‐napięciowych (I-V) ogniw<br />

słonecznych.<br />

Wyniki badań<br />

Na rysunku 2 przedstawiono topografię powierzchni płytki z wytworzoną<br />

laserowo teksturą, odpowiadającą równoległym rowkom<br />

a) b)<br />

Rys. 2. Topografia powierzchni krzemu polikrystalicznego: a) po<br />

obróbce laserowej, b) po obróbce laserowej i usunięciu warstwy<br />

uszkodzonej<br />

Eff [%]<br />

12<br />

10<br />

8<br />

6<br />

6,92<br />

10,6<br />

11,93<br />

6,45<br />

9,89<br />

8,44<br />

11,08<br />

10,03<br />

11,08<br />

10,12<br />

20 mm/s<br />

50 mm/s<br />

80 mm/s<br />

Pasywacja powierzchni<br />

4<br />

2,29<br />

Naniesiene warstwy antyrefleksyjnej<br />

Wypalanie tylnych kontaktów<br />

Selektywne spiekanie laserowe przednich kontaktów<br />

Rys. 1. Przykładowa technologia wytwarzania krzemowego ogniwa<br />

słonecznego<br />

2<br />

0<br />

0,16<br />

1,06<br />

1,22<br />

0 20 40 60 80<br />

grubość usuniętej warstwy [µm]<br />

Rys. 3. Zależność sprawności ogniw fotowoltaicznych wykonanych<br />

z płytek o teksturze odpowiadającej równoległym rowkom wytworzonym<br />

przy odstępie między rowkami 0,05 mm od prędkości skanowania<br />

wiązki laserowej i grubości warstwy usuniętej w wyniku trawienia<br />

Elektronika 6/2012 89


ezpośrednio po obróbce laserowej oraz po usunięciu uszkodzeń<br />

powstałych w jej wyniku.<br />

Laserowe teksturowanie powierzchni krzemu polikrystalicznego<br />

pogarsza właściwości elektryczne ogniw fotowoltaicznych<br />

wykonanych z przygotowanych w ten sposób płytek (rys. 3). Ogniwa<br />

słoneczne wykonane z płytek z laserowo teksturowaną powierzchnią,<br />

charakteryzują się bardzo niską sprawnością zależną<br />

od warunków obróbki laserowej oraz rodzaju wytworzonej tekstury.<br />

W związku z tym wprowadzono dodatkowy etap polegający<br />

na trawieniu teksturowanej powierzchni w roztworze 20% KOH<br />

w temperaturze 80°C. Usuwano wierzchnią warstwę materiału<br />

o grubości w zakresie 20…80 µm.<br />

Sprawność ogniw fotowoltaicznych przygotowanych w ten<br />

sposób zwiększa się o 1,72% w porównaniu do ogniw wykonanych<br />

z płytek nieteksturowanych i trawionych i o 1,11%<br />

w stosunku do ogniw wytworzonych z płytek teksturowanych<br />

alkalicznie.<br />

Selektywne spiekanie laserowe elektrody<br />

przedniej<br />

Dokonany przegląd piśmiennictwa [5–8] oraz własne badania<br />

[9] pozwalają na stwierdzenie, że warstwa elektrody powinna<br />

spełniać różne wymogi, aby zapewnić niską rezystancję strefy<br />

połączenia elektrody z podłożem. Szczególne znaczenie posiada<br />

odpowiedni dobór materiału (elektrody i podłoża), warunki jego<br />

wytworzenia, kształt oraz rozmiar elektrody, przyczepność elektrody<br />

do podłoża i morfologia podłoża.<br />

Materiał do badań<br />

Badania wykonano na płytkach z krzemu monokrystalicznego<br />

firmy Deutsche Solar AG domieszkowanego borem. Istotnym<br />

aspektem pracy było wytworzenie siatki kontaktowej elektrody<br />

przedniej na powierzchni ogniwa fotowoltaicznego o różnej morfologii,<br />

z zastosowaniem selektywnego spiekania laserowego.<br />

Elektrody przednie opracowano w celu określenia przydatności<br />

poszczególnych past srebrnych z wykorzystaniem technologii<br />

mikroobróbki laserowej. Wyboru składu chemicznego past dokonano<br />

doświadczalnie, a mieszanki przygotowano przy użyciu<br />

mieszadła mechanicznego. Dobór udziału poszczególnych<br />

składników pasty najlepiej dobranej do wytworzenia elektrody<br />

przedniej ogniwa fotowoltaicznego przedstawiono w tabeli 1.<br />

Układ testowy elektrod wytworzono podczas mikroobróbki<br />

laserowej pasty w selektywnie wybranych obszarach na powierzchni<br />

ogniwa fotowoltaicznego za pomocą lasera gazowego<br />

CO 2<br />

, będącego na wyposażeniu urządzenia EOSINT M 250<br />

Xtended. Do najważniejszych parametrów urządzenia zalicza<br />

się: moc lasera (270 W), prędkość skanowania wiązki lasera<br />

(max. 3,0 m/s).<br />

Tab. 1. Właściwości pasty opartej na bazie proszku srebra<br />

Metodyka badań<br />

W pracy dotyczącej mikroobróbki laserowej elementów krzemowych<br />

ogniw fotowoltaicznych z monokrystalicznego krzemu wykonano<br />

następujące badania:<br />

● opracowanie układu testowego elektrod I i II (gdzie rozmiar<br />

pasków – elektrod przednich dla układu I: 2 × 10 mm, a odległości<br />

pomiędzy nimi: 2,5; 5; 10; 20 mm, natomiast rozmiary<br />

elektrod przednich dla układu II wynoszą: 5 × 10 mm, a odległości<br />

pomiędzy nimi: 1; 2, 4; 8 mm);<br />

● właściwości elektrycznych elektrody przedniej przy zastosowaniu<br />

metody linii transmisyjnych TLM (ang. Transmission<br />

Line Model ) na stanowisku opracowanym w Instytucie Materiałów<br />

Inżynierskich i Biomedycznych;<br />

● badania topografii powierzchni wytworzonej elektrody przedniej<br />

oraz strefy jej połączenia z podłożem z wykorzystaniem<br />

mikroskopii konfokalnej CLSM 5 Exciter firmy Zeiss, wyposażonym<br />

w laser diodowy o mocy 25 mW emitujący promieniowanie<br />

o długości fali 405 nm.<br />

Wyniki badań<br />

W tej części pracy dokonano klasyfikacji właściwości elektrycznych<br />

układu testowego elektrod przednich ogniw fotowoltaicznych.<br />

Jako kryterium doboru warunków pracy lasera przyjęto minimalny<br />

przedział wartości rezystancji właściwej (minimum, maksimum)<br />

połączenia elektrody przedniej z podłożem krzemowym. Na podstawie<br />

wykresów przedstawiających rezystancję w zależności od<br />

odległości pomiędzy elektrodami obliczano rezystancję właściwą<br />

kontaktów ρ c<br />

oraz wyznaczono parametr L T<br />

(droga wpływu prądu).<br />

W niniejszym artykule ogniwa fotowoltaiczne z elektrodą zbierającą<br />

wykonano na podstawie uzyskanych najlepszych rezultatów<br />

rezystancji właściwej elektrod przednich zmierzonych metodą linii<br />

transmisyjnych TLM.<br />

Bazując na wynikach doświadczalnych serii stwierdzono, że<br />

spośród analizowanych elektrod przednich metodą linii transmisyjnych<br />

najmniejszą wartość rezystancji właściwej uzyskano<br />

w przedziale (tab. 2):<br />

● 0,17…0,50 Ω · cm 2 (min. ρ c<br />

= 0,17 Ω · cm 2 , P = 37,8 W) w przypadku<br />

układu testowego elektrod I,<br />

● 0,53…0,98 Ω · cm 2 (min. ρ c<br />

= 0,53 Ω · cm 2 , P = 37,8 W) w przypadku<br />

układu testowego elektrod II,<br />

na podłożu nieteksturowanym bez naniesionej warstwy antyrefleksyjnej<br />

ogniwa fotowoltaicznego o średniej grubości 35 µm.<br />

Bazując na wynikach doświadczalnych, stwierdzono że<br />

morfologia podłoża krzemowego ma duży wpływ na uzyskiwaną<br />

minimalną wartość rezystancji układu testowego elektrod,<br />

spiekanego laserowo z pasty A. Dla podłoża z teksturą<br />

jest ona większa niż dla podłoża bez tekstury, a związane jest<br />

to z występowaniem pustych obszarów pod kontaktami, gdyż<br />

średnia wysokość teksturowanej powierzchni dla Si (100) wynosi<br />

3…9 µm. Warstwa antyrefleksyjna zapobiega odbijaniu<br />

się światła i stratom energii, ale jednocześnie tworzy barierę<br />

Oznaczenie<br />

pasty<br />

Wielkość<br />

bazowego<br />

proszku<br />

srebra,<br />

[nm]<br />

Udział masowy składników<br />

w wytworzonych pastach, [%]<br />

Proszek<br />

bazowy<br />

Nośnik<br />

organiczny<br />

Szkliwo<br />

ceramiczne<br />

Powierzchnia<br />

fotoogniwa*<br />

Mikroobróbka laserowa<br />

A < 40 nm 83 15 2 1– 4<br />

* 1. nieteksturowana z naniesioną warstwą antyrefleksyjną,<br />

2. nieteksturowana bez naniesionej warstwy antyrefleksyjnej,<br />

3. teksturowana z naniesioną warstwą antyrefleksyjną,<br />

4. teksturowana bez naniesionej warstwy antyrefleksyjnej<br />

Rys. 4. Trójwymiarowa topografia powierzchni elektrody przedniej<br />

wykonanej z pasty A na powierzchni nieteksturowanej z naniesioną<br />

warstwą ARC przy mocy wiązki lasera 40,5 W i prędkości skanowania<br />

50 mm/s (CLSM)<br />

90<br />

Elektronika 6/2012


Tab. 2. Wyniki średnie, odchylenia standardowe oraz przedziały ufności<br />

dla 1 – α = 0,95 z pomiarów rezystancji właściwej elektrod przednich selektywnie<br />

spiekanych laserowo w zależności od mocy wiązki lasera przy<br />

prędkości skanowania 50 mm/s na powierzchniach nieteksturowanych<br />

bez naniesionych warstw antyrefleksyjnych ogniw fotowoltaicznych<br />

Symbol<br />

Moc<br />

wiązki<br />

lasera<br />

[W]<br />

L16 37,8<br />

Prąd<br />

[mA]<br />

Rezystancja<br />

ρ c<br />

[Ω·cm 2 ]<br />

10 0,167<br />

30 0,165<br />

50 0,168<br />

UKŁAD I<br />

UKŁAD II<br />

Średnia<br />

[Ω·cm 2 ]<br />

Odch.<br />

standard.<br />

Przedział<br />

ufności<br />

0,17 0,001 0,16<br />

Selektywne spiekanie laserowe elektrody przedniej monokrystalicznego<br />

ogniwa fotowoltaicznego stanowi innowacyjne<br />

rozwiązanie w porównaniu z konwencjonalną metodą wypalania<br />

w piecu taśmowym na podczerwień. Ponadto mikroobróbka laserowa<br />

elementów krzemowych ogniw fotowoltaicznych z monokrystalicznego<br />

krzemu o różnej morfologii, w tym selektywne<br />

spiekanie laserowe elektrody przedniej do jego powierzchni<br />

z użyciem lasera CO 2<br />

, wpływa na poprawę jakości ogniwa<br />

przez minimalizację rezystancji połączenia elektrody przedniej<br />

z podłożem. Tak wytworzone elektrody przednie wykazują lepsze<br />

właściwości elektryczne niż nanoszone tradycyjnie metodą<br />

sitodruku i wypalania w piecu taśmowym przy minimalnej wartości<br />

rezystancji połączenia elektrody przedniej z podłożem krzemowym.<br />

Spośród analizowanych elektrod przednich lepszymi<br />

właściwościami elektrycznymi oraz strukturalnymi charakteryzują<br />

się spiekane laserowo elektrody niż elektrody uzyskane<br />

w metodzie klasycznej.<br />

L16 37,8<br />

10 0,547<br />

30 0,517<br />

0,53 0,02 0,49<br />

Praca częściowo realizowana jest w ramach projektu<br />

NN 508 444 136 finansowanego przez Narodowe Centrum Nauki.<br />

50 0,520<br />

Literatura<br />

w strefie połączenia między warstwą elektrody i krzemem, co<br />

wpływa na zwiększenie rezystancji pomiędzy elektrodą, a krzemowym<br />

podłożem. Ponadto grubość nanoszonej warstwy wywiera<br />

wpływ na strukturę uzyskiwanej elektrody oraz na wartość<br />

rezystancji układu testowego elektrod. Stwierdzono, że<br />

spiekane laserowo układy testowe elektrod o średniej grubości<br />

35 µm stanowią zagęszczone i dobrze przylegające do podłoża<br />

krzemowego warstwy bez porów i nieciągłości. Natomiast układy<br />

testowe elektrod o średniej grubości 15 µm posiadają obszary<br />

całkowitego odsłonięcia krzemowego podłoża wewnątrz<br />

elektrod oraz uszkodzenia w postaci mikropęknięć powstałych<br />

podczas mikroobróbki laserowej. Rysunek 4 przedstawia przykładową<br />

topografię powierzchni elektrody przedniej wykonanej<br />

z nanopasty i spiekanej laserem przy odpowiednio dobranych<br />

zaleceniach technologicznych.<br />

Podsumowanie<br />

Chemiczna metoda teksturowania powierzchni przez trawienie<br />

w wodnych roztworach wodorotlenków stosowanych dla krzemu<br />

monokrystalicznego jest nieefektywna w przypadku krzemu polikrystalicznego<br />

z uwagi na przypadkową orientację krystaliczną<br />

poszczególnych ziarn. Teksturowanie powierzchni krzemu polikrystalicznego<br />

z zastosowaniem lasera neodymowego Nd:YAG,<br />

zapewnia zwiększenie absorpcji promieniowania słonecznego.<br />

Ponadto zastosowanie dodatkowej operacji technologicznej<br />

w produkcji ogniw fotowoltaicznych wytworzonych z płytek teksturowanych<br />

laserowo, polegającej na chemicznej mikroobróbce<br />

powierzchni krzemu w roztworze KOH, pozwala na poprawę<br />

właściwości elektrycznych w porównaniu do ogniw wykonanych<br />

z płytek nieteksturowanych lub z teksturą wykonaną przez trawienie<br />

w roztworze KOH:IPA:DIH 2<br />

O.<br />

[1] Dobrzański L.A., Drygała A., Panek P., Lipiński M., Zięba P.: Development<br />

<strong>of</strong> <strong>the</strong> laser method <strong>of</strong> multicrystalline silicon surface<br />

texturization. Archives <strong>of</strong> Materials Science and Enginerring 38/1<br />

(2009) 5–11.<br />

[2] Dobrzański L.A.: Niemetalowe materiały inżynierskie. WPŚ, Gliwice<br />

2008.<br />

[3] Goetzberger A., H<strong>of</strong>fmann V.U.: Photovoltaic solar energy generation.<br />

Springer, Berlin, 2005.<br />

[4] Nositschka W.A., Beneking C., Voigt O., Kurz H.: Texturisation <strong>of</strong><br />

multicrystalline silicon wafer for solar cells by reactive ion etching<br />

through colloidal masks. Solar Energy Materials and Solar Cells,<br />

76 (2003) 151–166.<br />

[5] Exner H., Regnefuss P., Hartwig L., Klötzer S., Ebert R.: Selective<br />

laser sintering with a Novel Process. <strong>Proceedings</strong> <strong>of</strong> 4 th<br />

International Symposium on Laser Precision Micr<strong>of</strong>abrication,<br />

Munich, 2003, 145–151.<br />

[6] Petch T., Regenfuβ P., Ebert R., Hartwig L., Klötzer S., Brabant<br />

T., Exner H.: Industrial laser micro sintering, <strong>Proceedings</strong> <strong>of</strong> <strong>the</strong><br />

23 rd International Congress on Applications <strong>of</strong> Lasers and Electro-Optics,<br />

2004.<br />

[7] Alemán M., Streek A., Regenfuβ P., Mette A., Ebert R., Exner H.,<br />

Glunz S.W., Willeke G.: Laser micro-sintering as a new metallization<br />

technique for silicon solar cells. <strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> 21 st<br />

<strong>European</strong> Photovoltaic Solar Energy Conference, Dresden, Germany,<br />

2006, 705.<br />

[8] Gautero L., H<strong>of</strong>mann M., Rentsch J., Lemke A., Mack S., Seiffe<br />

J., Nekarda J., Biro D., Wolf A., Bitnar B., Sallese J.M., Preu R.:<br />

All-screen-printed 120-µm-thin large-area silicon solar cells applying<br />

dielectric rear passivation and laser-fired contacts reaching<br />

18% efficiency. <strong>Proceedings</strong> <strong>of</strong> Photovoltaic Specialists Conference<br />

(PVSC), 34th IEEE, 2009, 1888–1893.<br />

[9] Dobrzański L.A., Musztyfaga M.: Effect <strong>of</strong> <strong>the</strong> front electrode<br />

metallisation process on electrical parameters <strong>of</strong> a silicon solar<br />

cell., Journal <strong>of</strong> Achievements in Materials and Manufacturing<br />

Engineering, Vol. 48/2, 2011, Issue 2, 115–144.<br />

Elektronika 6/2012 91


Recycling <strong>of</strong> silicon solar cells<br />

Piotr Ostrowski<br />

Państwowa Wyższa Szkoła Zawodowa w Krośnie, Instytut Politechniczny, Zakład Energetyki, Krosno<br />

W odpowiednio długiej perspektywie czasowej, wynoszącej średnio<br />

25 lat, krzemowe moduły PV podobnie jak inne przedmioty<br />

użytkowe stają się odpadem. Globalna tendencja zmierzająca do<br />

wykorzystania materiałów odpadowych, skłania do zagospodarowania<br />

materiałów odpadowych komercyjnych modułów PV. Efektywną<br />

metodą gospodarowania odpadami jest odzysk. Dotychczasowa<br />

praktyka polegająca na składowaniu zużytych modułów<br />

PV na składowiskach odpadów, w odniesieniu do modułów PV,<br />

na dłuższą metę nie jest do zaakceptowania.<br />

Eksperyment<br />

Strumień odpadów w sektorze fotowoltaicznym można podzielić<br />

na dwie grupy: odpady generowane na etapie produkcji oraz wynikające<br />

z procesów starzeniowych (odpad poeksploatacyjny). Na<br />

1000 wytworzonych krzemowych ogniw PV, średnio 50 sztuk nie<br />

spełnia stawianych im wymagań, zatem do produkcji modułów PV<br />

można wykorzystać 950 szt. pojedynczych ogniw, których parametry<br />

są zgodne z oczekiwanymi. Natomiast na 1000 wytworzonych<br />

modułów PV, średnio 10 sztuk nie spełni wymagań. Do eksploatacji<br />

trafi zatem 990 modułów PV. Dodatkowo ilość zużytych<br />

modułów PV zwiększa się w wyniku ich naturalnego starzenia<br />

podczas, którego zmianie ulęgają właściwości fizyko-chemiczne<br />

poszczególnych materiałów składowych. Na rys. 1 przedstawiono<br />

widok zużytych komercyjnych modułów PV.<br />

Rys. 1. Zużyte komercyjne moduły PV [1]<br />

Straty powstające na etapie produkcji w postaci uszkodzonych<br />

ogniw i modułów PV nie spełniających parametrów należy ograniczać<br />

poprzez stosowanie odzysku materiałowego. Z kolei odpad<br />

powstający w wyniku uzyskania przez moduły PV tzw. kresu<br />

eksploatacji, będącego następstwem procesów starzeniowych,<br />

należy odzyskiwać i ponownie wykorzystać. Recykling to forma<br />

odzysku polegająca na ponownym przetworzeniu materiałów<br />

w celu uzyskania materiału o przeznaczeniu pierwotnym lub przeznaczeniu<br />

alternatywnym. Rezultaty badań opisane w pracach<br />

[2, 3] wskazują iż recykling krzemowych ogniw PV jest możliwy.<br />

Poziom wykorzystania surowców wtórnych ogniw/modułów PV,<br />

można określać za pomocą wskaźników recyklingu.<br />

Wyniki<br />

Poniżej podano sposób określania wskaźników recyklingu komercyjnych<br />

ogniw i modułów PV w oparciu o zastosowanie metody<br />

masowej oraz metody strukturalnej.<br />

Podatność komercyjnego krzemowego ogniwa/modułu PV na<br />

recykling można wyznaczyć posługując się względnym udziałem<br />

92<br />

masowym poszczególnych elementów podatnych na recykling<br />

w odniesieniu do jego całkowitej masy (1).<br />

(1)<br />

gdzie: M ws<br />

– masowy wskaźnik podatności ogniwa/modułu PV na<br />

recykling, m ri<br />

– masa części podatnych na recykling, m – ogólna<br />

masa modułu PV.<br />

Rozdział wyc<strong>of</strong>anego z dalszej eksploatacji modułu PV na<br />

poszczególne grupy materiałowe polega głównie na demontażu<br />

jego elementów składowych. Rozdział ten do pewnego stopnia<br />

może być realizowany ręcznie lub mechanicznie. Natomiast separacja<br />

pojedynczych ogniw z modułu PV wymaga zastosowania<br />

obróbki termicznej lub chemicznej [4].<br />

Metoda masowa nie uwzględnia trudności związanych z koniecznością<br />

separacji (rozdzielenia) poszczególnych elementów<br />

wchodzących w skład modułu PV. Problemy złożoności demontażu<br />

modułów PV można ocenić wykorzystując do tego celu strukturalną<br />

metodę podatności na recycling. Metoda ta polega na dodatkowym<br />

wprowadzeniu do równania (1) współczynnik fs, odniesionego do<br />

struktury krzemowego modułu PV, jak w równaniu (2).<br />

(2)<br />

gdzie: S wr<br />

– strukturalny wskaźnik podatności ogniwa/modułu PV<br />

na recykling, fs – współczynnik struktury ogniwa/modułu PV.<br />

Równanie (2) uwzględnia wpływ struktury, sposób rozmieszczenia<br />

oraz typ powiązań standardowych krzemowych modułów<br />

PV, na podatność tych urządzeń na recykling. W przypadku<br />

uszkodzenia, zużycia czy osiągnięcia kresu eksploatacji krzemowego<br />

modułu PV należy wydzielić z niego poszczególne grupy<br />

materiałowe. Z tego względu ważnym zagadnieniem staje się<br />

proces demontażu zużytych modułów PV. Dopiero po tym etapie<br />

odzyskane poszczególne materiały składowe można kierować<br />

do procesu recyklingu dla danej grupy materiałowej (tworzywa<br />

sztuczne, aluminium, miedź, stal, szkło, krzem). Relacje zachodzące<br />

między elementami składowymi komercyjnych ogniw i modułu<br />

PV można opisano za pomocą grafu skierowany rys. 2.<br />

Określenie relacji połączeń elementów składowych krzemowych<br />

modułów PV jest pomocne przy określeniu współczynnika<br />

struktury fs. W tabeli 1 przedstawiono poszczególne elementy<br />

składowe komercyjnego modułu PV. Natomiast w tabeli 2<br />

przedstawiono elementy wchodzące w skład budowy komercyjnego<br />

krzemowego ogniwa PV. Tabela 3 przedstawia szacunkowe<br />

wskaźniki recyklingu uzyskiwane dla krzemowych ogniw<br />

i modułów PV.<br />

Rys. 2. Model zachodzących relacji między elementami składowymi<br />

ogniw/modułów PV<br />

Elektronika 6/2012


Tab. 1. Elementy składowe komercyjnego modułu PV<br />

Moduł<br />

PV<br />

Tab. 2. Elementy składowe komercyjnego ogniwa PV<br />

Ogniwo<br />

PV<br />

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8<br />

Rama<br />

Al<br />

2.1 2.2 2.3 2.4 2.5<br />

Łącznik<br />

Cu<br />

Metalizacja<br />

tylna<br />

Ag/Al<br />

Metalizacja<br />

przednia<br />

Ag<br />

Warstwa<br />

ARC<br />

Złącze<br />

p- n<br />

Podłoże<br />

bazowe<br />

Si<br />

Tab. 3. Szacunkowe wskaźniki recyklingu dla krzemowych ogniw i modułów<br />

PV<br />

Szacunkowy wskaźniki<br />

recyklingu<br />

krzemowego<br />

ogniwa PV<br />

krzemowego<br />

modułu PV<br />

Wartości w% ≈ 95 ≈ 80<br />

Zużyte ogniwo PV<br />

Śruby,<br />

Fe<br />

Krzemowe podłoże<br />

bazowe odzyskane<br />

ze zużytego ogniwa PV<br />

Kierunek procesu<br />

Rys. 3. Recykling polikrystalicznego ogniw PV<br />

Tworzywa<br />

Szkło EVA Tedlar ® sztuczne<br />

Przewody<br />

Cu<br />

Diody<br />

zabezp.<br />

Ogniwa<br />

PV<br />

Ogniwo PV wytworzone<br />

na bazie odzyskanego<br />

podłoża krzemowego<br />

Z uzyskanych wyników w tabeli 3 wynika że możliwe jest odzyskanie<br />

około 95% materiału z krzemowego ogniwa PV oraz<br />

niemalże 80% z krzemowego modułu PV. Odzyskane polikrystalicznego<br />

krzemowego podłoża bazowego wymaga zastosowania<br />

obróbki chemicznej umożliwiającej usuniecie z jego powierzchni<br />

metalizacji przedniej i tylnej, warstwy antyrefleksyjnej ARC oraz<br />

złącza półprzewodnikowego p- n. Rozdział zaś modułu PV na poszczególne<br />

grupy materiałowe najefektywniej zachodzi w procesach<br />

wysokotemperaturowych tj. w temperaturze około 500 o C.<br />

W celu odzyskania krzemowego podłoża bazowego ze zużytych<br />

ogniw PV należy usunąć z jego powierzchni wszystkie uprzednio<br />

nałożone substancje. W tym celu stosuje się sekwencyjną obróbkę<br />

chemiczną z wykorzystaniem takich substancji jak: wodorotlenek<br />

potasu KOH, kwas fluorowo-wodorowy HF, kwas azotowy HNO 3<br />

,<br />

kwas octowy CH 3<br />

COOH bądź nadtlenek wodoru H 2<br />

O 2<br />

[5]. Na<br />

rys. 3b przedstawiono widok podłoża krzemowego odzyskanego<br />

ze zużytego polikrystalicznego ogniwa PV. Ponowne wykorzystanie<br />

tak odzyskanego podłoża Si wymaga powtórnego wytworzenia<br />

na nim wszystkich wcześniejszych warstw. Technologia produkcji<br />

krzemowych ogniwa PV, na odzyskanym krzemowym podłożu bazowym,<br />

składają się m. in. z takich etapów jak:<br />

– kontrola jakości odzyskanego podłoża bazowego;<br />

– odtłuszczenie i oczyszczenie odzyskanej płytki Si;<br />

– przeprowadzenie procesu dyfuzji;<br />

– usunięcie szkliwa fosforowo-krzemowego;<br />

– osadzenie warstwy Al na tylnej powierzchni płytki;<br />

– osadzenie warstw Ti, Ag;<br />

– wypalenie kontaktów elektrycznych.<br />

Na rysunku 3c zaprezentowana widok ogniwa PV wytworzonego<br />

w Laboratorium Fotowoltaicznego w Kozach na odzyskanym<br />

krzemowym podłożu bazowym. Na rys. 3c widoczna jest<br />

powierzchnia przednia ogniwa PV z wytworzoną warstwą antyrefleksyjną<br />

ARC (kolor niebieski) oraz elektrodą przednią wykonana<br />

przy użyciu pasty srebrowej Ag. Powierzchnia wytworzonego ogniwa<br />

wynosi 25,00 cm 2 . Odzyskane podłoże krzemowe posiadało<br />

rezystywność 1,45 ∙ 10 -2 Ω·m, a jego grubość wyniosła 300 μm.<br />

Odzyskane podłoże krzemowe charakteryzowało się przewodnictwem<br />

dziurowym (typu- p) z tego względu, podczas etapu produkcyjnego<br />

domieszkowane było fosforem. Na powierzchni tylnej<br />

odzyskanego podłoża wykonano metalizację Al/Ag stanowiącą<br />

elektrodę tylną tak wytworzonego ogniwa.<br />

Podsumowanie<br />

Na rysunku 4 zaprezentowano podstawowe parametry eksploatacyjne<br />

wytworzonego polikrystalicznego ogniwa PV na odzyskanym<br />

podłożu bazowym. Wyniki uzyskano na stanowisku do<br />

wyznaczania charakterystyk I- U ogniw PV, zlokalizowanym w Laboratorium<br />

Fotowoltaicznym w Kozach. Pomiary ogniwa PV realizowano<br />

w warunkach testu standardowego STC tj.: E= 1000<br />

W/m 2 , t= 25 o C przy optycznej masie powietrza AM= 1,5. Z otrzymanej<br />

charakterystyki prądowo-napięciowej dla wytworzonego<br />

polikrystalicznego ogniwa PV wynika iż posiada ono bardzo dobre<br />

parametry nie odbiegające od tych uzyskiwanych dla ogniw wytwarzanych<br />

z materiałów pierwotnych. Polikrystaliczne ogniwo PV<br />

wytworzone z materiału wtórnego uzyskało sprawność konwersji<br />

fotowoltaicznej E ff<br />

= 13% oraz współczynnik wypełnienia charakterystyki<br />

FF= 0,74, co można uznać za poziom zadowalający.<br />

Rys. 4. Parametry polikrystalicznego ogniwa PV wytworzonego na<br />

krzemowym podłożu bazowym pochodzącym z odzysku<br />

gdzie: I sc<br />

–prąd zwarcia, V oc<br />

– napięcie obwodu otwartego,<br />

P m<br />

– wartość mocy maksymalnej, FF – współczynnik wypełnienia,<br />

E ff<br />

– sprawność konwersji.<br />

Wartości otrzymanych wyników pomiarowych potwierdzają, iż<br />

recykling krzemowych ogniw PV jest możliwy, a parametry uzyskiwane<br />

przez ogniwa wytworzone na odzyskanych krzemowych<br />

podłożach bazowych są nie gorsze jak te, które osiągają ogniwa<br />

PV wytwarzane na podłożach pierwszego użycia. Ponadto uzyskiwane<br />

wysokie wskaźniku recyklingu krzemowych ogniw i modułów<br />

fotowoltaicznych przemawiają za powszechnym wykorzystaniem<br />

tego typu możliwości.<br />

Serdeczne podziękowania składam kierownikowi oraz pracownikom<br />

Laboratorium Fotowoltaicznego w Kozach, a także Pani<br />

E. Klugmann-Radziemskiej pr<strong>of</strong>. nadzw. Politechniki Gdańskiej<br />

za owocną współpracę i życzliwość.<br />

Literatura<br />

[1] Flynn H., Bradford T.: Silicon Shortage: Supply Constrains Limit PV<br />

Growth Until 2008, Renewable Energy World, July 2006.<br />

[2] Klugmann-Radziemska E., P. Ostrowski, K. Drabczyk, P, Panek, M. Szkodo:<br />

Experimental Validation <strong>of</strong> <strong>the</strong> Chemical Recycling <strong>of</strong> Crystalline<br />

Silicon Solar Cells, Solar Energy Materials & Solar Cells 94 (2010).<br />

[3] Radziemska E., P. Ostrowski, F. Kozera: Sposób i urządzenie do kontrolowanego<br />

i automatycznego odzysku materiałów z krzemowych<br />

ogniw fotowoltaicznych. Zgłoszenie patentowe z dnia 10.11.2009.<br />

[4] Radziemska E., P. Ostrowski, A. Cenian, M. Sawczak: Chemical<br />

Thermal and Laser Processes in Recycling <strong>of</strong> Photovoltaic Solar<br />

Cells and Modules. Ecological Chemistry and Engineering S 2010,<br />

Vol 17, No.3, 385–391.<br />

[5] Klugmann –Radziemska E., Ostrowski P.: Chemical Treatment <strong>of</strong><br />

Crystalline Silicon Solar Cells as a Method <strong>of</strong> Recovering Pure Silicon<br />

From Photovoltaic Modules. Renewable Energy 2010, Vol. 35/8<br />

pp. 1751–1759, A.A.M. Saigh – USA: Elsevier, ISSN 0960-1481.<br />

Elektronika 6/2012 93


Evaluation <strong>of</strong> surface morphology <strong>of</strong> texturized Si wafers<br />

for solar cells applications<br />

Sławomir Białas, Politechnika Wrocławska, Wydział Elektroniki Mikrosystemów i Fotoniki<br />

Nauką zajmującą się pozyskiwaniem energii elektrycznej z promieniowania<br />

słonecznego jest fotowoltaika, a jej głównym kręgiem<br />

zainteresowań są ogniwa słoneczne (ogniwa fotowoltaiczne,<br />

ogniwa PV). Sprawność ogniwa słonecznego, a więc jego<br />

zdolność do efektywnej konwersji promieniowania świetlnego na<br />

prąd elektryczny, zależy od szeregu czynników takich jak:<br />

● rodzaj i grubość materiału użytego do wytworzenia ogniwa,<br />

● technologia wytworzenia,<br />

● zasłonięcie obszaru ogniwa pod elektrodą,<br />

● warunki pracy (widmo promieniowania, temperatura, zacienienie),<br />

● straty wynikające z nieabsorbowania fotonów wysoko- i niskoenergetycznych,<br />

● straty wynikające z rekombinacji nośników prądu i rezystancji<br />

pasożytniczych,<br />

● straty wynikające z odbicia promieniowania od powierzchni<br />

ogniwa.<br />

Ostatni z wymienionych czynników można minimalizować dzięki<br />

teksturyzacji, to znaczy rozwinięciu powierzchni w taki sposób,<br />

aby promieniowanie nie ulegało odbiciu, lecz pochłonięciu. Im<br />

więcej promieniowania zostanie pochłonięte, tym więcej fotonów<br />

dotrze w głąb materiału, a więc jest większe prawdopodobieństwo<br />

zajścia efektu fotowoltaicznego. Badania nad teksturyzacją<br />

powierzchni ogniw krzemowych prowadzone są przez autora niniejszej<br />

publikacji w ramach pracy magisterskiej w Zakładzie Mikroelektroniki<br />

i Nanotechnologii WEMiF Politechniki Wrocławskiej.<br />

Skupiono się nad znalezieniem najbardziej optymalnej geometrii<br />

tekstury, która wytworzona na powierzchni podłoża krzemowego<br />

zapewniałaby możliwie najmniejszy (a wręcz zerowy) współczynnik<br />

odbicia. W tym celu przeanalizowano zaprezentowane<br />

w literaturze symulacje komputerowe różnych struktur. Następnie<br />

zbadano (przy użyciu mikroskopii SEM) morfologię powierzchni<br />

podłoży krzemowych poddanych procesowi teksturyzacji. Celem<br />

analiz było skorelowanie geometrii powstającej tekstury ze spektralną<br />

charakterystyką współczynnika odbicia.<br />

Symulacje komputerowe tekstur ogniwa<br />

słonecznego<br />

Wpływ tekstury powierzchni na zmianę współczynnika odbicia<br />

uzależniony jest od szeregu czynników takich jak [1]: geometria<br />

tekstury, gęstość tekstury, kąt wierzchołkowy, wielkość elementów<br />

tworzących teksturę, regularność ułożenia elementów.<br />

Rys. 1. Zależność współczynnika odbicia od kąta padania promieniowania<br />

[2]; α – wielkość powierzchni nie steksturyzowanej, 0%, 10%,<br />

60% – rozrzut wysokości piramid<br />

W literaturze można znaleźć wiele propozycji kształtów elementów<br />

tworzących teksturę oraz symulacje ich wpływu na<br />

zmniejszenie współczynnika odbicia promieniowania od powierzchni<br />

ogniwa.<br />

W pracy [2] zbadano wpływ regularności ułożenia standardowej,<br />

piramidalnej tekstury, powstającej w wyniku anizotropowego<br />

trawienia krzemu w roztworach alkalicznych, na współczynnik<br />

odbicia światła. Przebadano dwa parametry: stopień pokrycia powierzchni<br />

teksturą (α) i rozrzut wielkości piramid. Na rysunku 1<br />

można zaobserwować, że przy określonej wielkości powierzchni<br />

podłoża niepokrytego teksturą, charakterystyki odbicia dla struktury<br />

o losowym i periodycznym rozkładzie tekstury w funkcji kąta<br />

padania promieniowania praktycznie się pokrywają. Można przyjąć,<br />

że tekstura o losowym rozmieszczeniu i wysokości elementów<br />

zachowuje się tak samo, jak analogiczna struktura o regularnym<br />

rozkładzie.<br />

Kolejną obserwacją wynikającą z symulacji przedstawionych<br />

w pracy [2] jest silna zależność reflektancji od wielkości powierzchni<br />

podłoża niepokrytej teksturą (współczynnik α na rys. 1).<br />

W stosunku do powierzchni nieteksturyzowanej, wprowadzenie<br />

tekstury na 75% powierzchni skutkuje obniżeniem współczynnika<br />

odbicia o blisko połowę (z 0,30 na 0,18). Teksturyzacja na całej<br />

powierzchni dodatkowo obniża odbicie do 15%. Parametr ten ma<br />

duże znaczenie w przypadku rzeczywistych ogniw, gdyż w procesie<br />

produkcji wytwarzana jest losowa tekstura, dla której istnieje<br />

duże prawdopodobieństwo występowania pustych obszarów pomiędzy<br />

piramidami.<br />

Wpływ kształtu wytrawionych na powierzchni krzemu elementów<br />

tekstury oraz wpływ rozwartości kąta wierzchołkowego na<br />

wartość współczynnika absorbcji światła zbadano w pracy [1].<br />

Autorzy przeprowadzili szereg symulacji, w których przeanalizowano<br />

następujące kształty tekstury: stożek (o dwóch różnych<br />

gęstościach rozmieszczenia na powierzchni), czworościan, ostrosłup<br />

o podstawie kwadratowej (piramida) oraz ostrosłup o podstawie<br />

sześciokątnej (rys. 2). Symulowane struktury charakteryzowały<br />

się wysokością 5 µm oraz kątem wierzchołkowym zależnym<br />

od wielkości podstawy figury. Na podstawie wyników symulacji<br />

można zauważyć, że struktury stożkowe charakteryzują się najmniejszą<br />

wartością absorbcji. Dla tekstury o mniejszym upakowaniu<br />

elementów, maksymalna wartość absorbcji to 0,8 (dla kątów<br />

wierzchołkowych o rozwartości do 80° i długości fali 550 nm<br />

– 1000 nm). W przypadku tekstury o gęstości upakowania 96,1%,<br />

w zakresie 900...1000 nm, na charakterystyce pojawia się obszar<br />

o wielkości współczynnika absorbcji na poziomie 0,9. Potwierdza<br />

to wniosek z poprzednio cytowanego artykułu, że powierzchnia<br />

ogniwa niepokrytego teksturą ma istotny wpływ na reflektancję.<br />

Im większa powierzchnia bez tekstury (tzn. im większe odstępy<br />

pomiędzy elementami składowymi), tym większy współczynnik<br />

odbicia. Współczynnikiem absorbcji na poziomie 0,9 w większym<br />

zakresie długości fali cechują się tekstury na bazie ostrosłupów.<br />

Maksimum absorbcji dla takich struktur występuje dla<br />

długości fali 640…1080 nm. Można także zauważyć, że zakres<br />

kąta wierzchołkowego dla obszaru o maksymalnej absorbcji jest<br />

największy dla czworościanu – od 0° do około 100°. Skutkuje to<br />

większą dowolnością przy projektowaniu procesu technologicznego<br />

prowadzącego do otrzymania takiej tekstury oraz zwiększa<br />

stabilność współczynnika reflektancji w przypadku rozrzutu wartości<br />

kąta. Standardowa tekstura zbudowana z piramid sprawuje<br />

się najlepiej dla kątów wierzchołkowych mniejszych niż 80°<br />

(rys. 2f ), a struktura ostrosłupów o podstawie sześciokątnej dla<br />

kątów mniejszych niż 60°.<br />

94<br />

Elektronika 6/2012


Rys. 2. Analizowane kształty tekstury [1]: a – stożki o upakowaniu 89,1%, b – stożki o upakowaniu 96,1%, c – przylegające do siebie ostrosłupy<br />

o podstawie sześciokąta, d – przylegające do siebie piramidy z powiększeniem pojedynczego elementu z zaznaczeniem kąta wierzchołkowego,<br />

e – przylegające do siebie czworościany, f – wyniki symulacji dla tekstury d<br />

Rys. 3. Symulowane struktury w pracy [3]: a – krzemowa tekstura piramidalna, b – krzemowa tekstura w postaci V-rowków, c – szklana tekstura<br />

w postaci V-rowków<br />

Autorzy pracy [3] zaproponowali zastąpienie struktur, zbudowanych<br />

z regularnej matrycy elementów, V-rowkami wytworzonymi<br />

wzdłuż całej długości ogniwa. Konstrukcję taką można<br />

wykonać bezpośrednio w krzemie lub poprzez połączenie<br />

krzemu z odpowiednio uformowanym szkłem (rys. 3). Wysokość<br />

symulowanych elementów składowych tekstury była stała<br />

i wynosiła 50 µm. W przypadku matrycy krzemowych V-rowków<br />

otrzymano, dla dwóch wartości kątów: 40°, 60° mniejsze odbicia<br />

niż w przypadku standardowej tekstury. Natomiast dla struktury<br />

o kącie wierzchołkowym 70…80° wyniki symulacji pokrywają<br />

się z wartościami otrzymanymi dla tekstury piramidalnej<br />

(rys. 4). Najmniejsza osiągnięta wartość współczynnika odbicia<br />

dla krzemowych V-rowków to około 2% (dla kąta wierzchołkowego<br />

wynoszącego 40°). Jest to bardzo dobra wartość<br />

gwarantująca wysoką sprawność ogniwa. Pozostałe kąty (większe<br />

od 80 o ) nie zapewniają dobrych parametrów – wartości<br />

reflektancji osiągają nawet 35%. Tekstura szklana nie zapewnia<br />

mniejszych strat odbiciowych niż standardowa tekstura<br />

piramidalna.<br />

Symulacje przybliżają rzeczywiste zachowanie się tekstury na<br />

powierzchni ogniwa. Dzięki temu możliwe są prace badawcze<br />

nad optymalizacją, które nie wymagają nakładów finansowych<br />

na materiały i technologię. Obecnie pracuje się nad minimalizacją<br />

odbicia promieniowania w całym zakresie spektrum światła<br />

słonecznego, co jest warunkiem zadawalającej sprawności ogniw.<br />

Z przedstawionych rozważań wynika, że aby tekstura spełniała<br />

powyższy warunek konieczne jest zachowanie następujących<br />

założeń:<br />

● całkowite pokrycie powierzchni frontowej teksturą (losowy lub<br />

regularny sposób rozmieszczenia elementów oraz rozrzut ich<br />

wysokości mają pomijalnie mały wpływ),<br />

● unikanie detali tekstury w kształcie stożka,<br />

● zachowanie następujących kątów wierzchołkowych w przypadku<br />

tekstury na bazie ostrosłupów:<br />

o podstawie trójkątnej (czworościan):


Rys. 4. Wyniki symulacji dla tekstury krzemowych V-rowków w porównaniu<br />

do tekstury piramidalnej [3]<br />

Korelacja pomiędzy parametrami tekstury<br />

a charakterystyką reflektancji<br />

Dokonano oceny morfologii siedmiu podłoży krzemowych o orientacji<br />

krystalograficznej [100], przewodnictwie typu n i rezystywności<br />

5…10 Ωcm teksturowanych w roztworach alkalicznych z dodatkiem<br />

surfaktantu 1,2-pentanediolu (rys. 5). Parametry procesu<br />

teksturyzacji zestawiono w tabeli 1.<br />

Rys. 6. Zależność współczynnika odbicia od temperatury procesu dla<br />

roztworu 1M KOH + 2% 1,2-pentanediolu, t = 20 min oraz zdjęcia SEM<br />

trawionych podłoży<br />

Tab. 1. Zestawienie parametrów trawienia badanych próbek [4]<br />

Nr<br />

podłoża<br />

Roztwór trawiący<br />

Czas procesu<br />

[min]<br />

Temperatura<br />

1<br />

1M KOH + 2%<br />

1,2-pentanediol<br />

10 90<br />

2<br />

1M KOH + 2%<br />

1,2-pentanediol<br />

15 90<br />

3<br />

1M KOH + 2%<br />

1,2-pentanediol<br />

20 90<br />

4<br />

1M KOH + 1%<br />

1,2-pentanediol<br />

20 90<br />

5<br />

6<br />

1M KOH + 4%<br />

1,2-pentanediol<br />

1M KOH + 2%<br />

1,2-pentanediol<br />

20 90<br />

20 80<br />

Rys. 7. Zależność współczynnika odbicia od czas trawienia dla roztworu<br />

1M KOH+2%1,2-pentanediolu, T = 90° oraz zdjęcia SEM trawionych<br />

podłoży<br />

7<br />

1M KOH + 1%<br />

1,2-pentanediol<br />

20 70<br />

Analizę morfologii przeprowadzono z wykorzystaniem mikroskopu<br />

SEM (mikroskop marki HITACHI, model SU6600). Następnie<br />

otrzymane wyniki zestawiono z pomiarami charakterystyk<br />

reflektancji wszystkich siedmiu podłoży. Wykresy podzielono ze<br />

względu na zmieniający się parametr procesu, tzn: temperaturę<br />

procesu, czas trwania procesu oraz stężenie procentowe dodanego<br />

surfaktantu (1,2-pentanediolu). Zestawienie przedstawiono<br />

na rys. 6–8.<br />

Rys. 8. Zależność współczynnika odbicia od koncentracji 1,2-pentanediolu,<br />

T = 90°, t = 20 min oraz zdjęcia SEM trawionych podłoży<br />

Rys. 5. Porównanie podłoża polerowanego i teksturyzowanego, pokrytego<br />

warstwą antyrefleksyjną<br />

96<br />

Przebadane struktury teksturujące (piramidy o podstawie<br />

kwadratowej) ograniczone były płaszczyznami {111} i miały kąt<br />

wierzchołkowy ok 70°. W wynikach przedstawionych symulacji<br />

otrzymano, że kąt wierzchołkowy takich piramid powinien być<br />

mniejszy od 80°, a więc kąt wierzchołkowy analizowanych struktur<br />

jest optymalny. Zaobserwowano również istotne znaczenie<br />

gęstości pokrycia powierzchni piramidami, co pokrywa się z wynikami<br />

symulacji.<br />

Elektronika 6/2012


Podsumowanie<br />

W wyniku badań doświadczalnych stwierdzono silny wpływ parametrów<br />

procesu na rodzaj otrzymywanej tekstury, a tym samym<br />

na otrzymywany współczynnik odbicia. Na gęstość pokrycia powierzchni<br />

teksturą ma wpływ w decydującym stopniu czas trawienia<br />

(rys. 6). Osiągana reflektancja podłoża maleje wraz z wydłużeniem<br />

czasu trawienia oraz z podwyższeniem temperatury. Istnieje<br />

optymalny skład roztworu, dla którego pokrycie powierzchni piramidami<br />

jest maksymalne, a kształty piramid najbardziej regularne<br />

(1M KOH + 2% 1,2-pentanediolu). Najbardziej optymalny proces<br />

trawienia uzyskuje się dla następujących parametrów: czas<br />

trawienia 20 min, temperatura kąpieli 90°C. Wiąże się to z uzyskaniem<br />

pełnego pokrycia piramidami o wielkości 10…17 µm.<br />

W wyniku takiej teksturyzacji otrzymuje się reflektancję na poziomie<br />

10%. Zastosowanie warstwy antyrefleksyjnej znacznie poprawia<br />

współczynnik reflektancji w całym zakresie stosowanych<br />

długości fal.<br />

Literatura<br />

[1] Xiao-She Hua i in.: The effect <strong>of</strong> texture unit shape on silicon surface on<br />

<strong>the</strong> absorption properties. Solar Energy Materials & Solar Cells 94 (2010)<br />

258–262.<br />

[2] Rodriguez J. M.: Random pyramidal texture modeling. Solar Energy Materials<br />

& Solar Cells 45 (1997) 241–253.<br />

[3] Yagi T. i in.: Ray-trace simulation <strong>of</strong> light trapping in silicon solar cell with texture<br />

structures. Solar Energy Materials & Solar Cells 90 (2006) 2647–2656.<br />

[4] Banaszczyk K. A.: Teksturyzacja powierzchni podłoży krzemowych w zastosowaniu<br />

do baterii słonecznych. Praca magisterska, Wydział Elektroniki<br />

Mikrosystemów i Fotoniki, Politechnika Wrocławska 2011.<br />

Printed transparent electrodes with graphene nanoplatelets<br />

Małgorzata JAKUBOWSKA 1, 2) , Marcin SŁOMA 1, 2) , Daniel JANCZAK 1) , Anna Młożniak 2) ,<br />

Grzegorz WRÓBLEWSKI 1)<br />

1)<br />

Department <strong>of</strong> Mechatronics, Warsaw University <strong>of</strong> Technology, Warszawa<br />

2)<br />

Institute <strong>of</strong> Electronic Materials Technology, Warszawa<br />

In last years notable development in research <strong>of</strong> sustainable, regenerative<br />

and ecological electrical energy generation solutions<br />

can be observed. This is caused by rising costs <strong>of</strong> fossil fuels,<br />

increasing demand for energy, especially in emerging countries<br />

and environmental problems for instance carbon dioxide amount<br />

in atmosphere, climate change or air contamination [1]. Nuclear<br />

plants could be a good alternative for fossil fuels plants, but <strong>the</strong>y<br />

are considered to be dangerous especially in regions <strong>of</strong> earth<br />

where earthquakes, tsunamis and o<strong>the</strong>r disasters can take place<br />

[2]. They are some regenerative electrical energy generation solutions<br />

but <strong>the</strong>y have also some disadvantages and <strong>the</strong>y are not<br />

totally environmental friendly. For example a hydropower plant<br />

needs large areas <strong>of</strong> terrain to be flooded – ecosystems changes<br />

[3] and need <strong>of</strong> homesteads relocations. Wind farms are not a<br />

perfect idea nei<strong>the</strong>r because <strong>the</strong>y are interfering with flying animals<br />

causing even fatality through collision with rotating turbine<br />

rotor blades [4]. Those drawbacks made authorities, researchers<br />

and investors looking for o<strong>the</strong>r sources <strong>of</strong> energy, and solar energy<br />

seems to have prominent future. Crystalline silicon solar cells<br />

are are actually well developed photovoltaic devices but still too<br />

expensive per kWh in comparison to o<strong>the</strong>r energy sources as regard<br />

mass energy generation. Therefore organic solar cells are<br />

investigated and can be a promising alternative for low cost energy<br />

generation. Fur<strong>the</strong>rmore using organic materials give great<br />

opportunity in elastic photovoltaic devices production.<br />

This paper is focused on transparent organic electrodes which<br />

are parts <strong>of</strong> photovoltaic cells. Indium tin Oxide (ITO) and fluorine<br />

tin oxide (FTO) have been common materials as window electrodes<br />

in optoelectronic devices [5], but <strong>the</strong>y have some disadvantages<br />

e.g. high price, chemical instability and low mechanical<br />

strength. They have been made many studies on o<strong>the</strong>r transparent<br />

contacting materials like conducting oxide ZnO:Al [6] or conducting<br />

polymer PEDOT:PSS used in organic light emitting diode<br />

application, and showed better results compared to ITO [7]. In<br />

last years a significant increase in interest about graphene can<br />

be observed because <strong>of</strong> great properties <strong>of</strong> this carbon one atom<br />

thick material. Some extraordinary properties like low resistance<br />

[8, 9], high elasticity [10], mechanical stability [11], and high transparency<br />

<strong>of</strong> light in visible lenghts [12–15] have been described<br />

in <strong>the</strong> literature locating graphene in <strong>the</strong> main point <strong>of</strong> interest <strong>of</strong><br />

researchers working on transparent electrodes. They have been<br />

made some approaches in obtaining transparent graphene films<br />

by chemical vapor deposition [16, 17], dip coating in hot Graphene<br />

Oxide (GO) aqueous dispersion [14], ink-jet printing [18, 19].<br />

We have already made some approaches to transparent electrodes<br />

in photovoltaics based on carbon nanotubes using screen<br />

printing method and compared results with indium tin oxide (ITO)<br />

electrodes made with <strong>the</strong> same method [20]. Here we present<br />

transparent electrodes based on graphene nanoplateletes using<br />

screen printing method because it seems to be <strong>the</strong> cheapest<br />

technology in mass production <strong>of</strong> large area and transparent films<br />

without vacuum participation. Printing pastes described in this<br />

paper consist <strong>of</strong> several graphene sheets stacked toge<strong>the</strong>r and<br />

mixed with PMM solution and appropriate solvent. Some similar<br />

approaches have been done and described using different carriers<br />

and solvents [21–24].<br />

Experiment<br />

Two types <strong>of</strong> graphene nanoplatelets were used in <strong>the</strong> present<br />

study as a reinforcing and conductive additive in composite with<br />

polymethylmethacrylate (PMMA) as a carrier with butyl carbitol<br />

acetate solvent. Both materials supplied by Cheap Tubes Inc.<br />

were obtained by chemical method. Dimensions graphene nanoplatelets<br />

type B and C (GNPB, GNPC) were observed on scanning<br />

electron microscope (SEM) AURIGA CrossBeam Workstation.<br />

Tabl. 1. Graphene nanoplatelets materials parameters used in investigation<br />

GNP<br />

Diameter<br />

[m 2 /g]<br />

Surface<br />

area [um]<br />

Grade<br />

Average thicknes<br />

[nm]<br />

GNP-B 100 2-5 2 10<br />

GNP-C 100 2-25 2 10<br />

Composite layers were prepared on elastic PET film using<br />

screen printing method. Pastes prepared for <strong>the</strong> experiment<br />

had graphene nanoplatelets loading varying from 0,1 to 3 wt%.<br />

Two kinds <strong>of</strong> fillers were used: 10wt% Polymethylmethacrylate<br />

(PMMA) with butyl carbitol acetate solvent and ready-made matrix<br />

no. 8155 produced by Diupont.<br />

To achieve uniformity <strong>of</strong> <strong>the</strong> filler in <strong>the</strong> PMMA matrix mixtures<br />

were sonicated for 60 min at room temperature. Then <strong>the</strong>y were<br />

rolled twice on tree roll mills with silicon carbide (SiC) roller. The<br />

printing process <strong>of</strong> <strong>the</strong> final mixtures was done using screen printer<br />

AMI Presco type 242. After printing samples were cured at<br />

120°C for 1 h.<br />

Elektronika 6/2012 97


a)<br />

a)<br />

transmissivity [%]<br />

b)<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

0<br />

200 400 600 800 1000 1200<br />

wavelength (nm)<br />

1% GNPC<br />

0,25% CNT<br />

1%GNPC +0,1%CNT<br />

1%GNPC+ 0,05%CNT<br />

b)<br />

transmissivity [%]<br />

90<br />

80<br />

70<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

0<br />

200 400 600 800 1000 1200<br />

wavelenght (nm)<br />

0,1 % GNPB<br />

0,3 % GNPB<br />

0,5 % GNPB<br />

Fig. 2. Transparency measurements<br />

a)<br />

Fig. 1. ITME image: a) graphene nanoplatelets GNP-B, 2-5 um, b) graphene<br />

nanoplatelets GNP-C 2-25 um<br />

To measure transparency optical spectrometer was used.<br />

Length <strong>of</strong> emitted waves varied from 1200 to 200 nm, but waves<br />

shorter than 300 nm were blocked by PET substrate. Prepared<br />

samples were compared with samples containing 0,25 wt% multiwalled<br />

carbon nanotubes (MWCNTs) and mixtures connecting<br />

1wt% GNPs-C with 0,05 or 0,1wt%MWCNTs.<br />

To verify <strong>the</strong> resistance <strong>of</strong> <strong>the</strong> conductive layer to cyclic bending<br />

mechanical fatigue tests were conducted. Prepared samples<br />

were compared with similar samples based on 0,25 wt%<br />

MWCNTs in 8 wt% PMMA 10 um layer and with 160 nm ITO on<br />

borosilicate glass.<br />

Results<br />

Transparency measurements<br />

Using GNPs in screen printed layers allows wavelengths with<br />

<strong>the</strong> same intensity pass through (in contrast to layers based on<br />

CNTs). This interesting property is characteristic for <strong>the</strong> GNPs layers<br />

and can by used in various types <strong>of</strong> sensors. Relatively high<br />

transparency <strong>of</strong> <strong>the</strong> layer containing <strong>the</strong> filler <strong>of</strong> carbon nanotubes<br />

for <strong>the</strong> waves in <strong>the</strong> upper measuring range is reduced by 50<br />

percent for <strong>the</strong> wave length <strong>of</strong> 300 nm (Fig. 2a).<br />

Layer <strong>of</strong> 1wt% GNPs-C + 0,05wt% CNT has resistance at<br />

80 kΩ/□. Transparency <strong>of</strong> 32% is sufficient for this layer to serve<br />

as an electrode for electroluminescent displays and shall be fur<strong>the</strong>r<br />

investigated as a possible fotovoltaic cell.<br />

Layers with GNP B loading varying from 0,1% to 0,5% have<br />

higher transmissivity. Resistance <strong>of</strong> 0,1 GNP B layer is 2,2 M Ω/□<br />

98<br />

b)<br />

Resistance [kΩ/ □ ]<br />

14<br />

12<br />

10<br />

8<br />

6<br />

4<br />

2<br />

0<br />

Results <strong>of</strong> mechanical fatigue tests.<br />

0 10000 20000 30000 40000 50000 60000 70000 80000<br />

number <strong>of</strong> cycles<br />

2%GNPC<br />

0,25% CNT<br />

Fig. 3. Results <strong>of</strong> mechanical fatigue tests: a) ITO based samples,<br />

b) comparison MWCNT layers with GNPC layer<br />

for layer based on Dupont 8155 matrix. Layers with PMMA-based<br />

matrix have resistance <strong>of</strong> 1,5 M Ω/□. High transparency <strong>of</strong><br />

75…80% will allow to print layers on elastic photovoltaic applications<br />

such as transparent conductive layers supporting classic<br />

electrodes.<br />

Elektronika 6/2012


a)<br />

b)<br />

Fig. 4. ITME image: a) graphene nanoplatelets agglomeration,<br />

b) screen printing GNP-C layers on PET film<br />

Mechanical properties <strong>of</strong> elastic graphene films<br />

Investigation proved that graphene layers have <strong>the</strong> same mechanical<br />

resistance as carbon nanotubes layers. After nearly 75 000<br />

cycles, <strong>the</strong> paths resistance <strong>of</strong> paste based on GNPs changed<br />

+/- 2% and <strong>the</strong> resistance <strong>of</strong> <strong>the</strong> paste paths based on carbon nanotubes,<br />

after initial improvement after <strong>the</strong> first 2000 cycles, consistently<br />

declined slightly, although value was higher than starting<br />

one. High mechanical resistance makes <strong>the</strong>m suitable to use in<br />

elastic fotovoltaic applications. Resistance <strong>of</strong> ITO based samples<br />

investigated in <strong>the</strong> same way changes after very first cycles. Fur<strong>the</strong>r<br />

bending quickly leads to destruction <strong>of</strong> <strong>the</strong> layer.<br />

Summary<br />

GNPs based layers prepared with screen printing technology proved<br />

to have good mechanical resistance and high transparency.<br />

GNPs agglomerates were observed in investigated samples and<br />

<strong>the</strong>y are <strong>the</strong> cause <strong>of</strong> <strong>the</strong> layer’s resistance raise. Fur<strong>the</strong>r investigation<br />

needs to be done to observe how such situation can be<br />

avoided.<br />

First experiments on carbon nanotubes layers showed that<br />

using dispersants prevents agglomerating. Without agglomerates<br />

appearing in <strong>the</strong> samples, conductivity improves consistently.<br />

References<br />

[1] R. Hilderman, “SIGNS OF CLIMATE CHANGE,” Mo<strong>the</strong>r Earth News,<br />

p. 60, 2011.<br />

[2] C. Perrow, “Fukushima and <strong>the</strong> inevitability <strong>of</strong> accidents,” Bulletin <strong>of</strong><br />

<strong>the</strong> Atomic Scientists, vol. 67, pp. 44–52, 2011.<br />

[3] J. E. Z. Innocent, N. Jari, and N. Matti, “Effects <strong>of</strong> a hydropower plant<br />

on Coleopteran diversity and abundance in <strong>the</strong> Udzungwa Mountains,<br />

Tanzania,” Biodiversity & Conservation, vol. 13, pp. 1453–1464,<br />

2004.<br />

[4] M. de Lucas, M. Ferrer, M. J. Bechard, and A. R. Muñoz, “Griffon vulture<br />

mortality at wind farms in sou<strong>the</strong>rn Spain: Distribution <strong>of</strong> fatalities<br />

and active mitigation measures,” Biological Conservation, vol. 147,<br />

pp. 184–189, 2012.<br />

[5] U. L. D. Bach, “Solid-state dye-sensitized mesoporous TiO... solar<br />

cells with high photon-to-electron conversion,” Nature, vol. 395, p.<br />

583, 1998.<br />

[6] K. Schulze, B. Maennig, K. Leo, Y. Tomita, C. May, J. Hupkes, E.<br />

Brier, E. Reinold, and P. Bauerle, “Organic solar cells on indium tin<br />

oxide and aluminum doped zinc oxide anodes,” Applied Physics Letters,<br />

vol. 91, p. 073521, 2007.<br />

[7] K. Fehse, K. Walzer, K. Leo, W. Lövenich, and A. Elschner, “Highly<br />

Conductive Polymer Anodes as Replacements for Inorganic Materials<br />

in High-Efficiency Organic Light-Emitting Diodes,” Advanced Materials,<br />

vol. 19, pp. 441–444, 2007.<br />

[8] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson,<br />

I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, “Two-dimensional<br />

gas <strong>of</strong> massless Dirac fermions in graphene,” Nature, vol. 438,<br />

pp. 197–200, 2005.<br />

[9] Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, “Experimental observation<br />

<strong>of</strong> <strong>the</strong> quantum Hall effect and Berry’s phase in graphene,”<br />

Nature, vol. 438, pp. 201–204, 2005.<br />

[10] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H.<br />

Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, “Large-scale pattern growth<br />

<strong>of</strong> graphene films for stretchable transparent electrodes,” Nature, vol.<br />

457, pp. 706–710, 2009.<br />

[11] L. Changgu, W. Xiaoding, J. W. Kysar, and J. Honel, “Measurement<br />

<strong>of</strong> <strong>the</strong> Elastic Properties and Intrinsic Strength <strong>of</strong> Monolayer Graphene,”<br />

Science, vol. 321, pp. 385–388, 2008.<br />

[12] W. Supinda, A. D. Dmitriy, S. Sasha, P. Richard, J. Inhwa, H. B. D.<br />

Ge<strong>of</strong>frey, E. Guennadi, W. Shang-En, C. Shu-Fang, L. Chuan-Pu, T.<br />

N. SonBinh, and S. R. Rodney, “Graphene−Silica Composite Thin<br />

Films as Transparent Conductors,” Nano Letters, vol. 7, pp. 1888–<br />

1892, 2007.<br />

[13] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth,<br />

T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant<br />

defines visual transparency <strong>of</strong> graphene,” Science (New York, N.Y.),<br />

vol. 320, pp. 1308–1308, 2008.<br />

[14] X. Wang, L. Zhi, and K. Mullen, “Transparent, Conductive Graphene<br />

Electrodes for Dye-Sensitized Solar Cells,” Nano Letters, vol. 8, pp.<br />

323–327, 2008/01/01 2007.<br />

[15] L. Xiaolin, Z. Guangyu, B. Xuedong, S. Xiaoming, W. Xinran, W. Enge,<br />

and D. Hongjie, “Highly conducting graphene sheets and Langmuir–<br />

Blodgett films,” Nature Nanotechnology, vol. 3, pp. 538–542, 2008.<br />

[16] L. Gomez De Arco, Y. Zhang, C. W. Schlenker, K. Ryu, M. E. Thompson,<br />

and C. Zhou, “Continuous, highly flexible, and transparent<br />

graphene films by chemical vapor deposition for organic photovoltaics,”<br />

ACS Nano, vol. 4, pp. 2865–2873, 2010.<br />

[17] Y.-Y. Choi, S. J. Kang, H.-K. Kim, W. M. Choi, and S.-I. Na, “Multilayer<br />

graphene films as transparent electrodes for organic photovoltaic devices,”<br />

Solar Energy Materials & Solar Cells, vol. 96, pp. 281–285,<br />

2012.<br />

[18] K.-Y. Shin, J.-Y. Hong, and J. Jang, “Flexible and transparent graphene<br />

films as acoustic actuator electrodes using inkjet printingElectronic<br />

supplementary information (ESI) available: A detailed experimental<br />

procedure, characterization, preparation <strong>of</strong> graphene films,<br />

AFM, TEM image <strong>of</strong>,” Chemical Communications, vol. 47, pp. 8527–<br />

8529, 2011.<br />

[19] F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo, T. S. Kulmala,<br />

G.-W. Hsieh, S. Jung, F. Bonaccorso, P. J. Paul, D. Chu, and A. C.<br />

Ferrari, “Inkjet-printed graphene electronics,” ACS Nano, vol. 6, pp.<br />

2992–3006, 2012.<br />

[20] J. M. Sibiński Maciej, Znajdek Katarzyna, Słoma Marcin, Guzowski<br />

Bartłomiej, “Carbon nanotube transparent conductive layers for solar<br />

cells application,” Optica Applicata, vol. XLI, pp. 375–381, 2011.<br />

[21] J. Ping, Y. Wang, K. Fan, J. Wu, and Y. Ying, “Direct electrochemical<br />

reduction <strong>of</strong> graphene oxide on ionic liquid doped screen-printed<br />

electrode and its electrochemical biosensing application,” Biosensors<br />

& Bioelectronics, vol. 28, pp. 204–209, 2011.<br />

[22] L. Zhang, Y. Li, L. Zhang, D.-W. Li, D. Karpuzov, and Y.-T. Long,<br />

“Electrocatalytic Oxidation <strong>of</strong> NADH on Graphene Oxide and Reduced<br />

Graphene Oxide Modified Screen-Printed Electrode,” International<br />

Journal <strong>of</strong> Electrochemical Science, vol. 6, pp. 819–829, Mar<br />

2011.<br />

[23] M. Qian, T. Feng, H. Ding, L. Lin, H. Li, Y. Chen, and Z. Sun, “Electron<br />

field emission from screen-printed graphene films,” Nanotechnology,<br />

vol. 20, Oct 21 2009.<br />

[24] D. W. Zhang, X. D. Li, S. Chen, H. B. Li, Z. Sun, X. J. Yin, and S. M.<br />

Huang, Graphene Nanosheet Counter-Electrodes for Dye-Sensitized<br />

Solar Cells, 2010.<br />

Elektronika 6/2012 99


Photoelectrical properties <strong>of</strong> photovoltaic structures<br />

based on CdTe/ZnO<br />

EUNIKA Zielony 1) , PAULINA Kamyczek 1) , PIOTR Biegański 1) , EWA Płaczek-Popko 1) ,<br />

RAFAŁ Pietruszka 2) , GRZEGORZ Łuka 2) , MAREK Godlewski 2)<br />

1)<br />

Wrocław University <strong>of</strong> Technology, Institute <strong>of</strong> Physics<br />

2)<br />

Institute <strong>of</strong> Physics, Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences, Warsaw<br />

<strong>Photovoltaics</strong> continues to be one <strong>of</strong> <strong>the</strong> fastest growing industries,<br />

with annual increase beyond 40% [1]. Photovoltaic (PV)<br />

solar cells convert incoming solar radiation directly into electricity<br />

and produce electricity as each conventional source <strong>of</strong> energy,<br />

but <strong>the</strong>y are very attractive for <strong>the</strong>y are environment friendly.<br />

Zinc oxide (ZnO) is nowadays worldwide extensively studied<br />

for optoelectronics and photovoltaics application. It is predicted<br />

that <strong>the</strong>re will be a mass production <strong>of</strong> conducting ZnO layers as<br />

transparent electrodes in solar cells. It is said that ZnO layers will<br />

soon replace indium transparent oxide (ITO) because <strong>of</strong> high cost<br />

and limited supply <strong>of</strong> indium. Yet <strong>the</strong>re is ano<strong>the</strong>r very attractive<br />

field <strong>of</strong> ZnO application. It may be used as <strong>the</strong> n-type partner<br />

for <strong>the</strong> organic materials. There is a bright future in front <strong>of</strong> <strong>the</strong><br />

PV cells based on such a hybrid structures for <strong>the</strong>ir flexibility and<br />

a very low cost <strong>of</strong> production.<br />

The use <strong>of</strong> ZnO in <strong>the</strong> novel electronic and PV devices demands<br />

low or extremely low processing temperature [2–4]. There<br />

are many different technologies used to obtain ZnO layers. These<br />

are chemical vapor deposition (CVD), molecular beam epitaxy<br />

(MBE), sputtering, electron-beam evaporation, pulse laser deposition<br />

(PLD), hydro<strong>the</strong>rmal method and many o<strong>the</strong>rs, however<br />

<strong>the</strong>se techniques are unsuitable for <strong>the</strong> aforementioned applications<br />

as <strong>the</strong>y are run at too high temperature. In this context<br />

<strong>the</strong> Low Temperature Atomic Layer Deposition method (LT ALD)<br />

technique has proven to be <strong>the</strong> most promising [5]. Moreover,<br />

<strong>the</strong> technique is fairly cheap and allows ZnO growth with atomic<br />

resolution.<br />

Experiment<br />

The subject <strong>of</strong> investigations was a test semiconductor cell based<br />

on ZnO grown by ALD method on CdTe substrate. The structure<br />

<strong>of</strong> <strong>the</strong> studied sample is depicted in Fig. 1. The undoped layer <strong>of</strong><br />

ZnO is <strong>the</strong> n-type partner for p-type CdTe whereas <strong>the</strong> ZnO:Al<br />

layer is transparent conductive layer. The thickness <strong>of</strong> <strong>the</strong> ZnO<br />

layers was determined from interference spectra <strong>of</strong> reflectance<br />

measurements [6].<br />

oxygen precursor. Zinc oxide is created as a result <strong>of</strong> a doubleexchange<br />

chemical reaction that takes place at <strong>the</strong> surface:<br />

Zn(C 2<br />

H 5<br />

) 2<br />

+ H 2<br />

O → ZnO + 2C 2<br />

H 6<br />

Substrate temperature was varied between 60°C and 240°C. For<br />

dimethylzinc (DMZn) as zinc precursor a growth temperature could<br />

be reduced to a room temperature [7].<br />

Rectifying properties <strong>of</strong> <strong>the</strong> ZnO/CdTe junction have been<br />

investigated by current-voltage (I-V) characteristics measured<br />

in darkness and after illumination. Spectral characteristics<br />

<strong>of</strong> photocurrent have been carried out within spectral range <strong>of</strong><br />

300…1100 nm using PV Quantum Efficiency system, Bentham<br />

U.K. From <strong>the</strong> I-V measurements series resistance (R s<br />

) was<br />

calculated. From <strong>the</strong> spectral characteristics <strong>of</strong> photocurrent<br />

spectral sensitivity was determined. Additionally, based on <strong>the</strong><br />

spectral characteristics <strong>of</strong> photocurrent <strong>the</strong> thickness <strong>of</strong> <strong>the</strong> layers<br />

was determined.<br />

Results<br />

Current-voltage characteristics obtained for <strong>the</strong> studied n-ZnO/p-<br />

CdTe junctions are shown in Fig. 2. They were carried out at room<br />

temperature, in darkness and under illumination. The sample was<br />

illuminated by a Xe lamp.<br />

The I-V curves show rectifying properties, and light-electricity<br />

conversion (both better visible in Fig. 3. on a semilogarithmic plot)<br />

but <strong>the</strong> I-V characteristics are shifted towards negative voltage<br />

bias. From <strong>the</strong> linear slope <strong>of</strong> <strong>the</strong> I-V curves given in Fig. 2 for<br />

<strong>the</strong> studied junction series resistance was determined. Its value<br />

equals to 36 Ω. Such a high value <strong>of</strong> series resistance as well as<br />

some leakage current (low shunt resistance) are presumably due<br />

to <strong>the</strong> observed shift.<br />

In future work <strong>the</strong> fill factor must be enhanced by reducing <strong>the</strong><br />

series resistance and increasing <strong>the</strong> shunt resistance.<br />

Figures 4 presents spectral characteristics <strong>of</strong> photocurrent<br />

measured at room temperature. In spite <strong>of</strong> poor efficiency <strong>of</strong> <strong>the</strong><br />

test diode its spectral characteristics is promising for photovoltaic<br />

application.<br />

Fig. 1. The layer structure <strong>of</strong> n-ZnO/p-CdTe sample<br />

In <strong>the</strong> ALD process, precursors are sequentially introduced to<br />

<strong>the</strong> growth chamber, where <strong>the</strong>y reach a surface <strong>of</strong> <strong>the</strong> growing<br />

film. The ALD process consists <strong>of</strong> repeating <strong>of</strong> four deposition<br />

steps: 1) deposition <strong>of</strong> <strong>the</strong> first precursor, 2) purging <strong>the</strong> reaction<br />

chamber with an inert gas, 3) deposition <strong>of</strong> <strong>the</strong> second precursor<br />

and 4) purging <strong>of</strong> <strong>the</strong> reaction chamber [7]. Two precursors<br />

are at present commonly used for ZnO growth by ALD method:<br />

diethylzinc (DEZn) as a zinc precursor and deionized water as an<br />

Fig. 2. I-V characteristics for n-ZnO/p-CdTe structure measured<br />

in darkness and under illumination (white light)<br />

100<br />

Elektronika 6/2012


To obtain <strong>the</strong> total thickness <strong>of</strong> <strong>the</strong> ZnO films following recurrence<br />

equations were considered for interference maxima, corresponding<br />

to k and m orders:<br />

<br />

and for interference minima:<br />

<br />

d<br />

m<br />

d<br />

m<br />

=<br />

( k − m)<br />

⋅<br />

λ<br />

2 ⋅ n<br />

n<br />

( λ )<br />

⎛ 1 ⎞ λ<br />

= ⎜k<br />

− m + ⎟ ⋅<br />

⎝ 2 ⎠ 2 ⋅ n<br />

n<br />

n<br />

,<br />

( λ)<br />

n<br />

,<br />

Fig. 3. Semilogarithmic plot <strong>of</strong> I-V characteristics for n-ZnO/p-CdTe<br />

structure measured in darkness and under illumination<br />

Fig. 4. Spectral response <strong>of</strong> <strong>the</strong> n-ZnO/p-CdTe diode<br />

Efficient photoresponse is observed up to ~3 AW -1 at 920<br />

nm. However <strong>the</strong> photoresponse is lower than expected at short<br />

wavelength range. This fact may be related to strong recombination<br />

at <strong>the</strong> interface between ZnO and CdTe; also, interdiffusion<br />

between <strong>the</strong>se layers may have some effect on <strong>the</strong><br />

spectral response in this wavelength range. It can be noticed<br />

also that <strong>the</strong> spectrum is extended towards longer wavelength<br />

with comparison to <strong>the</strong> CdTe/CdS based thin film solar cells for<br />

which <strong>the</strong> long wavelength cut-<strong>of</strong>f is around 850 nm (CdTe band<br />

gap) (cf. for example Handbook <strong>of</strong> Photovoltaic Science and<br />

Engineering, edited by A. Luque and S. Hegedus, 2003 John<br />

Wiley&Sons, Ltd ISBN: 0-471-49196-9.) This suggests that <strong>the</strong><br />

bandgap <strong>of</strong> CdTe has decreased most likely as a result <strong>of</strong> Zn<br />

diffusion into this layer.<br />

The band <strong>of</strong> <strong>the</strong> spectral responsivity exhibits interference<br />

fringes related to <strong>the</strong> total ZnO films thickness (cf. Fig. 4 ). The<br />

fringes appear because <strong>the</strong> light ray passes through <strong>the</strong> ZnO<br />

layers, undergoes multiple reflection on <strong>the</strong>ir borders and subsequently<br />

– interference takes place. Electrical response <strong>of</strong> <strong>the</strong><br />

investigated semiconductor structure is proportional to <strong>the</strong> intensity<br />

<strong>of</strong> <strong>the</strong> light beam which changes with <strong>the</strong> wavelength due<br />

to <strong>the</strong> interference effects.<br />

where λ – measured wavelength, d – total ZnO films thickness,<br />

k, m – interference orders and n – refractive index (for ZnO n<br />

changes from 2.1 to 1.9 with increasing <strong>the</strong> wavelength [8]).<br />

In order to determine <strong>the</strong> thickness d <strong>of</strong> <strong>the</strong> layers, an iteration<br />

procedure has been applied until <strong>the</strong> standard deviation<br />

for all <strong>the</strong> obtained thicknesses d achieved minimal value. The<br />

thickness <strong>of</strong> ZnO layers was subsequently calculated as an<br />

average value <strong>of</strong> all values d found for various wavelengths.<br />

The value equals to ~630 nm (±90 nm) taking into account<br />

possibly phase shift on <strong>the</strong> CdTe/ZnO border. Thus it may be<br />

concluded that <strong>the</strong> value <strong>of</strong> ZnO thickness determined from<br />

<strong>the</strong> photocurrent interference is close to <strong>the</strong> value determined<br />

from reflectance measurements.<br />

Summary<br />

The n-type ZnO layers were grown by ALD method on p-type<br />

CdTe substrate. I-V characteristics verified rectifying properties<br />

<strong>of</strong> <strong>the</strong> test ZnO/CdTe solar cell diode and exhibited photovoltaic<br />

effect when <strong>the</strong> junction was exposed to light. The series resistance<br />

<strong>of</strong> <strong>the</strong> diode, determined from <strong>the</strong> I-V curves, equals to 36<br />

Ω. Such a high value is responsible for low value <strong>of</strong> fill factor and<br />

efficiency <strong>of</strong> <strong>the</strong> solar cell. Photoresponse properties <strong>of</strong> <strong>the</strong> studied<br />

junction were measured at room temperature. Efficient photoresponse<br />

was observed within wavelength range <strong>of</strong> 400–1000<br />

nm. These results indicate that n-ZnO/p-CdTe junction is suitable<br />

for <strong>the</strong> fabrication <strong>of</strong> efficient solar cells. It was shown that <strong>the</strong><br />

thickness <strong>of</strong> <strong>the</strong> ZnO layers can be also determined with <strong>the</strong> help<br />

<strong>of</strong> interference fringes <strong>of</strong> photoresponse analysis. Fur<strong>the</strong>r work<br />

will involve a better understanding <strong>of</strong> <strong>the</strong> properties <strong>of</strong> window<br />

layer and junction formation processes.<br />

This work has been supported by <strong>the</strong> project <strong>of</strong> National Laboratory<br />

<strong>of</strong> Quantum Technologies (POIG. 02.02.00-00-003/08-00).<br />

References<br />

[1] Godlewski M., A. Wójcik-Głodowska, E. Guziewicz, S. Yatsunenko,<br />

A. Zakrzewski, Y. Dumont, E. Chikoidze, M. R. Phillips:<br />

Optical Materials 31, 1768 (2009).<br />

[2] Klingshirn C., J. Fallert, H. Zhou, J. Sartor, C. Thiele, F. Maier-Flaig,<br />

D. Schneider, H. Kalt, Phys. Stat. Sol. B 247, 1424<br />

(2010).<br />

[3] Morkoc H.,U. Ozgur: Zinc Oxide: Fundamentals, materials and<br />

Devices Technology, ed. Wiley-VCH (2008).<br />

[4] Huby N., S. Ferrari, E. Guziewicz, M. Godlewski, V. Osinniy:<br />

Appl. Phys. Lett. 92, 023502 (2008).<br />

[5] Guziewicz E., I. A. Kowalik, M. Godlewski, K. Kopalko, V. Osinniy,<br />

A. Wójcik, S. Yatsunenko, E. Łusakowska, W. Paszkowicz:<br />

J. Appl. Phys. 103, 033515 (2008).<br />

[6] http://info.ifpan.edu.pl/rn_ifpan/Luka-doktorat.pdf<br />

[7] Godlewski M., E. Guziewicz, G. Łuka, T. Krajewski, M.<br />

Łukasiewicz, Ł. Wachnicki, A. Wachnicka, K. Kopalko, A. Sarem,<br />

B. Dalati: Thin Solid Films 518, 1145 (2009).<br />

[8] http://refractiveindex.info/?group=CRYSTALS&material=ZnO<br />

Elektronika 6/2012 101


Multifunctional coatings for solar cells application<br />

Karolina Sieradzka, Michał Mazur, Danuta Kaczmarek, Jarosław Domaradzki,<br />

Damian Wojcieszak<br />

Wroclaw University <strong>of</strong> Technology, Faculty <strong>of</strong> Microsystem Electronics and Photonics<br />

In recent years, an increased interest <strong>of</strong> nanocrystalline oxide materials<br />

was been observed [1, 2]. Such oxides join a few selected,<br />

well-defined properties such as high transparency, good electrical<br />

conductivity, hydrophilic or hydrophobic properties, antireflective<br />

properties, etc. [3, 4]. In contrary to conventional semiconductors,<br />

such thin oxide films, prepared for transparent electronics<br />

or solar cells application, combine mainly two specific features<br />

[5]: high transparency in visible light and <strong>the</strong> ability <strong>of</strong> electrical<br />

conduction at room temperature. These oxides, depending on <strong>the</strong><br />

level <strong>of</strong> electrical resistivity are divided into two groups <strong>of</strong> materials:<br />

Transparent Conducting Oxide (TCO) or Transparent Oxide<br />

Semiconductor. However, in <strong>the</strong> literature, <strong>the</strong>re is increasing<br />

number <strong>of</strong> reports about trying to get <strong>the</strong> additional properties,<br />

but most showed examples <strong>of</strong> multilayers. For example, it might<br />

be a thin oxide films with additional antireflective properties, prepared<br />

as TiO 2<br />

/SiO 2<br />

multilayers [4, 6] in order to obtain <strong>the</strong> largest<br />

possible reduction <strong>of</strong> light reflectance. Pemble et al. described<br />

dual functionality self-cleaning <strong>the</strong>rmochromic films prepared by<br />

APCVD method. The multilayers based on VO 2<br />

and TiO 2<br />

revealed<br />

good degradation <strong>of</strong> stearic acid under UV radiation and <strong>the</strong>rmochromic<br />

properties with <strong>the</strong> switching temperature <strong>of</strong> 55 o C.<br />

In <strong>the</strong> first part <strong>of</strong> this paper, <strong>the</strong> possibilities <strong>of</strong> characterization<br />

<strong>of</strong> nanostructures used in <strong>the</strong> Laboratory <strong>of</strong> Optoelectrical<br />

Diagnostics <strong>of</strong> Nanomaterials located at Wroclaw Univeristy <strong>of</strong><br />

Technology have been showed. Then antireflective, photoactive<br />

nanocrystalline Ti-V oxide as a multifunctional thin film has been<br />

presented.<br />

Directions <strong>of</strong> current research<br />

The scope <strong>of</strong> current work carried out in <strong>the</strong> Laboratory involves<br />

<strong>the</strong> research for new, nanocrystalline oxide materials with unique<br />

electrical and optical properties, which might be applied as multifunctional<br />

coatings for example solar cells, glasses for special<br />

application or optoelectronic elements.<br />

The subject <strong>of</strong> interest <strong>of</strong> staff from our Laboratory are closely<br />

related to preparation and analysis <strong>of</strong> mainly <strong>the</strong> optical, electrical,<br />

hydrophilic and hydrophobic and antistatic properties <strong>of</strong> thin oxide<br />

films. A modified magnetron sputtering method [7], developed<br />

and patented by <strong>the</strong> staff from our Division enabled to achieve<br />

transparent and semiconducting oxide based on TiO 2<br />

doped with<br />

different metal ions with n- or p-type <strong>of</strong> electrical conduction.<br />

Review <strong>of</strong> measurements apparatus for thin<br />

oxide films characterization used in Laboratory<br />

Optical properties are analysed based on spectrophotometry<br />

measurements. Transmittance and reflectance spectra are studied<br />

in <strong>the</strong> spectral range from 200 nm to 2600 nm at normal<br />

incident. The spectra are recorded with <strong>the</strong> use <strong>of</strong> Ocean Optics<br />

spectrophotometers (QE65000 and NIR-256.2.1) and coupled<br />

DH-2000-BAL deuterium-halogen or XE-450 xenon lamps as an<br />

excitation light source. Additionally, <strong>the</strong> optical measurements<br />

performed with <strong>the</strong> aid <strong>of</strong> Janis ST-100H cryostat in a varied temperature<br />

conditions (from ~2K up to 700K) allow for <strong>the</strong> analysis<br />

<strong>of</strong> gasochromic and <strong>the</strong>rmochromic properties.<br />

Electrical properties are examined in <strong>the</strong> wide temperature range<br />

in order to determine <strong>the</strong> basic electrical parameters, e.g. resistivity,<br />

type <strong>of</strong> electrical conductivity. The electrical temperature<br />

dependent measurements are performed in <strong>the</strong> range from 25 o C<br />

to 300 o C using air-cooled <strong>the</strong>rmal chuck operated by ERS SP72<br />

controller. In case <strong>of</strong> heterojunction structure <strong>the</strong> fundamental property<br />

is <strong>the</strong>ir current-to-voltage performance. The measurements<br />

102<br />

are performed using Keithley 4200-SCS semiconductor characterization<br />

system equipped with Cascade MicroTech probe station<br />

with <strong>the</strong>rmal chuck heater. The system allows <strong>the</strong> measurements<br />

from 100 aA up to 100 mA within a voltage source-measure range<br />

from 1 mV up to 210 V. Additonally, measurements performed at<br />

different temperature with and without light illumination provide to<br />

study <strong>of</strong> <strong>the</strong>ir influence on electrical performance <strong>of</strong> solar cells. A<br />

special s<strong>of</strong>tware Keithley Interactive Test Environment enables for<br />

<strong>the</strong> rapid determination <strong>of</strong> solar cells parameters such as: shortcircuit<br />

current, open circuit voltage, maximum power, maximum<br />

current, maximum voltage, fill factor and cell efficiency.<br />

The analysis <strong>of</strong> electrical conduction mechanisms are done<br />

using <strong>the</strong> impedance spectroscopy (IS) method. Capacitance and<br />

conductivity data measured in a wide frequency range from 40 Hz<br />

to 110 MHz are recorded with a help <strong>of</strong> an Agilent Technologies<br />

4294A precision impedance analyser.<br />

Electrical investigations are completed with stationary and<br />

transient photoelectrical measurements. The measurements performed<br />

with a monochromatic light on <strong>the</strong> range <strong>of</strong> 200 nm up to<br />

1400 nm allow to investigate photoelectrical effect at <strong>the</strong> prepared<br />

heterostructures and to study <strong>the</strong>irs spectral sensitivity. Investigation<br />

<strong>of</strong> photoelectrical properties performed by a scanned<br />

light beam allows building two-dimensional maps <strong>of</strong> photocurrent<br />

distribution at <strong>the</strong> active area <strong>of</strong> <strong>the</strong> junction. These optical beam<br />

induce current (OBIC) maps usually allow to study generation-recombination<br />

centres in junctions, inter-level shorts and a latch-up<br />

mechanism.<br />

The surface wettability <strong>of</strong> <strong>the</strong> thin films are evaluated by contact<br />

angle measurement <strong>of</strong> deionized water under room temperature<br />

and daylight illumination. The measurement are carried out<br />

with a computer controlled Theta Lite goniometer system manufactured<br />

by Attension.<br />

The antistatic properties <strong>of</strong> thin films are investigated using a<br />

modern measurement station – JCI 155v5 Charge Decay Test<br />

Unit operated in <strong>the</strong> chamber with controlled humidity and temperature.<br />

The ability for static charge dissipation from samples<br />

surface vs. time were measured.<br />

Results <strong>of</strong> <strong>the</strong> measurments<br />

Thin oxide films were prepared mainly based on TiO 2<br />

, which is<br />

in particular non-toxic, transparent to light in wide spectral range<br />

and <strong>the</strong>rmally, chemically, mechanically stable material. The thin<br />

films were deposited in pure oxygen atmosphere by developed,<br />

modified high energy reactive magnetron sputtering (HE RMS)<br />

method. As a result <strong>the</strong> nanocrystalline oxides with <strong>the</strong>rmodynamically<br />

stable rutile form with average crystallites size under<br />

10 nm (Fig. 1) has been achieved.<br />

Typically, in a conventional magnetron sputtering process, directly<br />

after deposition <strong>of</strong> TiO 2<br />

thin films, have <strong>the</strong> anatase phase,<br />

which is less <strong>the</strong>rmodynamically stable. Then, <strong>the</strong> crystalline rutile<br />

form <strong>of</strong> TiO 2<br />

is received only when additional <strong>the</strong>rmal heating<br />

at 750 o C has been used. These achievements have contributed<br />

to a number <strong>of</strong> unique and desirable properties. These films,<br />

in addition to high transparency for light in <strong>the</strong> visible range, revealed<br />

among o<strong>the</strong>rs increased hardness. The results <strong>of</strong> hardness<br />

measurements using nanoindenter (Fig. 2) showed that <strong>the</strong> nanocrystalline<br />

TiO 2<br />

thin films deposited in HE RMS process has<br />

a high hardness <strong>of</strong> 14.3 GPa, what is comparable to DLC materials.<br />

While in case <strong>of</strong> TiO 2<br />

films sputtered in <strong>the</strong> conventional<br />

process <strong>the</strong> hardness was 3.5 GPa and 7.9 GPa respectively for<br />

thin films after deposition and after annealing at 800 o C.<br />

Elektronika 6/2012


Optical and electrical parameters <strong>of</strong> thin oxide films based on TiO 2<br />

Thin film [% at.]<br />

T 550<br />

[%]<br />

λ cut<strong>of</strong>f<br />

[nm]<br />

ρ 300K<br />

[Ω cm]<br />

S<br />

[mV/K]<br />

TiO 2<br />

83 338 1·10 11 – –<br />

TiO 2<br />

:(5.5 Pd) 50 370 9.6·10 2 -11<br />

Ti-V oxides (3.0 V) 73 381 2.7·10 5 -200<br />

Ti-V oxides (23.0 V) 73 430 5,6·10 2 -20 n type<br />

TiO 2<br />

:(0.9 Eu, 5.8 Pd) 35 450 2.2·10 -1 -90<br />

TiO 2<br />

:(10.2 V, 6.9 Pd) 40 400 1.5 -410<br />

Ti-V oxides (19.0 V) 73 380 8·10 4 +685<br />

TiO 2<br />

:(15.8 Co, 6.9 Pd) 29 * 560 3.5·10 3 +77.5 p type<br />

TiO 2<br />

:(0.6 Tb, 9.0 Pd) 22 * 510 1·10 5 +120<br />

* average transmission determined at <strong>the</strong> wavelength λ=650 nm<br />

Fig. 1. AFM image <strong>of</strong> <strong>the</strong> nanocrystalline TiO 2<br />

thin films<br />

Fig. 2. Nanoindenter images <strong>of</strong> TiO 2<br />

thin films surface after hardness<br />

measurement<br />

From <strong>the</strong> point <strong>of</strong> view <strong>of</strong> oxide materials prepared for application<br />

in transparent electronics very important is a possibility to produce<br />

<strong>the</strong> transparent oxide semiconductors both with electron and hole<br />

type <strong>of</strong> electrical conduction. In Tabl. 1 <strong>the</strong> basic optical and electrical<br />

parameters <strong>of</strong> thin films based on TiO 2<br />

with different material<br />

compositions, determined from optical and electrical measurements<br />

were summarized. On <strong>the</strong> basis <strong>of</strong> electrical results it can be stated<br />

that <strong>the</strong> electrical conductivity <strong>of</strong> prepared films is primarily activated<br />

by <strong>the</strong> presence <strong>of</strong> Pd dopant [8]. Unfortunately, simultaneously with<br />

increasing concentration <strong>of</strong> Pd in <strong>the</strong> TiO 2<br />

matrix (from 5.5 to 9.0 at.<br />

%) <strong>the</strong>re was a significant decrease in transparency, up to 22% for<br />

TiO 2<br />

: (Tb,Pd) thin films (Tabl.). In addition, doping <strong>of</strong> TiO 2<br />

matrix with<br />

Pd element resulted in <strong>the</strong> shift <strong>of</strong> optical absorption edge towards<br />

longer wavelength. The authors in previous work [8] have found that<br />

improved electrical conductivity in TiO 2<br />

:Pd thin films is <strong>the</strong> results<br />

<strong>of</strong> <strong>the</strong> presence <strong>of</strong> nanocrystalline islands <strong>of</strong> mettalic Pd. Ano<strong>the</strong>r<br />

example are TiO 2<br />

:(Co,Pd) thin films, in which increase conductivity<br />

was due to <strong>the</strong> substitution <strong>of</strong> Ti 4+ ions located in <strong>the</strong> crystal lattice<br />

<strong>of</strong> TiO 2<br />

by dopant ions Co 3+ and Co 2+ . Thus, within <strong>the</strong> TiO 2<br />

energy<br />

band gap appeared <strong>the</strong> acceptor levels, which in consequence<br />

resulted decrease <strong>of</strong> resistivity [8]. Average transparency in <strong>the</strong> visible<br />

wavelength for TiO 2<br />

: (Co,Pd) thin films is 29% and it is much<br />

lower compared to <strong>the</strong> undoped TiO 2<br />

(Tabl.). As it can be seen, good<br />

electrical conductivity <strong>of</strong> TiO 2<br />

: Pd and TiO 2<br />

: (Co,Pd) thin films were<br />

obtained at <strong>the</strong> cost <strong>of</strong> <strong>the</strong>ir transparency level to light.<br />

Type <strong>of</strong> electrical conductivity was determined on <strong>the</strong> basis<br />

<strong>of</strong> Seebeck coeficient (S). A positive value <strong>of</strong> S indicates p-type<br />

conductivity in oxide. When <strong>the</strong> sign is negative <strong>the</strong> conductivity is<br />

dominated by n-type conductivity. As it can be observed from <strong>the</strong><br />

characteristics <strong>of</strong> S (1000/T) shown in Fig. 3, <strong>the</strong> doping <strong>of</strong> thin<br />

films based on TiO 2<br />

makes it possible to receive oxide semiconductors,<br />

both <strong>the</strong> n-type (Fig. 3a) and <strong>the</strong> p-type (Fig. 3b). N-type <strong>of</strong><br />

electrical conductivity was obtained for TiO 2<br />

:Pd, TiO 2<br />

:V (3 at. % <strong>of</strong><br />

V), Ti-V oxides (23 at. % <strong>of</strong> V), TiO 2<br />

:(Eu,Pd) oraz TiO 2<br />

:(V,Pd). While<br />

<strong>the</strong> opposite type <strong>of</strong> electrical conductivity revealed in case <strong>of</strong> Ti-V<br />

oxides (19 at. % <strong>of</strong> V), TiO 2<br />

:(Tb,Pd) and TiO 2<br />

:(Co,Pd) thin films.<br />

In Fig. 3 also interesting property <strong>of</strong> Ti-V oxides it has been<br />

observed, in which by different V concentration <strong>the</strong> type <strong>of</strong> electrical<br />

conductivity has been controlled. However, <strong>the</strong> analysis <strong>of</strong><br />

<strong>the</strong>rmoelectic properties in case <strong>of</strong> vanadium oxides are very<br />

complex, because <strong>of</strong> multivalence <strong>of</strong> vanadium oxides, ranging<br />

from 2 + to 5 + and <strong>the</strong>ir structural transformation in specified<br />

temperature. For example, n-type conduction is possible by replacing<br />

Ti 4+ ions in <strong>the</strong>ir site position in TiO 2<br />

with reduced to +4<br />

valence state vanadium ions (V4+). It is well known that titanium<br />

and vanadium dioxides may form V 1−x<br />

Ti x<br />

O 2<br />

solid solution in<br />

a broad range <strong>of</strong> x values [9]. Therefore, in such films <strong>the</strong>re are<br />

available extra electrons able to move in external electric field,<br />

giving n-type conduction. N-type conduction is also observed<br />

a)<br />

b)<br />

S [ µV/K]<br />

S [µV/K]<br />

550 500 450 400 350<br />

100<br />

N-type Ti:V oxides<br />

(23 at. % <strong>of</strong> V)<br />

0<br />

-100<br />

-200<br />

-300<br />

-400<br />

500<br />

400<br />

300<br />

200<br />

100<br />

0<br />

TiO 2<br />

:V<br />

(3 at. % <strong>of</strong> V)<br />

TiO 2<br />

:Pd<br />

TiO 2<br />

:(V, Pd)<br />

T [K]<br />

TiO 2<br />

:(Eu,Pd)<br />

2,0 2,5 3,0 3,5<br />

1000/T [K -1 ]<br />

550 500 450 400 350<br />

P-type<br />

Ti:V oxides<br />

(19 at. % <strong>of</strong> V)<br />

TiO 2<br />

:(Co,Pd)<br />

T [K]<br />

TiO 2<br />

:(Tb,Pd)<br />

2,0 2,5 3,0 3,5<br />

1000/T [K -1 ]<br />

Fig. 3. Characteristics <strong>of</strong> Seebeck coefficient <strong>of</strong> semiconducting<br />

thin-film based on TiO 2<br />

: a) with electron and b) hole type <strong>of</strong> electrical<br />

conduction<br />

Elektronika 6/2012 103


Photocurrent [A]<br />

a)<br />

b)<br />

104<br />

10,0p<br />

8,0p<br />

6,0p<br />

4,0p<br />

2,0p<br />

R λ<br />

[%]<br />

0,0<br />

0 500 1000 1500 2000<br />

R λ<br />

(%)<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

X [ µm]<br />

pure, polished Si<br />

TOS-Si<br />

U=0 V<br />

λ=399 nm<br />

λ=466 nm<br />

λ=547 nm<br />

λ=589 nm<br />

λ=632 nm<br />

λ=928 nm<br />

Fig. 4. Characteristics <strong>of</strong> photocurrent obtained in effect <strong>of</strong> single<br />

scan “in line” <strong>of</strong> light beam across <strong>the</strong> area between <strong>the</strong> electrodes<br />

on Ti-V (3 at. % <strong>of</strong> V) oxides/Si heterostructure with <strong>the</strong> different light<br />

wavelength<br />

400 600 800 1000<br />

λ [nm]<br />

pure, polished and texturized Si<br />

TOS-Si<br />

R λ<br />

=18 %<br />

400 600 800 1000<br />

λ (nm)<br />

Fig. 5. Reflection spectra <strong>of</strong> mixed Ti-V oxide thin films deposited on<br />

different Si substrates: a) polished and b) polished, texturized<br />

in TiO 2<br />

–V 2<br />

O 5<br />

system [10] containing V 5+ vanadium ions. However<br />

thin films with vanadium reduced to +3 valence state (V 3+ ) inV 2<br />

O 3<br />

structure may display p-type conduction [11]. In Ti–V mixed oxide<br />

system with such reduced vanadium ions, formation <strong>of</strong> V 2<br />

TiO 5<br />

phase is possible. P-type conduction in this case is due to vanadium<br />

vacancies, which may act as acceptor levels and introduce<br />

holes in <strong>the</strong> valence band.<br />

In order to complete <strong>the</strong> study it was necessary to confirm<br />

that prepared thin TOSs films can form heterojunction with silicon<br />

(p-type) substrate. As a hterojunction example, Ti-V oxides<br />

(3 at. % <strong>of</strong> V) has been presented in Fig. 3. The photoelectrical<br />

characteristics I ph<br />

(x) were recorded using <strong>the</strong> OBIC measurements<br />

performed at six different wavelengths <strong>of</strong> 399, 466, 547,<br />

589, 632 and 928 nm (Fig. 4). The measurements were carried<br />

out at room temperature, with <strong>the</strong> light beam <strong>of</strong> ca. 30 µm in<br />

diameter and frequency modulation <strong>of</strong> f = 180 Hz. The highest<br />

photocurrent vs. scaning beam position was recorded for infrared<br />

light (λ = 928 nm).<br />

In Fig. 5 <strong>the</strong> comparision <strong>of</strong> reflection spectra <strong>of</strong> Ti-V (3 at. %<br />

<strong>of</strong> V) oxides deposited on different monocrystalline silicon substrate<br />

have been presented. Monocrystalline silicon substrate<br />

has a reflection coefficient (R λ<br />

) in <strong>the</strong> range <strong>of</strong> 30-35 % and<br />

usually it is reduced mainly through texturization process and<br />

additionally by a special antireflective film deposited on <strong>the</strong> silicon<br />

surface. The polished silicon substrate has R λ<br />

=30 % in <strong>the</strong><br />

visible spectral range. Never<strong>the</strong>less, after deposition <strong>of</strong> mixed<br />

vanadium-titanium oxides on such silicon substrate, <strong>the</strong> reflection<br />

coefficient was reduced <strong>of</strong> about 10 % (Fig. 5a). Observed<br />

behaviour testifies about antireflective function <strong>of</strong> Ti-V oxide thin<br />

films in <strong>the</strong> range <strong>of</strong> 300...1000 nm. Ano<strong>the</strong>r example concerns<br />

polished and texturized silicon substrate. Due to more diversivied<br />

surface <strong>of</strong> textured silicon, <strong>the</strong> minimalization <strong>of</strong> R λ<br />

down to<br />

10% can been achieved. Covering <strong>the</strong> polished and texturized<br />

Si with <strong>the</strong> same film like in <strong>the</strong> previous case causes fur<strong>the</strong>r<br />

reduction <strong>of</strong> <strong>the</strong> reflection coefficient dwon to several percent<br />

(Fig. 5b).<br />

Summary<br />

This paper briefly presents <strong>the</strong> main scientific directions and measurement<br />

opportunities <strong>of</strong> <strong>the</strong> staff from Laboratory <strong>of</strong> Optoelectrical<br />

Diagnostics <strong>of</strong> Nanomaterials situated at Wroclaw Univeristy<br />

<strong>of</strong> Technology. All discussed thin oxide films based on TiO 2<br />

were<br />

prepared by High Energy Reactive Magnetron Sputtering method.<br />

As it was showed, control <strong>of</strong> material composition allows to obtain<br />

nanocrystalline, transparent semiconductors with particular<br />

type <strong>of</strong> conductivity. Additionally, in such thin films o<strong>the</strong>r desirable<br />

properties can be achieved simultaneously, e.g. photoactivity and<br />

reduced reflection. It testifies about great application potential <strong>of</strong><br />

nanocrystalline TOS thin films as coatings for architectural glasses,<br />

windscreens, solar cells application.<br />

This work was financed from <strong>the</strong> sources granted by <strong>the</strong> NCN<br />

in <strong>the</strong> years 2011-2013 as a supervisors research project number<br />

N N515 4970 40.<br />

Authors would like to thank to E.L. Prociow from our Faculty<br />

for his help in <strong>the</strong> experimental part <strong>of</strong> this work.<br />

References<br />

[1] Wang C., T. Wang, S. Zheng, “Investigation <strong>of</strong> <strong>the</strong> photoreactivity <strong>of</strong><br />

nanocrystalline TiO 2<br />

thin film by ion-implantation technique”, Physica<br />

E, vol. 14, p. 242, 2002.<br />

[2] Banerjee A.N., “The design, fabrication, and photocatalytic utility <strong>of</strong><br />

nanostructured semiconductors: focus on TiO 2<br />

-based nanostructures”,<br />

Nanotechnology, Science and Applications, vol. 4, p. 35, 2011.<br />

[3] Parkin I.P., R.G. Palgrave, “Self-cleaning coatings”, Journal <strong>of</strong> Materials<br />

Chemistry, vol. 15, p. 1689, 2005.<br />

[4] Lien S-Y., D-S. Wuu, W-C. Yeh, J-C. Liu, “Tri-layer antireflection coatings<br />

(SiO 2<br />

/SiO 2<br />

-TiO 2<br />

/TiO 2<br />

) for silicon solar cells using a sol-gel technique”,<br />

Solar Energy Materials & Solar Cells, vol. 90, p. 2710, 2006.<br />

[5] Facchetti A., T.J. Marks, “Transparent Electronics – From Syn<strong>the</strong>sis<br />

to Applications”, John Wiley &Sons Ltd., United Kingdom, 2010.<br />

[6] Prado R., G. Beobide, A. Marcaide, J. Goikoetxea, A. Aranzabe, “Development<br />

<strong>of</strong> multifunctional sol-gel coatings: Anti-reflection coatings<br />

with enhanced self-cleaning capacity”, Solar Energy Materials & Solar<br />

Cells, vol. 94, p. 1081, 2010.<br />

[7] Prociow E.L., J. Domaradzki, D. Kaczmarek, T. Berlicki, Polish patent<br />

No P382163, 2007.<br />

[8] Domaradzki J., A. Borkowska, D. Kaczmarek, E. Prociów, “Transparent<br />

oxide semiconductors based on TiO 2<br />

doped with V, Co and Pd elements”,<br />

J. Non-Crystall. Solids, vol. 352, nr 23–25, p. 2324, 2006.<br />

[9] Beteille F., R. Morineau, J. Livage, M. Nagano, „Switching properties<br />

<strong>of</strong> V 1−x<br />

Ti x<br />

O 2<br />

thin films deposited from alkoxides”, Mater. Res. Bull.,<br />

vol. 32, p. 1109, 1997.<br />

[10] Szabo A., A. Urda, M. Alifanti, Ann. Univ. Buc. Chim. 15 (2006) 85.<br />

[11] Habel D., O. Goerke, M. Tovar, E. Kondratenko, H. Schubert, “Phase<br />

Relations in <strong>the</strong> System TiO 2<br />

-V 2<br />

O x<br />

under Oxidizing and Reducing<br />

Conditions”, J. Phase Equilib. Diffus., vol. 29, p. 482, 2008.<br />

Elektronika 6/2012


Antireflection coating with plasmonic metal nanoparticles<br />

for photovoltaic applications<br />

Zbigniew Starowicz, Marek Lipiński<br />

Institute <strong>of</strong> Metallurgy and Materials Science <strong>of</strong> Polish Academy <strong>of</strong> Science, Cracow<br />

<strong>Photovoltaics</strong> is dynamically growing field <strong>of</strong> science and industry.<br />

Every year significant growth <strong>of</strong> installed power <strong>of</strong> photovoltaic<br />

systems is observed. Two major factors stand up against<br />

world wide popularity <strong>of</strong> PV: low efficiency and high production<br />

cost, which is mainly concerned with materials costs. Many technological<br />

and physical tricks have been implanted to <strong>the</strong> solar<br />

cells to improve <strong>the</strong>ir capabilities <strong>of</strong> conversion solar radiation<br />

into electricity. To deal with high materials costs photovoltaic<br />

thin film technology have been invented. One <strong>of</strong> <strong>the</strong> methods <strong>of</strong><br />

improving cells efficiency is reduction <strong>of</strong> reflected and non-absorbed<br />

photons. Conventionally antireflection coating and surface<br />

texturisation is used for that matter. Texturisation <strong>of</strong> semiconductor<br />

surface means creating a few micron-high geometric<br />

figures, shapes on <strong>the</strong> surface to enable multiple reflection <strong>of</strong><br />

light beams. For thin film technology implementation <strong>of</strong> surface<br />

texture is impossible due to size <strong>of</strong> figures exceeding total<br />

cell thickness. Here plasmonics as a new solution have been<br />

proposed.<br />

Plasmonics is a new branch <strong>of</strong> science which provides tools<br />

for confinement <strong>of</strong> light in nanoscale objects. Generally speaking<br />

special type <strong>of</strong> interaction <strong>of</strong> light with metallic objects is <strong>the</strong> origin<br />

<strong>of</strong> many phenomena. In <strong>the</strong> solar cells <strong>the</strong>y can provide improvement<br />

<strong>of</strong> carriers generation due to near field around <strong>the</strong> object or<br />

efficient scattering <strong>of</strong> light and extending <strong>the</strong> optical path length<br />

[1]. This paper refers only to second feature <strong>of</strong> plasmonic structures.<br />

These structures can be used in any kind <strong>of</strong> solar cells<br />

[2]. In <strong>the</strong> last few years many authors have reported <strong>the</strong>ir achievements.<br />

For example Ma<strong>the</strong>u in his work announced 2,8 and<br />

8,8% increase <strong>of</strong> efficiency after addition <strong>of</strong> 100 and 150 nm silver<br />

nanospheres [3].<br />

Physical explanation <strong>of</strong> this phenomenon is closely related to<br />

internal structure <strong>of</strong> metals. Immovable atomic cores sit in <strong>the</strong><br />

node <strong>of</strong> crystalline net and electron cloud is filling <strong>the</strong> space around.<br />

Electric field applied to small metallic object cause <strong>the</strong> electron<br />

move according to field direction, leaving positively charged<br />

cores behind, <strong>the</strong>n <strong>the</strong> object act as a dipole. When light wave<br />

falls on <strong>the</strong> particle smaller than wavelength, oscillating electric<br />

field cause <strong>the</strong> electron cloud is oscillating too (Fig. 1). The<br />

quantum <strong>of</strong> oscillation <strong>of</strong> different sign charges in metal is a quasi-particle<br />

known as “plasmon”. For wave <strong>of</strong> frequency close to<br />

metal plasmon frequency (for spherical shape equals square<br />

www.livenano.org/wp-content/uploads/2011/03/plasmon.jpeg<br />

Fig. 1. Electromagnetic wave passing by two metal nanospheres<br />

root <strong>of</strong> three times value <strong>of</strong> plasmon frequency) this oscillations<br />

take <strong>the</strong> form <strong>of</strong> resonance. For photovoltaic applications great<br />

role play aluminum and silver because <strong>of</strong> <strong>the</strong>ir high density <strong>of</strong><br />

valance electrons [6], also gold and copper as <strong>the</strong>ir resonance<br />

is placed in visible range.<br />

Oscillating electrons scatter <strong>the</strong> incident beam in <strong>the</strong> direction<br />

<strong>of</strong> semiconductor substrate preferable, as <strong>the</strong>re is much greater<br />

number <strong>of</strong> optical modes than in air. Placing particles on both<br />

sides <strong>of</strong> <strong>the</strong> cell this can provide light trapping effect as <strong>the</strong> is an<br />

angular scattering.<br />

Scattering abilities can be calculated from <strong>the</strong> following formula:<br />

(1)<br />

as well as absorption from:<br />

(2)<br />

Where alpha is polarization:<br />

(3)<br />

dependent on particle volume (size) and particle and surrounding<br />

medium dielectric functions. Scattering abilities change with frequency<br />

<strong>of</strong> incident light. In <strong>the</strong> Figure 2 it was shown how scattering<br />

cross-section normalized to <strong>the</strong> particles size varies with<br />

different wavelengths and size for silver spheres.<br />

This plot should be understood as for example: 20nm diameter<br />

sphere scatters light from 18 times larger area than its geometrical<br />

size at <strong>the</strong> resonance wavelength <strong>of</strong> 475 nm.<br />

Crucial feature <strong>of</strong> <strong>the</strong> particle is scattering abilities better than<br />

<strong>the</strong> absorption ones.<br />

Q sca<br />

= C sca<br />

/(C sca<br />

+ C abs<br />

) (4)<br />

On that basis we can sey that small particles absorb stronger than<br />

bigger. For 100nm sphere scattering is large in <strong>the</strong> broad part<br />

<strong>of</strong> spectrum and <strong>the</strong> value <strong>of</strong> Q sca<br />

exceeds 90%. Larger particles<br />

have also resonances <strong>of</strong> <strong>the</strong> higher mode than dipole – quadrupole,<br />

hexapole – recognized as picks moved towards shorter wavelengths.<br />

Second important parameter is effectiveness <strong>of</strong> scattering<br />

<strong>the</strong> light to substrate that equals:<br />

f = f substrate<br />

/f total<br />

(5)<br />

It is more likely that light would be send to medium <strong>of</strong> higher value<br />

<strong>of</strong> refractive index, this aspect was well covered by Catchepole<br />

and Polman. Position <strong>of</strong> <strong>the</strong> particle above semiconductor<br />

substrate is compromise between scattering cross-section which<br />

grow as apart increases and parameter f which drops down [4].<br />

Final optical properties <strong>of</strong> <strong>the</strong> plasmonic structure depends on<br />

many variables like: particle size, shape, position above substrate<br />

and distribution as well as <strong>the</strong> refractive indexes <strong>of</strong> surrounding<br />

media [5].<br />

Plasmonic structures composed <strong>of</strong> array <strong>of</strong> nanoparticles fixed<br />

to <strong>the</strong> substrate are obtained by thorough, time demanding<br />

and expensive methods. We chose to our experiments nanoparticles<br />

from colloidal solutions. In that way size <strong>of</strong> particles can<br />

be precisely fixed. Particle will be placed in <strong>the</strong> semiconductor<br />

substrate by <strong>the</strong> spin-coating method or deep coating method.<br />

Elektronika 6/2012 105


Even greater enhancement was observed for PV cell with<br />

nano-spheres made from pure silver. After two stages <strong>of</strong> deposition<br />

efficiency increased from 9,8% to 10,6% which means about<br />

7,6% enhancement <strong>of</strong> efficiency (Table 2). Next attempt <strong>of</strong> far<strong>the</strong>r<br />

growth <strong>of</strong> particles density on <strong>the</strong> surface resulted in drop <strong>of</strong> efficiency.<br />

Tabl. 2. Solar cells parameters obtained before and after each stage<br />

<strong>of</strong> 70 nm Ag sphere deposition<br />

Cell number<br />

I sc<br />

[mA]<br />

V oc<br />

[mV]<br />

FF<br />

[%]<br />

E ff<br />

[%]<br />

1-15-50 1040 558 74.7 9.8<br />

1-15-50-02 1093 560 74.4 10.3<br />

1-15-50-03 1097 560 75.8 10.6<br />

1-15-50-04 1075 558 74.7 10.1<br />

Fig. 2. Normalized scattering cross-section for spherical silver nanoparticles<br />

with diameter <strong>of</strong> 20 nm (blue), 70 nm (green), 100 nm (purple),<br />

200 nm (orange) in medium <strong>of</strong> refractive index n = 2<br />

This methods are relatively cheap and <strong>of</strong> wide throughput which<br />

is very important in industrial stage.<br />

Experiment<br />

We used mono crystalline silicon circular shape solar cell with an<br />

area equals to 44 centimeter squared. Nonoparticles source ware<br />

colloidal solutions <strong>of</strong> 70 nm spheres <strong>of</strong> Ag and Ag coated with Cu.<br />

Metals concentration in solution was 1080 ppm. Deposition technique<br />

was spin coating. Solution was injecting on wafer spinning<br />

with speed <strong>of</strong> 4000 rotates per minute. In <strong>the</strong> first stage 200 µl <strong>of</strong><br />

colloidal solution was injected and <strong>the</strong> same amount in <strong>the</strong> o<strong>the</strong>r<br />

stages. Measurements <strong>of</strong> cell parameters were taken before and<br />

after deposition <strong>of</strong> combined Ag-Cu nanoparticles and between<br />

stages <strong>of</strong> deposition in case <strong>of</strong> pure Ag spheres. Measurements<br />

were carried out on measuring position equipped with halogen<br />

lamp simulating solar spectrum.<br />

Results and discussion<br />

Theory in this case was confirmed by experimental procedure,<br />

and any time when cell efficiency increased it was connected mainly<br />

with increase <strong>of</strong> photocurrent as it was expected. For solar cell<br />

without texturisation and antireflection coating (it can be treated<br />

as equivalent <strong>of</strong> typical thin film cell surface) deposited plasmonic<br />

nanoparticles <strong>of</strong> Ag-Cu caused increase <strong>of</strong> efficiency. Results <strong>of</strong><br />

I-V measurements are shown in Table 1. In this case overall efficiency<br />

enhancement was about 4.38%.<br />

Tabl. 1. Solar cells parameters obtained before and after Ag-Cu nanoparticles<br />

deposition<br />

Cell number<br />

Isc<br />

[mA]<br />

V oc<br />

[mV]<br />

FF<br />

[%]<br />

E ff<br />

[%]<br />

1-15-42 978 556 71.6 8.8<br />

1-15-42<br />

Ag-Cu<br />

1028 557 70.9 9.2<br />

Where: I sc<br />

– short circuit current, V oc<br />

– open circuit voltage, FF – fill factor,<br />

E ff<br />

– conversion efficiency.<br />

This example <strong>of</strong> cell response point to conclusion that for any<br />

case exist maximum <strong>of</strong> particles concentration. Over this concentration<br />

particles begin reflect and absorb more light than is scattered<br />

to <strong>the</strong> substrate.<br />

For cell with antireflection coating efficiency enhancement was<br />

observed as well, but effect was smaller. Coated and texturised<br />

cell already possess enough reduction <strong>of</strong> reflection losses that<br />

our method did not work anymore.<br />

Summary<br />

The paper contains:<br />

● broad introduction to <strong>the</strong> topic <strong>of</strong> plasmonics for photovoltaic<br />

application,<br />

● <strong>the</strong> purpose <strong>of</strong> research was given as well as <strong>the</strong> description <strong>of</strong><br />

one <strong>of</strong> alternative methods <strong>of</strong> nanoparticle deposition,<br />

● Addition <strong>of</strong> plasmonic metal nanospheres resulted in photocurrent<br />

and efficiency enhancement,<br />

● Using <strong>of</strong> relatively smaller spheres <strong>of</strong> 70nm diameter and mixtures<br />

<strong>of</strong> two metals is novel approach. This opens up new are<br />

<strong>of</strong> research <strong>of</strong> <strong>the</strong> best metal mixture.<br />

● Far<strong>the</strong>r studies have to be done to explain explicitly o<strong>the</strong>r features<br />

like optimal size and apart between nanoparticles<br />

References<br />

[1] Atwater H., A. Polman Plasmonics for improved photovoltaic devices.<br />

Nature Materials doi: 10.1038/nmat2629<br />

[2] Pillain S., M.A.Green Plasmonics for photovoltaic applications<br />

S. Pillai n, M.A.Green Solar Energy Materials & Solar Cells 94<br />

(2010) 1481–1486.<br />

[3] Ma<strong>the</strong>u P., S.H.Lim, D. Derkacs, C.McPheeters, E.T. Yu, Metal<br />

and dielectric nanoparticles scattering for improved optical absorption<br />

in photovoltaic devices, Appl. Phys. Lett. 93 (2008)<br />

113108-1-113108-3.<br />

[4] Catchpole K., A. Polman, “Plasmonic Solar cell” Optics Express<br />

/Vol. 16, No. 26 / December 2008.<br />

[5] Catchpole K. R., A. Polman “Design principles for particle plasmon<br />

enhanced solar cells” Applied Physics Letters 93, 191113<br />

2008.<br />

[6] Akimov Yuriy A., Wee Shing Koh, “Design <strong>of</strong> Plasmonic Nanoparticles<br />

for Efficient Subwavelength Light Trapping in Thin-Film<br />

Solar Cells” Plasmonics (2011) 6:155–16 Doi: 10.1007/s11468-<br />

010-9181-4.<br />

106<br />

Elektronika 6/2012


Technological issues and optimization processes<br />

<strong>of</strong> junctions in a multijunction pv cell based<br />

on ingap/ingaas/ge materials<br />

Piotr Knyps, Institute <strong>of</strong> Electronics Materials Technology, Warsaw, Warsaw University <strong>of</strong> Technology, IMiO<br />

At present, photovoltaics is <strong>the</strong> most dynamically developing<br />

method for obtaining sources <strong>of</strong> renewable energy in <strong>the</strong> world. It<br />

is confirmed by <strong>the</strong> figures about <strong>the</strong> new installed power in 2011,<br />

which shows 27,7 GW p<br />

growth only in one year. In comparison to<br />

<strong>the</strong> year before, it is a 70% increase [1]. Such fast growth in photovoltaic<br />

industry is connected with intensive research on increasing<br />

<strong>the</strong> efficiency <strong>of</strong> photovoltaic conversion in solar cells. The<br />

higher efficiency will help to produce energy cheaper, and create<br />

new applications for photovoltaics.<br />

The most popular single-junction solar cell achieves efficiencies<br />

up to 25,0% [2], although in comparison to traditional coal-fired power<br />

plants, which have efficiency <strong>of</strong> over 40%, it is still far lower. That<br />

is <strong>the</strong> reason why research activities focus on developing multijunction<br />

solar cells technologies, which utilize different spectrum wavelengths<br />

<strong>of</strong> solar energy in a better way. Among many applications <strong>of</strong><br />

multijunction solar cells, terrestrial concentrators, which focus light<br />

even 500 times on <strong>the</strong> surface <strong>of</strong> a PV cell, systems to power space<br />

satellites and rovers belong to <strong>the</strong> most important ones.<br />

Work objectives and established results<br />

The main objective <strong>of</strong> <strong>the</strong> project „Advanced materials and technologies<br />

<strong>of</strong> <strong>the</strong>ir production” is <strong>the</strong> preparation <strong>of</strong> <strong>the</strong> epitaxial<br />

structure <strong>of</strong> triple junction solar cells based on III-V semiconductor<br />

compounds <strong>of</strong> . An additional task is to improve <strong>the</strong> construction<br />

<strong>of</strong> a solar cell and increase its efficiency.<br />

Epitaxial technology<br />

The structure <strong>of</strong> triple junction solar cells showed in Fig. 1 is divided<br />

into 3 p-n junction regions consisting <strong>of</strong> A III<br />

-B V<br />

compounds.<br />

These are Ge, InGaP, InGaAs compounds with <strong>the</strong> energy<br />

Fig. 1. Structure <strong>of</strong> triple junction solar cell Ge/InGaAs/InGaP<br />

bandgap <strong>of</strong> 0,66 eV, 1,4 eV, 1,86 eV. As a result, <strong>the</strong> solar spectrum<br />

is divided into tree different regions, each <strong>of</strong> which is absorbed<br />

by ano<strong>the</strong>r junction. Between active regions <strong>the</strong>re are<br />

layers <strong>of</strong> tunnel junctions, which are highly doped regions with<br />

an opposite direction <strong>of</strong> p ++ /n ++ polarization (AlGaAs/GaInP)<br />

between 3rd and 2nd junction and p ++ /n ++ polarization (GaAs/<br />

GaAs) between 2 and 1st junction. The tunnel junction acts as<br />

a low resistive contact between p-n junctions, creating carrier<br />

flow as a result <strong>of</strong> <strong>the</strong> tunnel effect. Tunnel junctions are very<br />

thin, up to 50 nm, and <strong>the</strong>y are optically transparent and do not<br />

absorb <strong>the</strong> solar spectrum.<br />

All epitaxial processes were performed in an AIXTRON AIX<br />

200/4 reactor. Phosphine (PH 3<br />

) was <strong>the</strong> source <strong>of</strong> V group elements<br />

, trimethylgallium (TMGa) and trimethylindium (TMIn) were<br />

<strong>the</strong> source <strong>of</strong> group III elements . “P” and “n” layers were doped<br />

with silicon and zinc respectively.<br />

First junction Ge<br />

The first junction is made on a p-type germanium substrate. The<br />

thickness <strong>of</strong> wafers used in reactors is 150 µm. The junction is<br />

created by diffusion <strong>of</strong> phosphorus from an InGaP layer doped<br />

with silicon. Phosphorus atoms are shallow donors and during<br />

<strong>the</strong> diffusion process in p-type materials <strong>the</strong>y create an n-type<br />

surface layer.<br />

Second junction InGaAs<br />

The middle junction made <strong>of</strong> InGaAs absorbs solar spectrum in<br />

<strong>the</strong> range 650…950 nm. In that junction a back surface field region,<br />

which reduces <strong>the</strong> diffusion <strong>of</strong> holes through <strong>the</strong> potential<br />

barrier, can be distinguished. That layer also reduces recombination<br />

on <strong>the</strong> interface between <strong>the</strong> tunnel junction and <strong>the</strong> second<br />

junction <strong>of</strong> <strong>the</strong> cell. A layer <strong>of</strong> a 3 µm thick base and a 100 nm thick<br />

emitter is doped up to 1*10 17 and 1*10 18 respectively.<br />

Third junction InGaP<br />

The top junction is composed <strong>of</strong> three InGaP elements with a<br />

band gap <strong>of</strong> 1,86 eV. The absorbed spectrum is in range between<br />

300 and 650 nm, and higher wavelengths are transmitted to deeper<br />

layers <strong>of</strong> <strong>the</strong> cell. The BSF layer is made <strong>of</strong> InAlP doped with<br />

zinc up to 2×10 18 . The active region <strong>of</strong> <strong>the</strong> p-n junction is 0,5 µm<br />

thick, and its top surface is passivated by an InAlP:Si layer, which<br />

also acts as a window layer.<br />

Postprocessing<br />

The obtained epitaxial structures were postprocessed, which included<br />

<strong>the</strong> manufacture <strong>of</strong> electrical contacts. Front electrodes<br />

were created by photolithography <strong>of</strong> <strong>the</strong> mask pattern on <strong>the</strong><br />

deposited layer <strong>of</strong> aluminum with <strong>the</strong> thickness <strong>of</strong> 300 nm. The<br />

patterns <strong>of</strong> front electrodes were projected in Fig. 2 to choose<br />

<strong>the</strong> optimal dimensions <strong>of</strong> <strong>the</strong> grid: <strong>the</strong> width <strong>of</strong> fingers and <strong>the</strong><br />

space between <strong>the</strong>m. The projected patterns have <strong>the</strong> width <strong>of</strong><br />

fingers in <strong>the</strong> range between 0.1 mm and 0.01 mm and densities<br />

between 20 and 100 lines per 1 cm. We also made structures<br />

with a radial pattern and “an inverted square” [3]. The best results<br />

for <strong>the</strong> finger shape pattern were obtained with metallization,<br />

in <strong>the</strong> case <strong>of</strong> which <strong>the</strong> width <strong>of</strong> fingers was 5 µm and<br />

space between <strong>the</strong>m reached 25 µm.<br />

Elektronika 6/2012 107


Fig. 4. Current-voltage curve <strong>of</strong> single junction InGaAs cell<br />

Fig. 2. Photolithographic mask for front electrode<br />

Results <strong>of</strong> electro-optical measurements<br />

All three separately made structures <strong>of</strong> p-n junctions were measured<br />

in terms <strong>of</strong> <strong>the</strong>ir electrical parameters and spectral efficiency.<br />

Current-voltage characteristics were measured in a solar simulator<br />

PhotoEmission ss300B at Warsaw University <strong>of</strong> Technology.<br />

Epitaxial structures were not cut out because that could create<br />

shunts on edges. In long term, <strong>the</strong> front electrode will be created<br />

on <strong>the</strong> whole surface <strong>of</strong> <strong>the</strong> wafer. Fig. 3 shows light current-voltage<br />

curves <strong>of</strong> <strong>the</strong> PV cell with <strong>the</strong> area <strong>of</strong> 1 cm 2 under Standard<br />

Test Conditions (irradiation 1000 W/m 2 , spectrum AM1.5, cell temperature<br />

25°C) for different pressures in <strong>the</strong> reactor during epitaxial<br />

growth. With pressure increasing up to 400 mbar, <strong>the</strong> surface<br />

quality was enhanced and <strong>the</strong> surface roughness decreased,<br />

which has influence on obtaining better electrical parameters.<br />

Open circuit voltage increased up to 230 mV and <strong>the</strong> efficiency <strong>of</strong><br />

a single Ge junction was 5,5%.<br />

The middle InGaAs junction on <strong>the</strong> GaAs substrate has open<br />

circuit voltage equal to 0.78 V and <strong>the</strong> efficiency up to 9.9%. GaAs<br />

substrates have good surface quality , and <strong>the</strong>re are no problems<br />

with epitaxial growth <strong>of</strong> layers with low roughness. Fig. 4 shows<br />

current-voltage and power characteristics <strong>of</strong> <strong>the</strong> cell. An evident<br />

influence <strong>of</strong> shunt resistance, <strong>the</strong> value <strong>of</strong> which is about 50 Ω,<br />

is observed. Such an increase would be possible after etching <strong>of</strong><br />

edges, which will be done for <strong>the</strong> purpose <strong>of</strong> future works.<br />

Fig. 5. External quantum efficiency InGaP and InGaAs cell<br />

The external quantum efficiency <strong>of</strong> InGaP and InGaAs is showed<br />

in Fig. 5. The InGaAs cell absorbs photons in <strong>the</strong> range between<br />

400 and 900 nm, although after stacking junctions one on<br />

<strong>the</strong> top <strong>of</strong> <strong>the</strong> o<strong>the</strong>r, spectrum in <strong>the</strong> range between 400 and 650<br />

nm will be absorbed by <strong>the</strong> top InGaP junction.<br />

For an efficient operation <strong>of</strong> <strong>the</strong> cell it is important to match<br />

current flows through all three junctions, so that absorption coefficients,<br />

junction thickness, energy bandgaps are matched. It will<br />

cause uniform current flow and will reduce recombination loses.<br />

Top junction made <strong>of</strong> InGaP, as an independent structure has<br />

<strong>the</strong> efficiency <strong>of</strong> 11.1% and open circuit voltage 0.92 V.<br />

Summary<br />

Epitaxial structures made from 3 different materials act as photovoltaic<br />

solar cells in different spectrum ranges. Their electrical<br />

parameters are still improved by optimization <strong>of</strong> technological processes.<br />

We also plan to interconnect <strong>the</strong> separate junction with<br />

<strong>the</strong> stacked triple junction cell by tunnel junctions.<br />

This work was financed within <strong>the</strong> project POIG.01.03.01-00-<br />

015/09 „Advanced materials and Technologies <strong>of</strong> <strong>the</strong>ir production.<br />

Development <strong>of</strong> technology <strong>of</strong> photovoltaics materials<br />

production.”<br />

References<br />

Fig. 3. Improvement <strong>of</strong> I-V curve caused by increase <strong>of</strong> pressure<br />

in reactor from 100 to 400 mbar<br />

[1] Market Report 2011, <strong>European</strong> Photovoltaic Industry Association,<br />

http://www.epia.org/publications/publications.html<br />

[2] Martin A. Green, Keith Emery, Yoshihiro Hishikawa and Wilhelm Warta,<br />

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICA-<br />

TIONS Prog. Photovolt: Res. Appl. 2011; 19:84–92 Published online in<br />

Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1088.<br />

[3] G.M.M.W. Bissels*, M.A.H. Asselbergs, E.J. Haverkamp, N.J.<br />

Smeenk and J.J. Schermer, A new circular contact grid pattern, designed<br />

for solar cells in a mechanical stack.<br />

108<br />

Elektronika 6/2012


GaAsN as a photovoltaic material – photoelectrical<br />

characterization<br />

Paulina Kamyczek 1) , Ewa Placzek-Popko 1) , Piotr Biegański 1) , Eunika Zielony 1) ,<br />

Beata Sciana 2) , Marek TłaczaŁa 2)<br />

1)<br />

Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology<br />

2)<br />

Faculty <strong>of</strong> Microsystem Electronics and Photonics, Wroclaw University <strong>of</strong> Technology<br />

GaAsN and InGaAsN materials have attracted considerable attention<br />

due to <strong>the</strong>ir unique physical properties and wide range <strong>of</strong><br />

<strong>the</strong>ir possible application in optoelectronics, especially in infrared<br />

laser diodes for 1.3 and 1.55 mm [1, 2] and high efficiency<br />

multi-junction (MJ) solar sells [3, 4], where <strong>the</strong>se low-band-gap<br />

materials can, in principle, be used to efficiently collect <strong>the</strong> lowphoton-energy<br />

portion <strong>of</strong> <strong>the</strong> solar spectrum [4].<br />

In this paper <strong>the</strong> results <strong>of</strong> studies on <strong>the</strong> layers <strong>of</strong> GaAs 1-x<br />

N x<br />

grown on (100)-oriented Si-doped n-type GaAs substrates by atmospheric<br />

pressure metal organic vapour phase epitaxy APMO-<br />

VPE are presented. In <strong>the</strong> first step <strong>the</strong> layers <strong>of</strong> GaAs 1-x<br />

N x<br />

were<br />

characterized with <strong>the</strong> use <strong>of</strong> optical methods, <strong>the</strong>n Schottky diodes<br />

were realized and <strong>the</strong> rectifying properties <strong>of</strong> <strong>the</strong> diodes were<br />

studied with electrical methods. The diodes exhibit light-energy<br />

conversion effect. Basic parameters <strong>of</strong> <strong>the</strong> solar cells ( short-circuit<br />

current I sc<br />

, open circuit-voltage, V oc<br />

, and fill factor, FF) were<br />

determined. Obtained results confirm that <strong>the</strong> diodes are promising<br />

as efficient solar cells.<br />

Samples<br />

The investigated structures were grown on (100)-oriented Sidoped<br />

n-type GaAs substrates by APMOVPE with AIX200 R&D<br />

AIXTRON horizontal reactor. Trimethylgallium (TMGa), tertiarybutylhydrazine<br />

(TBHy) and arsine (AsH3) were used as <strong>the</strong> growth<br />

precursors. High purity hydrogen with <strong>the</strong> total flow rate <strong>of</strong><br />

9.6 l/min was employed as a carrier gas. The hydrogen flow rate<br />

through <strong>the</strong> saturator was changed during runs with TBHy - VH2/<br />

TBHy = 1500...2500 ml/min. Stable parameters were as follows:<br />

<strong>the</strong> growth temperature was Tg = 566°C , <strong>the</strong> arsine flow rate<br />

VAsH3 = 50 ml/min (for GaAsN) and 300 ml/min (for GaAs), <strong>the</strong><br />

organic source temperatures: TTMGa = −10°C, TTBHy = 30°C.<br />

The thickness <strong>of</strong> Si-doped n-type GaAs substrates was set to<br />

350 μm. Subsequently GaAs buffer layer <strong>of</strong> <strong>the</strong> thickness <strong>of</strong> 450<br />

nm was grown. The layer <strong>of</strong> 200...300 nm thick GaAs 1−x<br />

N x<br />

was<br />

grown on top <strong>of</strong> <strong>the</strong> buffer layer. Gold Schottky contacts <strong>of</strong> 0.5<br />

mm 2 area were prepared by electrolitography technique on <strong>the</strong><br />

front side <strong>of</strong> <strong>the</strong> GaAs 1−x<br />

N x<br />

layer. An AuGe served as <strong>the</strong> ohmic<br />

contact to <strong>the</strong> n-type GaAs substrates (cf. Fig. 1).<br />

In this paper two kinds <strong>of</strong> samples labeled as N42N, N48N with<br />

various nitrogen content were investigated. Nitrogen content was<br />

determined from <strong>the</strong> spectral characteristics <strong>of</strong> transmittance (T) ,<br />

reflectance (R) and photocurrent (PC).<br />

Experimental<br />

The optical properties were analysed using transmittance, reflectance<br />

and photocurrent spectral room-temperature (RT) measurements.<br />

The transmitted and reflected light was dispersed by<br />

BENTHAM spectrometer and detected by <strong>the</strong> Ge and Si detectors<br />

with a lock-in amplifier. The same system was used to perform<br />

photocurrent measurements. The latter were realized on Schottky<br />

Au–GaAs 1−x<br />

N x<br />

/GaAs diodes. The solar cell figures <strong>of</strong> merit (Isc,<br />

Voc and FF) were determined from current-voltage characteristics<br />

carried out in darkness and after illumination with halogen lamp.<br />

The dark and illuminated current-voltage (I-V) characteristics<br />

were measured by using Keithley 2601 current source meter.<br />

From transmittance or reflectance spectra <strong>the</strong> excitonic band<br />

gap can be obtained. The first derivative <strong>of</strong> <strong>the</strong> absorption coefficient<br />

has a maximum in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong> band gap [5] whereas<br />

reflectance spectrum exhibits a “dip” at <strong>the</strong> same wavelength. The<br />

value <strong>of</strong> <strong>the</strong> band gap (GaAs 1-x<br />

N x<br />

layers) can be also determined<br />

from <strong>the</strong> photocurrent spectrum more specifically from <strong>the</strong> midpoint<br />

<strong>of</strong> <strong>the</strong> PC drop. Once <strong>the</strong> energy <strong>of</strong> <strong>the</strong> excitonic band gap<br />

is determined, <strong>the</strong> GaAs 1-x<br />

N x<br />

nitrogen content x can be extracted<br />

from <strong>the</strong> equation:<br />

2/3<br />

∆ E = 3.91x<br />

(1)<br />

where ΔE g<br />

= E g<br />

(GaAs) – E g<br />

(GaAs 1x<br />

N x<br />

) [5, 6] (assuming RT E g<br />

(GaAs) = 1.42 eV).<br />

From I-V characteristics basic parameters characterizing solar<br />

cell can determined: open circuit voltage V OC<br />

and short circuit current<br />

I SC<br />

and a point <strong>of</strong> maximum power P max<br />

. Using this parameters<br />

a fill factor FF can be calculated:<br />

(2)<br />

Results and discussion<br />

g<br />

Optical properties <strong>of</strong> <strong>the</strong> studied heterostructures were verified by<br />

<strong>the</strong> room temperature (RT) transmittance (T), reflectance (R) and<br />

photocurrent (PC) spectral measurements. A comparison <strong>of</strong> <strong>the</strong><br />

reflectance spectra with <strong>the</strong> derivative <strong>of</strong> transmission for N42N,<br />

N48 are given in Fig. 2a and 2b. The arrows correspond to <strong>the</strong> excitonic<br />

transition <strong>of</strong> GaAs 1-x<br />

N x<br />

and <strong>the</strong> dotted lines indicate GaAs<br />

band gap shifted with respect to <strong>the</strong> expected value <strong>of</strong> 1.42 eV<br />

due to Urbach tail.<br />

Figure 3 shows PC spectra <strong>of</strong> <strong>the</strong> structures with different nitrogen<br />

content in <strong>the</strong> GaAs 1−x<br />

N x<br />

epilayers (N42N and N48N). The<br />

photocurrent generated in GaAs 1−x<br />

N x<br />

can be seen above <strong>the</strong> absorption<br />

edge <strong>of</strong> <strong>the</strong> GaAs. In Table 1 <strong>the</strong> nitrogen content x obtained<br />

for <strong>the</strong> studied samples (N42N, N48N) from transmittance<br />

and reflectance as well as from PC measurements are collected.<br />

Tabl. 1. Nitrogen content obtained for <strong>the</strong> studied samples<br />

Sample x (%) T x(%)PC<br />

N42 1.08 1.2<br />

N48 1.88 1.9<br />

Fig. 1. A diagram <strong>of</strong> a GaAs 1−x<br />

N x<br />

/GaAs heterostructure<br />

The dark and illuminated I-V curves <strong>of</strong> <strong>the</strong> two structures are<br />

shown in Fig. 4, while <strong>the</strong> basic solar cell parameters <strong>of</strong> <strong>the</strong> diodes<br />

obtained from <strong>the</strong> curves are reported in Table 2.<br />

Elektronika 6/2012 109


a)<br />

a)<br />

b)<br />

b)<br />

Fig. 2. Reflectance and derivative <strong>of</strong> <strong>the</strong> transmittance spectra for<br />

a) N42N, b) N48N samples. The arrows point to <strong>the</strong> excitonic band gaps<br />

<strong>of</strong> <strong>the</strong> GaAs 1-x<br />

N x<br />

layers, and <strong>the</strong> dotted lines indicate GaAs band gap<br />

a)<br />

b)<br />

Fig. 3. PC spectra <strong>of</strong> GaAs 1−x<br />

N x<br />

110<br />

Fig. 4. Dark and illuminated I-V characteristics <strong>of</strong> <strong>the</strong> sample a) N42N,<br />

b) N48N<br />

Tabl. 2. Characteristics <strong>of</strong> <strong>the</strong> studied samples<br />

Sample J SC<br />

(mA/cm 2 ) V OC<br />

(V) P max<br />

(W) FF<br />

N42N 0.23 0.42 -1.71x10 -7 0.35<br />

N48N 0.23 0.38 -1.41x10 -7 0.33<br />

Our results are comparable with GaAsN homo-junction solar<br />

cells fabricated by CBE [7]. The short circuit current for <strong>the</strong> latter<br />

is bigger (8.99 mA/cm 2 ) but open circuit voltage and fill factor are<br />

0.48 V and 0.43, respectively [7].<br />

Conclusions<br />

In summary, we present in this study <strong>the</strong> results <strong>of</strong> studies on <strong>the</strong><br />

layers <strong>of</strong> GaAs 1-x<br />

N x<br />

grown on n-type GaAs substrates by atmospheric<br />

pressure metal organic vapour phase epitaxy (APMOVPE).<br />

Using transmittance, reflectance, photocurrent measurements and<br />

empirical expression for E g<br />

a new nitrogen content for <strong>the</strong> studies<br />

samples was obtained. Using dark and illuminated I-V characteristics<br />

<strong>the</strong> main parameters <strong>of</strong> <strong>the</strong> Schottky contacts ( short-circuit<br />

current I sc<br />

, open circuit-voltage, V oc<br />

, and fill factor, FF) were determined.<br />

Obtained contacts are promising for solar cell application.<br />

References<br />

[1] Kondow M. et al.: Japan. J. Appl. Phys. 35, 1273 (1996).<br />

[2] Nakahara K. et al.: IEEE Photonics Technol. Lett. 10, 487 (1998).<br />

[3] Friedman D.J. et al.: J. Cryst. Growth 195, 409 (1998).<br />

[4] Kurtz S.R. et al.: 26th IEEE PVSEC (1997) 875.<br />

[5] Taliercio T., R. Intartaglia, B. Gil, P. Lefebvre, T. Bretagnon, U. Tisch,<br />

E. Finkman, J. Salzman, M.-A Pinault, M. Laugt and E. Tournie: Phys.<br />

Rev. B 69, (2004)073303.<br />

[6] Zhang Y., A. Mascarenhas, H.P. Xin, and C.W. Tu: Phys. Rev. B 63,<br />

(2001) 161303.<br />

[7] Suzuki H., K. Nishimura, T. Hashiguchi, K. Saito, B. Balasubramanian,<br />

S. Yamamoto, M. Inagaki, Y. Ohshita, N. Kojima and M. Yamaguchi:<br />

Fabrication <strong>of</strong> GaAsN homo-junction solar cells by chemical<br />

beam epitaxy, Photovoltaic Specialists Conference, 2008. PVSC ‘08.<br />

33rd IEEE.<br />

Elektronika 6/2012


AP-MOVPE technology <strong>of</strong> AIIIBV-N heterostructures<br />

for photovoltaic applications<br />

Wojciech Dawidowski 1) , Beata Ściana 1) , Damian Pucicki 1) , Damian Radziewicz 1) ,<br />

Jarosław Serafińczuk 1) , Magdalena Latkowska 2) , Marek Tłaczała 1)<br />

1)<br />

Faculty <strong>of</strong> Microsystem Electronics and Photonics, Wroclaw University <strong>of</strong> Technology<br />

2)<br />

Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology<br />

Properties <strong>of</strong> dilute nitrides material system like large discontinuity<br />

<strong>of</strong> a conduction band (due to high electronegativity <strong>of</strong><br />

nitrogen) [1], large band gap bowing coefficient [2], increased<br />

electron effective mass [3], large scattering rate (connected with<br />

a small radius <strong>of</strong> nitrogen atom in comparison with <strong>the</strong> o<strong>the</strong>rs V<br />

group atoms) [4] provide a way to novel applications: telecommunications<br />

lasers [1], heterojunction bipolar transistors [5] and<br />

very efficient solar cells [6]. InGaAsN is a very promising, lattice<br />

matched to GaAs and In 0.5<br />

Ga 0.5<br />

P material with band gap about<br />

1 eV. The control <strong>of</strong> indium and nitrogen amounts in <strong>the</strong> InGaAsN<br />

quaternary alloy <strong>of</strong>fers <strong>the</strong> ability to tailor <strong>the</strong> value <strong>of</strong> band gap<br />

and positions <strong>of</strong> valence and conduction band edges. Main advantage<br />

<strong>of</strong> dilute nitrides is a large reduction <strong>of</strong> band gap caused<br />

by nitrogen introduction (about ~150 meV/% <strong>of</strong> N). The technology<br />

<strong>of</strong> GaAs is better developed and still much cheaper than<br />

indium phosphide and allow to fabricate much better Bragg reflectors<br />

(based on <strong>the</strong> high refractive index contrast GaAs/AlAs<br />

heterostructure) for laser applications. Moreover, this material<br />

system guarantees a good temperature performance <strong>of</strong> light<br />

emitters and a high internal quantum efficiency <strong>of</strong> solar cells.<br />

On <strong>the</strong> o<strong>the</strong>r hand, growth <strong>of</strong> InGaAsN quaternary alloy causes<br />

some technological problems. The most serious technological<br />

problem results from a large miscibility gap between GaAs and<br />

GaN binary alloys, which is connected with both a high electronegativity<br />

and a small radius <strong>of</strong> N atom. It complicates <strong>the</strong> growth<br />

<strong>of</strong> dilute nitrides– <strong>the</strong> incorporation <strong>of</strong> a small amount <strong>of</strong> nitrogen<br />

strongly deteriorates <strong>the</strong> optical properties <strong>of</strong> epilayers [3].<br />

Material growth with higher nitrogen concentration would cause<br />

phase segregation (to GaN and GaAs) which could be a nonradiative<br />

recombination centres [7]. Introduction <strong>of</strong> nitrogen into<br />

GaAs or InGaAs crystalline lattice requires a low growth temperature<br />

(in comparison with GaN deposition), for MOVPE method<br />

it is below 600 ˚C. This technological limitation causes non-homogeneous<br />

distribution <strong>of</strong> <strong>the</strong> nitrogen atoms in epilayers and<br />

also favours to forming <strong>of</strong> various defects, which are <strong>the</strong> centres<br />

<strong>of</strong> non-radiative recombination and reduce a photoluminescence<br />

efficiency [8]. Split interstitials N-N and N-As, vacancies V Ga<br />

and<br />

V As<br />

, antisites As Ga<br />

are commonly occurred points defects in dilute<br />

nitrides. Ano<strong>the</strong>r problem connected with <strong>the</strong> epitaxial growth is<br />

high concentration <strong>of</strong> impurities such as: carbon, oxygen and<br />

hydrogen. In MOVPE technique occurs unintentional doping by<br />

carbon, which is a component <strong>of</strong> all organometallic precursors.<br />

The main sources <strong>of</strong> hydrogen contamination are metaloorganic<br />

compounds and a carrier gas.<br />

Experiment<br />

The series <strong>of</strong> undoped InGaAsN/GaAs heterostructures were<br />

grown by AP-MOVPE (atmospheric pressure metal organic vapour<br />

phase epitaxy) technique with AIX 200 R&D Aixtron reactor<br />

on n-type and semi insulating (100) oriented GaAs substrates.<br />

The grown multi quantum well (MQW) structures consisted <strong>of</strong><br />

500 nm buffer, triple dilute nitrides quantum wells (about 15 nm)<br />

sandwiched between GaAs barriers (about 30 nm) and capped<br />

by 40…50 nm thick GaAs layer (~75 nm with last barrier). The<br />

typical MQW epitaxial structure is shown in Fig. 1.<br />

Trimethylindium (TMIn), trimethylgallium (TMGa), tertiarybutylhydrazine<br />

(TBHy) and 10% mixture <strong>of</strong> arsine (AsH 3<br />

) in<br />

H 2<br />

were used as growth precursors. As a carrier gas was applied<br />

a high purity hydrogen. The growth temperature T g<br />

was<br />

changed in <strong>the</strong> range <strong>of</strong> 566 to 585˚C in order to optimize <strong>the</strong><br />

properties <strong>of</strong> investigated epitaxial structures. For dilute nitrides<br />

<strong>the</strong> arsine and hydrogen (passed through <strong>the</strong> bubbler with<br />

nitrogen source – TBHy) flows were stable and equal 50 ml/min<br />

and 1500 ml/min, respectively. Low flow <strong>of</strong> AsH 3<br />

ensured better<br />

incorporation <strong>of</strong> nitrogen into layer. The arsine flow during <strong>the</strong><br />

epitaxy <strong>of</strong> GaAs barrier was equal 300 ml/min. Structural and<br />

optical properties <strong>of</strong> <strong>the</strong> obtained MQW structures were examined<br />

by high resolution X-ray diffraction (HRXRD), photoluminescence<br />

(PL) and contactless electro-reflectance spectroscopy<br />

(CER). Optical measurements were carried out at room<br />

temperature (RT).<br />

Fig. 1. Scheme <strong>of</strong> <strong>the</strong> typical MQW epitaxial structure<br />

Results<br />

The structural analysis based on XRXRD measurements <strong>of</strong> diffraction<br />

curves <strong>of</strong> <strong>the</strong> symmetric (004) reflection in ω/2θ geometry.<br />

Obtained curves for NI 43 and NI 46 samples are shown in<br />

Fig. 2. Keenness <strong>of</strong> peaks with highest intensity (<strong>the</strong>se peaks<br />

represent GaAs substrate) suggests sharp interfaces between<br />

layers. The occurrence <strong>of</strong> secondary oscillations, also known<br />

as <strong>the</strong> Pendellösung oscillations allow to determine thickness<br />

<strong>of</strong> layers and <strong>the</strong>ir composition by comparison <strong>of</strong> <strong>the</strong> measured<br />

curves with simulations results. Complete set <strong>of</strong> parameters determined<br />

in that way is listed in Table 1. The indium content in<br />

<strong>the</strong> InGaAsN quantum wells is varied from 12.1 to 17% while<br />

<strong>the</strong> nitrogen amount does not exceed0.6%. Based on <strong>the</strong> data<br />

determined from XRXRD <strong>the</strong> temperature dependence <strong>of</strong> <strong>the</strong><br />

efficiency <strong>of</strong> nitrogen incorporation into InGaAsN quantum wells<br />

was estimated. This relationship is shown in Fig. 3. The content<br />

<strong>of</strong> nitrogen decreases with increasing a growth temperature, as<br />

it was expected.<br />

From <strong>the</strong> measured PL and CER spectra <strong>the</strong> ground state<br />

energy (E GS<br />

) in <strong>the</strong> investigated quantum wells was determined.<br />

The obtained values from <strong>the</strong> both mentioned methods are comparable<br />

and listed in Table 2.<br />

Elektronika 6/2012 111


Fig. 2. The symmetric (004) HRXRD scans for <strong>the</strong> MQW structures<br />

Tabl. 1. Parameters <strong>of</strong> investigated MQW structures grown on n-type doped<br />

substrates determined from HRXRD measurements<br />

Sample<br />

Thickness [nm] QW composition [%]<br />

Quantum<br />

well<br />

Barrier Indium Nitrogen<br />

NI 43n 14.5 29 14.5 0.38<br />

NI 45n 14.7 31 16.6 0.5<br />

NI 46n 14.8 30.2 17 0.55<br />

NI 58n 6 29.5 12.1 0.41<br />

Fig. 3. The nitrogen content in In y<br />

Ga 1-y<br />

As 1-x<br />

N x<br />

quantum wells grown on<br />

SI GaAs substrate as a function <strong>of</strong> <strong>the</strong> growth temperature T g<br />

--+++<br />

Tabl. 2. The ground state energy values <strong>of</strong> MQW structures determined from<br />

PL and CER spectra<br />

Sample<br />

PL<br />

E GS<br />

[eV]<br />

CER<br />

NI 43n 14.5 1.201<br />

NI 45n 1.183 1.184<br />

NI 46n 1.177 1.176<br />

NI 58n 1.218 1.212<br />

Fig. 5. PL spectra <strong>of</strong> InGaAsN/GaAs MQW structures grown at different<br />

temperatures<br />

Fig. 4. CER spectra <strong>of</strong> InGaAsN/GaAs MQW structures grown at different<br />

temperatures<br />

112<br />

Measured CER and PL spectra <strong>of</strong> <strong>the</strong> MQW structures grown<br />

at different temperatures are shown in Fig. 4 and 5, respectively.<br />

Optical transitions marked in Fig. 4, testify a good optical quality<br />

<strong>of</strong> <strong>the</strong> MQW region.<br />

The maximum intensity <strong>of</strong> <strong>the</strong> photoluminescence is observed<br />

for NI 46 structure, which was grown in highest temperature <strong>of</strong><br />

585°C. Good material quality <strong>of</strong> this structure was also confirmed<br />

by HRXRD measurements.<br />

Conclusions<br />

This paper presents <strong>the</strong> influence <strong>of</strong> <strong>the</strong> AP-MOVPE epitaxial process<br />

growth temperature on <strong>the</strong> optical and structural properties<br />

<strong>of</strong> heterostructures containing InGaAsN quantum wells. The best<br />

Elektronika 6/2012


optical and structural features were observed for MQW structure<br />

grown in highest temperature. This structure (sample NI 46) was<br />

applied in <strong>the</strong> test p-i-n solar cell construction. Measured dc I-V<br />

characteristics exhibit electrical response under <strong>the</strong> optical excitation<br />

by a discrete laser diode with λ = 980 nm [9], what confirms<br />

<strong>the</strong> usability <strong>of</strong> InGaAsN semiconductor compounds in solar cell<br />

applications.<br />

The future work will be focused on determination <strong>of</strong> <strong>the</strong> influence<br />

<strong>of</strong> <strong>the</strong> nitrogen concentration in <strong>the</strong> reactor gas phase on<br />

InGaAsN alloys properties.<br />

This work was co-financed by Polish National Science Centre<br />

under <strong>the</strong> grant no. N N515 607539, by <strong>the</strong> <strong>European</strong> Union within<br />

<strong>European</strong> Regional Development Fund, through grant Innovative<br />

Economy (POIG.01.01.02-00-008/08-04), by Wroclaw University<br />

<strong>of</strong> Technology statutory grant S10019 and Slovak-Polish<br />

International Cooperation Program no. SK-PL-0017-09.<br />

References<br />

[1] Kondow M.et al.: GaInNAs: A Novel Material for Long-Wavelength<br />

Semiconductor Lasers. IEEE J. Select. Topic Quantum Electron. 3<br />

(719), 1997.<br />

[2] Wei S., A. Zunger: Giant and Composition-Dependent Optical Bowing<br />

Coefficient in GaAsN Alloys. Phys. Rev. Lett. 76, 664–667, 1996.<br />

[3] Buyanova I., W. Chen [eds]: Physics and Applications <strong>of</strong> Dilute Nitrides.<br />

Garland Science, 2004.<br />

[4] Erol A. [ed]: Dilute III-V Nitride Semiconductors and Material Systems:<br />

Physics and Technology. Springer, 2008.<br />

[5] Chang P. et al.: InGaP/InGaAsN/GaAs NpN double-heterojunction<br />

bipolar transistor. Appl. Phys. Lett. 76, 2262, 2000.<br />

[6] Kurtz S. et al.: InGaAsN solar cells with 1.0 eV band gap, lattice<br />

matched to GaAs. Appl. Phys. Lett. 74, 729, 1999.<br />

[7] Gambin V., PhD <strong>the</strong>sis: Long wavelength luminescence from GaIn-<br />

NAsSb on GaAs. Stanford University, 2002.<br />

[8] Buyanova I., W. Chen, C. Tu, “Defects in dilute nitrides”, J. Phys.:<br />

Condens. Matter 16 S3027, 2004.<br />

[9] Dawidowski W.et al.: Application <strong>of</strong> InGaAsN in construction <strong>of</strong> p-i-n solar<br />

cell. Elektronika – konstrukcje, technologie, zastosowania, in press.<br />

Rare earth activated YAM materials as solar spectrum<br />

converters for photovoltaics<br />

BARTOSZ FetliŃski, ZUZANNA Boruc, MICHAŁ Malinowski<br />

Warsaw University <strong>of</strong> Technology, Faculty <strong>of</strong> Electronics and Information Technology, Division <strong>of</strong> Optoelectronics<br />

One <strong>of</strong> <strong>the</strong> most important factors limiting efficiency and energy<br />

yield <strong>of</strong> solar cells is mismatch between incident solar spectrum<br />

and <strong>the</strong>ir spectral response curve. It was calculated that under<br />

a standard AM1.5G spectrum crystalline silicon PV cell with<br />

Eg =1.12 eV losses ~55% [1] <strong>of</strong> incoming energy <strong>of</strong> solar radiation<br />

due to this effect. There are numerous concepts <strong>of</strong> limiting <strong>the</strong> losses<br />

caused by this effect, notably <strong>the</strong> multi-junction solar cells, hot<br />

carriers collecting contacts, impurity photovoltaics, <strong>the</strong>rmophotovoltaics,<br />

multi-exciton generation and photon conversion. These<br />

concepts vary substantially in maturity and perspectives for widespread<br />

application in foreseeable future. The most mature <strong>of</strong><br />

<strong>the</strong> above mentioned concepts are multijunction solar cells widely<br />

used in space and terrestrial concentrators application. However,<br />

current mismatch between junctions in case <strong>of</strong> spectrum different<br />

than one cell was designed for, complicated manufacturing resulting<br />

in very high unit price, need for sun tracking mechanism<br />

for concentrator’s optics and usage limited to regions with high<br />

insolation with dominant direct beam significantly limit more widespread<br />

use <strong>of</strong> this technology. O<strong>the</strong>r concepts, require serious<br />

modification <strong>of</strong> cells structure, are hampered by numerous problems<br />

and trade-<strong>of</strong>fs (for example impurity PV cells while enabling<br />

absorption <strong>of</strong> IR light also provide new paths <strong>of</strong> parasitic non-radiative<br />

recombination). The photon conversion concept, especially<br />

downshifting and down-conversion, has distinct advantage <strong>of</strong><br />

possibility <strong>of</strong> enhancing photovoltaic system’s efficiency without<br />

interference with already existing, optimized and commercially<br />

available photovoltaic cells.<br />

The photon conversion concept permits three ways <strong>of</strong> reshaping<br />

<strong>the</strong> incident solar spectrum: up-conversion <strong>of</strong> low energy<br />

IR photons and down-conversion or down-shifting <strong>of</strong> high energy<br />

photons. Basic downside <strong>of</strong> up-conversion is that it is highly<br />

non-linear process with probability <strong>of</strong> two photon up-conversion<br />

being squarely dependent on incident light intensity and for three<br />

photon processes being dependent to <strong>the</strong> third power. This limits<br />

possible application <strong>of</strong> up-converting layers to concentrators,<br />

such as luminescent solar concentrators providing concentration<br />

level <strong>of</strong> up to 10 suns [2] or means need to accept poor efficiency<br />

<strong>of</strong> up-converting layers under natural illumination. Also, since<br />

such a layer would exhibit absorption in range <strong>of</strong> higher energy<br />

photons it should be placed under <strong>the</strong> PV cell. This solution has<br />

advantage <strong>of</strong> up-converting layer not interfering with <strong>the</strong> incident<br />

sunlight before it reaches surface <strong>of</strong> <strong>the</strong> PV cell, which means that<br />

any current gain resulting from light emitted by this layer is real<br />

gain <strong>of</strong> <strong>the</strong> device. On <strong>the</strong> o<strong>the</strong>r hand positioning <strong>the</strong> up-converting<br />

layer in <strong>the</strong> rear <strong>of</strong> PV cell may require significant redesign<br />

<strong>of</strong> <strong>the</strong> cell structure (i.e. for obtaining maximum efficiency bifacial<br />

cell should be used) which diminishes advantage <strong>of</strong> nonintrusive<br />

integration <strong>of</strong> up-converting layers into existing solar cells – a basic<br />

advantage <strong>of</strong> <strong>the</strong> photon conversion concept. The 1.12 eV<br />

bandgap <strong>of</strong> classic silicon PV cells is relatively narrow and fur<strong>the</strong>r<br />

limits applicability <strong>of</strong> up-converting layers in systems without light<br />

concentrator, as only at about 1.25 eV sub-bandgap losses are<br />

greater than <strong>the</strong> <strong>the</strong>rmalisation losses under standard AM1.5G<br />

illumination [3].<br />

Concepts utilizing opposite part <strong>of</strong> <strong>the</strong> solar spectrum are<br />

down-conversion and downshifting. The difference between those<br />

is that down-conversion results in emission <strong>of</strong> two photons in long<br />

λ range for every photon <strong>of</strong> E > 2E g<br />

, while downshifting assumes<br />

emission <strong>of</strong> single long λ photon for every short λ photon. Downconversion<br />

and quantum cutting notions are <strong>of</strong>ten used interchangeably<br />

but in fact down-conversion requires two different ions to<br />

take part in photon conversion process while in photon cutting <strong>the</strong><br />

two low energy photons are emitted by <strong>the</strong> same single ion that<br />

absorbed <strong>the</strong> high energy photon.<br />

There are three main ways <strong>of</strong> achieving downshifting: by utilization<br />

<strong>of</strong> rare earths, organic dyes or quantum dots. Among those,<br />

rare earths <strong>of</strong>fer capability <strong>of</strong> tuning absorption and emission<br />

in wide range. Shortcoming <strong>of</strong> rare earths’ based converters is<br />

weak absorption coefficient and narrow absorption bands. In Re 3+<br />

based converters it is also possible to achieve down-conversion,<br />

mainly in Re 3+ ions pairs with Yb 3+ .<br />

Experiment<br />

For above mentioned reasons we choose down-conversion and<br />

down-shifting <strong>of</strong> <strong>the</strong> solar spectrum in Re 3+ activated host as <strong>the</strong><br />

main path <strong>of</strong> obtaining efficient photon converting layer for PV<br />

cells. In this work we present preliminary data <strong>of</strong> Tb 3+ ,Yb 3+ activated<br />

YAM (Y 4<br />

Al 2<br />

O 9<br />

– Yttrium Aluminum Monoclinic). YAM exhibits<br />

maximum phonon energy <strong>of</strong> <strong>the</strong> order <strong>of</strong> 812 cm -1 , slightly more<br />

than most <strong>of</strong> <strong>the</strong> oxide hosts. However, since <strong>the</strong> 5 D 4<br />

and 7 F 0<br />

levels<br />

are separated by about 5660 cm -1 gap, non-radiative processes<br />

do not significantly depopulate <strong>the</strong> 5 D 4<br />

level.<br />

Elektronika 6/2012 113


Fig. 1. Mechanisms <strong>of</strong> downshifting and down-conversion in Tb 3+ , Yb 3+ and crossrelaxation mechanisms within<br />

Tb 3+ iob and Tb 3+ , Yb 3+ ions pair<br />

Y 4<br />

Al 2<br />

O 9<br />

is one <strong>of</strong> <strong>the</strong> four crystalline phases in <strong>the</strong> Y 2<br />

O 3<br />

–Al 2<br />

O 3<br />

system with <strong>the</strong> formation <strong>of</strong> monoclinic, maintaining Y 2<br />

O 3<br />

:Al 2<br />

O 3<br />

ratio <strong>of</strong> 2:1. O<strong>the</strong>r phases are cubic garnet with ratio <strong>of</strong> 3:5<br />

(Y 3<br />

Al 5<br />

O 12<br />

, abbreviated YAG), orthorhombic perovskite with ratio<br />

<strong>of</strong> 1:1 (YAlO 3<br />

, abbreviated YAP) and a metastable hexagonal structure<br />

phase YAH, with <strong>the</strong> same stoichiometry as YAM, observed<br />

during <strong>the</strong> syn<strong>the</strong>sis by s<strong>of</strong>t chemistry methods. Crystals grown<br />

by standard Czochralski method crack during cooling because <strong>of</strong><br />

phase transition in YAM. In our investigations <strong>the</strong> modified solgel<br />

method was used to obtain YAM:Tb 3+ +Yb 3+ samples, in form<br />

<strong>of</strong> nanopowders. The samples were obtained in <strong>the</strong> Institute <strong>of</strong><br />

Electronic Materials Technology in Warsaw. The measurements<br />

were performed using Photon Technology International spectrophotometer.<br />

The properties <strong>of</strong> Tb 3+ activated YAM hosts were subject <strong>of</strong><br />

few investigations. In case <strong>of</strong> YAM: Tb 3+ +Yb 3+ system <strong>the</strong> main<br />

routes for down-conversion and downshifting are presented in<br />

<strong>the</strong> Fig. 1. <strong>the</strong> 5 D 4<br />

Tb level plays critical role in down-conversion<br />

process. The energy <strong>of</strong> <strong>the</strong> Tb 3+ 5 D 4<br />

→ 7 F 6<br />

transition is twice larger<br />

than <strong>the</strong> energy <strong>of</strong> <strong>the</strong> Yb 3+ 2 F 5/2<br />

→ 2 F 7/2<br />

transition. This suggests<br />

a possibility <strong>of</strong> efficient energy transfer between 5 D 4<br />

level <strong>of</strong> single<br />

Tb 3+ ion and 2 F 5/2<br />

levels <strong>of</strong> two Yb 3+ ions. The emission spectrum<br />

<strong>of</strong> Yb 3+ ion possesses two distinct peaks at 976 nm and 1026<br />

nm, which suit transitions from Stark sub-levels <strong>of</strong> <strong>the</strong> 2 F 5/2<br />

level.<br />

This emission may be excited by direct excitation <strong>of</strong> Yb 3+ ion at<br />

908 nm or via down-conversion <strong>of</strong> excitation <strong>of</strong> Tb 3+ ion at about<br />

274 nm. Such wavelength corresponds to spin-allowed transition<br />

<strong>of</strong> electron from 4f 8 to 4f 7 5d 1 configuration [4]. This down-conversion<br />

mechanism is <strong>the</strong> most desirable due to emission from Yb 3+<br />

coinciding with <strong>the</strong> range <strong>of</strong> high spectral response <strong>of</strong> <strong>the</strong> standard<br />

crystalline silicon cells.<br />

The down-conversion is not <strong>the</strong> only mechanism occurring in<br />

Tb 3+ +Yb 3+ doped system. Ano<strong>the</strong>r way <strong>of</strong> relaxation <strong>of</strong> exited 4f 7 5d 1<br />

configuration is by emission o light at wavelengths corresponding<br />

to transitions from 5 D 3<br />

→ 7 F J<br />

and longer wavelength emissions from<br />

5<br />

D 4<br />

→ 7 F J<br />

(J=6,5,4,3,2). The strongest <strong>of</strong> those emissions occurs in<br />

case <strong>of</strong> 5 D 4<br />

→ 7 F 5<br />

transition, which gives UV irradiated terbium materials<br />

a distinct green glow. Spectrum <strong>of</strong> above mentioned emissions<br />

are presented in Fig. 3, along with excitation spectrum <strong>of</strong> <strong>the</strong><br />

5<br />

D 4<br />

→ 7 F 5<br />

emission. Shape <strong>of</strong> <strong>the</strong> spectrum, again with dominant<br />

feature at around 274 nm confirms that both Yb 3+ NIR and Tb 3+ VIS<br />

emissions primarily result from <strong>the</strong> same UV excitation and are<br />

thus competitive processes. From point <strong>of</strong> view <strong>of</strong> obtaining maximally<br />

efficient down-conversion process emissions from higher<br />

energy 5 D 3<br />

level to 7 F J<br />

multiplets are particularly unfavorable, since<br />

<strong>the</strong>y provide pathway <strong>of</strong> depopulation <strong>of</strong> this level before excited<br />

electrons non-radiatively decay to <strong>the</strong> lower 5 D 4<br />

level, where<br />

<strong>the</strong>y have chance to take part in <strong>the</strong> cooperative energy transfer<br />

Fig. 2. Excitation spectrum <strong>of</strong> Yb 3+ IR emission at 1026 nm (black) and<br />

emission spectrum from excitation <strong>of</strong> Tb 3+ ion at 274 nm indicates<br />

energy transfer from Tb 3+ to Yb 3+ ions<br />

114<br />

Fig. 3. Emission spectrum resulting from excitation <strong>of</strong> Tb 3+ ion at 274<br />

nm and excitation spectrum <strong>of</strong> green emission at 544 nm<br />

Elektronika 6/2012


chanism, as it provides alternative explanation for Yb 3+ emission<br />

under Tb 3+ UV excitation. Due to required multiphonon relaxation<br />

intensity <strong>of</strong> this pathway may be reduced in lower phonon energy<br />

hosts, which will be subject <strong>of</strong> fur<strong>the</strong>r investigations.<br />

Conclusions<br />

Emission <strong>of</strong> NIR light from Yb 3+ ions after excitation <strong>of</strong> <strong>the</strong> Tb 3+<br />

was observed indicating existence <strong>of</strong> transfer mechanisms. Relative<br />

intensities <strong>of</strong> <strong>the</strong>se down-conversion and down-shifting<br />

via single Yb 3+ ion mechanisms is yet to be established. Efficient<br />

down-shifting resulting in visible light emission was observed. Relatively<br />

narrow absorption spectrum with peak at 274 nm limits<br />

potential for application <strong>of</strong> this system in PV systems, due to little<br />

sunlight energy contained within that part <strong>of</strong> <strong>the</strong> solar spectrum.<br />

Fig. 4. Influence <strong>of</strong> terbium ions quenching effects on shape <strong>of</strong> <strong>the</strong><br />

Tb 3+ VIS emission spectrum<br />

with Yb 3+ ions. There is however mechanism <strong>of</strong> cross relaxations<br />

within Tb 3+ ion that at high terbium concentration quenches emissions<br />

from <strong>the</strong> 5 D 3<br />

level and excites 5 D 4<br />

. This mechanism reveals<br />

itself at Tb 3+ concentrations larger than 5% at., as presented<br />

in Fig. 4 (results in YAM: Tb 3+ not containing Yb 3+ ).<br />

The third possible mechanism <strong>of</strong> depopulation <strong>of</strong> <strong>the</strong> 5 D 4<br />

level,<br />

also shown at Fig. 1, is Tb 3+ Yb 3+ non-resonant cross relaxation<br />

resulting in excitation <strong>of</strong> single Yb 3+ ion and multi-phonon decay<br />

to <strong>the</strong> Tb 3+ 7 F 0<br />

level [5]. Existence <strong>of</strong> this pathway complicates calculation<br />

<strong>of</strong> energy transfer efficiency <strong>of</strong> <strong>the</strong> down-conversion me-<br />

References<br />

[1] Macdonald D. H.: Recombination and Trapping in Multicrystalline Silicon<br />

Solar Cells. PhD. Thesis, The Australian National University, May<br />

2001.<br />

[2] Giebink N. C., G. P. Wiederrecht, M. R. Wasielewski: Resonanceshifting<br />

to circumvent reabsorption loss in luminescent solar concentrators.<br />

Nature Photonics 5, 694–701(2011).<br />

[3] Shalav A., B.S. Richards, M.A. Green: Luminescent layers for enhanced<br />

silicon solar cell performance: Up-conversion. Solar Energy<br />

Materials & Solar Cells 91 (2007) 829–842.<br />

[4] Li Li, Wei Xiantao, Chen Yonghu, Guo Changxin, Yin Min, “Energy transfer<br />

in Tb3+,Yb3+ codoped Lu 2<br />

O 3<br />

near-infrared down-conversion nanophosphors”,<br />

Journal Of Rare Earths, Vol. 30, No. 3, Mar. 2012, P. 197.<br />

[5] Terra I.A.A. et. al., “Down-conversion process in Tb 3+ –Yb 3+ co-doped<br />

Calibo glasses”, Journal <strong>of</strong> Luminescence 132 (2012) 1678–1682.<br />

Photocatalytic degradation <strong>of</strong> <strong>the</strong> organic compounds<br />

enhanced by chemical oxidants<br />

Justyna Dziedzic, Paweł Nowak, Piotr Warszyński, Jerzy Haber<br />

Institute <strong>of</strong> Catalysis and Surface Chemistry Polish Academy <strong>of</strong> Sciences, Krakow<br />

Water for drinking and domestic purposes must not contain harmful<br />

substances. It should be transparent, colorless, odorless, have<br />

a pleasant and refreshing taste and cannot contain pathogenic<br />

bacteria [1]. One <strong>of</strong> <strong>the</strong> basic problems <strong>of</strong> water treatment is <strong>the</strong> removal<br />

<strong>of</strong> organic species, especially humic substances (HS) – compounds,<br />

which have not as yet been properly chemically defined.<br />

HS represent a major fraction <strong>of</strong> natural organic matter (NOM) in<br />

ground and surface waters. Their presence causes growth <strong>of</strong> microorganisms<br />

resulting in undesirable odor and change <strong>of</strong> color<br />

and turbidity <strong>of</strong> water [2]. NOM occurrence in water, especially that<br />

part which cannot be removed by coagulation, determines demand<br />

for chlorine in chlorination process. However, toxicological studies<br />

indicate that after chlorination carcinogenic byproducts, such as<br />

trihalomethanes, can be found. Therefore, <strong>the</strong> efficient removal <strong>of</strong><br />

NOM leads to reduction <strong>of</strong> <strong>the</strong> required amount <strong>of</strong> disinfectant,<br />

decreases <strong>the</strong> risk <strong>of</strong> formation trihalomethanes and also prevents<br />

<strong>the</strong> formation <strong>of</strong> bi<strong>of</strong>ilm [3]. For <strong>the</strong>se reasons <strong>the</strong>re exists a strong<br />

demand for new methods for NOM removal. As photocatalysis, is<br />

a promising method for removing organic compounds from water,<br />

in recent years, TiO 2<br />

based photocatalysis <strong>of</strong> humic acids (HAs)<br />

has been extensively investigated [4–7].<br />

In this work, <strong>the</strong> photocatalytic removal <strong>of</strong> humic acid (HA) under<br />

artificial sun light (ASL) and UV irradiation was examined by<br />

monitoring changes in <strong>the</strong> UV absorbance at 254 nm (UV 254<br />

). That<br />

absorbance is <strong>the</strong> widely accepted measure for determination <strong>of</strong><br />

<strong>the</strong> degradation rate <strong>of</strong> humic acid and is used as <strong>the</strong> surrogate<br />

parameter for <strong>the</strong> total organic carbon (TOC) – usually applied to<br />

determine degree <strong>of</strong> HA photodegradation [8]. As <strong>the</strong> alternative<br />

method, <strong>the</strong> measurements <strong>of</strong> <strong>the</strong> chemical oxygen demand<br />

(COD) were also performed. We considered <strong>the</strong> effectiveness <strong>of</strong><br />

TiO 2<br />

photocatalytic oxidation for HS removal from water with <strong>the</strong><br />

use <strong>of</strong> titanium dioxide powder (Degussa P – 25) with <strong>the</strong> addition<br />

<strong>of</strong> oxidants – sodium persulphate. and hydrogen peroxide. The<br />

main novelty was observed synergistic effect occurring when TiO 2<br />

and Na 2<br />

S 2<br />

O 8<br />

were used simultaneously, leading to significant<br />

increase <strong>of</strong> efficiency <strong>of</strong> HA removal.<br />

Experiment<br />

Materials<br />

Degussa P – 25 Aeroxide TiO 2<br />

powder (surface area – 50 m 2 g -1 )<br />

was kindly supplied by Evonic – Degussa and used as a photocatalyst<br />

without any modification. Humic acid sodium salt (technical<br />

grade) was supplied by Sigma – Aldrich. HA working solution <strong>of</strong><br />

40 mg l -1 for photocatalytic degradation experiments was always<br />

freshly prepared by dilution <strong>of</strong> <strong>the</strong> stock solution (500 mg l −1 ) with<br />

water. More details concerning preparation <strong>of</strong> HA stock solution<br />

were given in our previous work [9]. The oxidants – sodium persulphate<br />

and hydrogen peroxide were provided by Sigma – Aldrich.<br />

Photodegradation experiments<br />

The photodegradation experiments were performed in two quartz<br />

flat bottomed batch reactors <strong>of</strong> <strong>the</strong> volume 40 ml as shown in<br />

Fig. 1. In one <strong>of</strong> <strong>the</strong> reactors halogen lamp (150 W, Philips) was<br />

used as <strong>the</strong> artificial sunlight (ASL) source, in <strong>the</strong> o<strong>the</strong>r one, dedicated<br />

for ultraviolet (UV) region, <strong>the</strong> high pressure xenon arc lamp<br />

(250 W, Optel) was applied. Both lamps were used without any<br />

cut-<strong>of</strong>f filters, except <strong>the</strong> layer <strong>of</strong> cooling water flowing below <strong>the</strong><br />

bottom <strong>of</strong> <strong>the</strong> cell. The cell illumination was monitored by <strong>the</strong> radiometer<br />

Radiometer RD 0.2/2/100 (Optel). The irradiation intensity<br />

was 68.8 mW cm -2 for ASL and 48.8 mW cm -2 for UV source. The<br />

photocatalyst suspension was stirred during <strong>the</strong> experiments with<br />

a mechanical stirrer at a constant rate <strong>of</strong> 300 rpm. The tempera-<br />

Elektronika 6/2012 115


Fig. 2. Changes <strong>of</strong> absorbance at 254 nm <strong>of</strong> HA solution under UV<br />

and ASL irradiation at <strong>the</strong> presence <strong>of</strong> TiO 2<br />

. Concentration <strong>of</strong> photocatalyst:<br />

100 mg l -1 , concentration <strong>of</strong> HA: 40 mg l -1<br />

Fig. 1. The scheme <strong>of</strong> <strong>the</strong> quartz flat bottomed photoreactor<br />

ture was stabilized at 25 ±0.5°C. Toge<strong>the</strong>r with <strong>the</strong> sample used<br />

in a photocatalytic experiment an identical reference sample was<br />

always prepared and kept in dark during <strong>the</strong> same period <strong>of</strong> time.<br />

After <strong>the</strong> experiment three portions <strong>of</strong> both suspension and reference<br />

samples were taken, subjected to 25 min <strong>of</strong> centrifugation<br />

at 15000 rpm to separate TiO 2<br />

and <strong>the</strong>n analyzed for <strong>the</strong> concentration<br />

<strong>of</strong> humic acid. In that way <strong>the</strong> influence <strong>of</strong> HA adsorption<br />

on <strong>the</strong> TiO 2<br />

surface on <strong>the</strong> uptake <strong>of</strong> <strong>the</strong> contaminant from <strong>the</strong><br />

solution was eliminated. The absorption <strong>of</strong> <strong>the</strong> supernatant at <strong>the</strong><br />

wavelength <strong>of</strong> 254 nm was measured using Specord 40 (Analytic<br />

Jena) single beam spectrophotometer. COD values were determined<br />

according to Standard Metod PN-85/C-04578.02.<br />

The photooxidation experiment included a sets <strong>of</strong> tests with<br />

different objectives. First two sets <strong>of</strong> experiments was carried in<br />

order to determine <strong>the</strong> degree <strong>of</strong> HA degradation under UV and<br />

ASL irradiation in <strong>the</strong> absence and in presence <strong>of</strong> photocatalyst<br />

(initial HA concentration in that series <strong>of</strong> experiments was always<br />

40 mg l -1 ). The details <strong>of</strong> <strong>the</strong>se experiments were given in our<br />

previous work [9]. Then <strong>the</strong> measurements <strong>of</strong> <strong>the</strong> efficiency <strong>of</strong> <strong>the</strong><br />

photocatalytic process for simultaneous application <strong>of</strong> chemical<br />

oxidants (Na 2<br />

S 2<br />

O 8<br />

, H 2<br />

O 2<br />

) and titanium dioxide were performed.<br />

Results<br />

Decomposition <strong>of</strong> HA during UV/ASL irradiation and in UV<br />

(ASL)/TiO 2<br />

system<br />

The effect <strong>of</strong> ASL or UV illumination on humic acid degradation<br />

was described in detail in our previous work [9]. We observed no<br />

degradation under visible light and a very low degradation rate<br />

under UV as after 9 hr irradiation with <strong>the</strong> UV lamp only 5.3% decrease<br />

<strong>of</strong> <strong>the</strong> HA concentration (as measured by <strong>the</strong> UV absorbance<br />

at 254 nm (A 254<br />

) was observed. Since <strong>the</strong> ultraviolet light alone<br />

wasn’t enough to decompose humic acid, <strong>the</strong> effect <strong>of</strong> simultaneous<br />

action <strong>of</strong> irradiation and application <strong>of</strong> photocatalyst – TiO 2<br />

was determined. In <strong>the</strong> experiment <strong>the</strong> suspension consisting <strong>of</strong><br />

40 mg l -1 <strong>of</strong> HA sodium salt and 100 mg l -1 <strong>of</strong> TiO 2<br />

was exposed<br />

to <strong>the</strong> ASL and UV radiation. The results are shown in Fig. 2. and<br />

compared with <strong>the</strong> blank experiment.<br />

After 20 min <strong>of</strong> UV irradiation 15% decrease <strong>of</strong> A 254<br />

was observed,<br />

whereas one hour UV irradiation <strong>of</strong> HA in TiO 2<br />

suspension<br />

resulted in nearly 50% drop <strong>of</strong> HA concentration (Fig. 2) [9]. The<br />

correlation between <strong>the</strong> results <strong>of</strong> absorbance, A[254 nm], and<br />

COD determination during 5 – hours <strong>of</strong> UV irradiation process is<br />

illustrated in Fig. 3.<br />

As it can be seen, <strong>the</strong> normalized COD and A[254 nm] are<br />

well correlated, which substantiate <strong>the</strong> selected spectroscopic<br />

detection method. However, for low concentrations <strong>of</strong> humic acid<br />

COD was still measurable, whereas decrease <strong>of</strong> absorbance was<br />

almost 100%. As it was explained in our previous work [9], HA is<br />

116<br />

Fig. 3. Changes <strong>of</strong> normalized absorbance at 254 nm and COD values<br />

<strong>of</strong> humic acid solution under UV irradiation<br />

composed <strong>of</strong> at least two components, one can adsorb easily at<br />

<strong>the</strong> TiO 2<br />

surface, <strong>the</strong> o<strong>the</strong>r one does not adsorb at all or adsorbs<br />

to a very limited extent. Moreover, <strong>the</strong> photodegradation reaction<br />

<strong>of</strong> such complicated compounds as HS does not probably occur<br />

in one stage, so intermediate species formed during degradation<br />

may not absorb UV light in <strong>the</strong> range near 254 nm. Therefore,<br />

it seems that non-zero COD value indicate exactly on <strong>the</strong>se parts<br />

<strong>of</strong> humic substances, which do not degrade in <strong>the</strong> first step and<br />

consequently <strong>the</strong>ir decay is not detected by UV spectrometry.<br />

Decomposition <strong>of</strong> HA by simultaneous application <strong>of</strong> TiO 2<br />

and sodium persulphate<br />

First <strong>the</strong> suspensions consisting <strong>of</strong> HA sodium salt (40 mg l -1 )<br />

and different dosages <strong>of</strong> sodium persulphate (1×10 -3 mol l -1 ,<br />

5×10 -4 mol l -1 , 2×10 -4 mol l -1 , 1×10 -4 mol l -1 , 5×10 -5 mol l -1 ) were UV<br />

and ASL irradiated for 60 min. Then same procedure was repeated<br />

for <strong>the</strong> suspension containing additionally 100 mg l -1 <strong>of</strong> TiO 2<br />

.<br />

The results <strong>of</strong> <strong>the</strong>se experiments are showed in Fig. 4.<br />

Fig. 4. The effectiveness <strong>of</strong> <strong>the</strong> oxidation <strong>of</strong> HA by simultaneous<br />

action <strong>of</strong> UV or ASL radiation, TiO 2<br />

and sodium persulphate. Irradiation<br />

time: 60 min, concentration <strong>of</strong> HA: 40 mg l -1 , concentration <strong>of</strong><br />

TiO 2<br />

: 100 mg l -1 ,data for experiment without Na 2<br />

S 2<br />

O 8<br />

– 40 mg l -1 HA +<br />

100 mg l -1 TiO 2<br />

in dark<br />

Elektronika 6/2012


The experimental results showed in Fig. 4 indicate that in <strong>the</strong><br />

presence <strong>of</strong> UV light <strong>the</strong> degradation rate <strong>of</strong> HA increases with<br />

increasing Na 2<br />

S 2<br />

O 8<br />

concentration. Without photocatalyst it reaches<br />

23.6% in concentration <strong>of</strong> 1×10 -3 mol l -1 <strong>of</strong> sodium persulphate.<br />

Much higher efficiency <strong>of</strong> HA decomposition were obtained<br />

when TiO 2<br />

and Na 2<br />

S 2<br />

O 8<br />

were used simultaneously: nearly 90%<br />

conversion was attained after 1 h <strong>of</strong> UV irradiation. It is also worth<br />

to note that after <strong>the</strong> visible light illumination c.a. 30% decrease <strong>of</strong><br />

<strong>the</strong> A[254 nm] signal is observed. Humic acid does not decompose<br />

under ASL alone. Table 1 provides <strong>the</strong> collected results <strong>of</strong> measurements<br />

<strong>of</strong> <strong>the</strong> decomposition <strong>of</strong> HA in systems: TiO 2<br />

/UV, Na-<br />

S O /UV and TiO /Na S O /UV. These data clearly indicate on <strong>the</strong><br />

2 2 8 2 2 2 8<br />

existence <strong>of</strong> synergy between <strong>the</strong> UV light-activated TiO 2<br />

photocatalysis<br />

and sodium persulphate oxidation <strong>of</strong> humic substances.<br />

Tabl. 1. The results indicating on <strong>the</strong> synergistic effect <strong>of</strong> TiO 2<br />

and sodium<br />

persulphate in photocatalytic degradation <strong>of</strong> HA<br />

That synergy can be observed in both spectroscopic and COD<br />

measurements, although again <strong>the</strong> COD results indicate on <strong>the</strong><br />

uncompleted decomposition <strong>of</strong> HA in <strong>the</strong> photocatalytic process.<br />

Literature reports [10, 11] confirm that sodium persulphate is <strong>the</strong><br />

suitable oxidant, which exhibits synergistic effect with photocatalytic<br />

oxidation on titania. The reactions <strong>of</strong> persulphate ions (S 2<br />

O 8<br />

2-<br />

),<br />

which are created before dissociation <strong>of</strong> persulphate salts in water<br />

and have a strong oxidation potential (E o = 2.01 eV) with various<br />

organic and inorganic compounds have been extensively studied<br />

[10–12]. During UV irradiation very strong oxidizing species, sulphate<br />

radical anions – SO 4<br />

•−<br />

are produced (<strong>the</strong> oxidation potential<br />

E ◦ = 2.6 eV). They may react with photogenerated electrons<br />

and with water molecules producing additionally hydroxyl radicals.<br />

SO 4<br />

•−<br />

reacts with many organic compounds as an oxidant more<br />

effectively than OH • . The reason would be that sulphate radical<br />

anion is more selective to oxidation while OH • would react quickly<br />

by hydrogen abstraction or addition [11]. Taking into account<br />

<strong>the</strong> presence in solution both sulphate and hydroxyl radicals, <strong>the</strong><br />

efficiency improvement <strong>of</strong> organic contaminants degradation process,<br />

which was observed in our investigations, can be expected.<br />

Photocatalytic degradation <strong>of</strong> HA with H 2<br />

O 2<br />

/UV and H 2<br />

O 2<br />

/TiO 2<br />

/UV<br />

The suspension consisting <strong>of</strong> 40 mg l -1 <strong>of</strong> HA sodium salt and hydrogen<br />

peroxide (1×10 -3 mol l -1 ) was exposed to UV and ASL illumination<br />

for 60 minutes.. In <strong>the</strong> same experimental conditions <strong>the</strong><br />

HA solution <strong>of</strong> <strong>the</strong> concentration 40 mg l -1 with <strong>the</strong> addition <strong>of</strong> TiO 2<br />

(100 mg l -1 ) and hydrogen peroxide (1×10 -3 mol l -1 ) was irradiated<br />

by UV and ASL for <strong>the</strong> 1h and 2h. Results <strong>of</strong> <strong>the</strong>se experiments are<br />

collected in Table 2.<br />

Tabl. 2. Results <strong>of</strong> photocatalytic degradation <strong>of</strong> HA in <strong>the</strong> presence<br />

<strong>of</strong> H 2<br />

O 2<br />

in solution (A 254<br />

measurements)<br />

They show that hydrogen peroxide can be used as oxidant supporting<br />

degradation <strong>of</strong> HA in <strong>the</strong> photocatalytic process However,<br />

it is less effective than sodium persulphate. At <strong>the</strong> same oxidant<br />

concentration <strong>of</strong> 1×10 -3 mol l -1 , after 1 h <strong>of</strong> UV irradiation 60.7%<br />

decrease <strong>of</strong> absorbance was obtained for hydrogen peroxide in<br />

comparison with 90% for Na 2<br />

S 2<br />

O 8<br />

. The same conclusions concern<br />

ASL irradiation. After 1h exposition <strong>the</strong> observed decrease <strong>of</strong><br />

<strong>the</strong> A 254<br />

signal is two times lower than for potassium persulphate.<br />

The COD measurements also indicated on <strong>the</strong> synergistic effect<br />

<strong>of</strong> hydrogen peroxide in <strong>the</strong> photocatalytic degradation <strong>of</strong> humic<br />

substances. The 48% decrease <strong>of</strong> HA concentration (v.s. 63.2%<br />

in case <strong>of</strong> Na 2<br />

S 2<br />

O 8<br />

) was observed in TiO 2<br />

/H 2<br />

O 2<br />

/UV system after<br />

1h UV irradiation.<br />

Summary<br />

We demonstrated that more than 50% <strong>of</strong> humic acid may be removed<br />

from <strong>the</strong> solution simply by its adsorption on <strong>the</strong> titania<br />

surface if <strong>the</strong> concentration <strong>of</strong> TiO 2<br />

in suspension is high enough.<br />

100% removal <strong>of</strong> HA can be achieved even at relatively low dosage<br />

<strong>of</strong> TiO 2<br />

(100 mg l -1 ) if UV illumination <strong>of</strong> <strong>the</strong> suspension is<br />

applied for sufficiently long time (at least 3 hours). Addition <strong>of</strong> oxidant<br />

to <strong>the</strong> solution containing titania suspension and dissolved<br />

humic acid greatly enhances <strong>the</strong> activity <strong>of</strong> TiO 2<br />

in <strong>the</strong> process <strong>of</strong><br />

decomposition <strong>of</strong> organic substances. We showed that more than<br />

90% conversion <strong>of</strong> HA can be obtained when sodium persulphate<br />

was applied as chemical oxidant already after one hour. Therefore,<br />

<strong>the</strong> synergistic effect was observed when TiO 2<br />

and Na 2<br />

S 2<br />

O 8<br />

were used simultaneously for <strong>the</strong> removal <strong>of</strong> humic substances.<br />

The photodecomposition efficiency <strong>of</strong> in <strong>the</strong> UV/TiO 2<br />

/Na 2<br />

S 2<br />

O 8<br />

system<br />

was greater than <strong>of</strong> one with H 2<br />

O 2<br />

used as oxidant. Moreover,<br />

contrary to hydrogen peroxide, sodium persulphate is ra<strong>the</strong>r<br />

stable in room temperature. Therefore, use <strong>of</strong> Na 2<br />

S 2<br />

O 8<br />

as oxidant<br />

supporting photocatalytic removal <strong>of</strong> humic substances from potable<br />

water should be preferred.<br />

References<br />

[1] Decree <strong>of</strong> <strong>the</strong> Ministry <strong>of</strong> Health <strong>of</strong> 29 March on <strong>the</strong> quality <strong>of</strong> water<br />

intended for human consumption (Dz.U.07.61.417).<br />

[2] McDonald S., Bishop A.G., Prenzler P.D., Robards K.: Analytical<br />

chemistry <strong>of</strong> freshwater humic substances, Analytica Chimica Acta<br />

527 (2004) 105–124.<br />

[3] Chow C. W. K., Fitzgerald F., Holmes M.: The impact <strong>of</strong> natural organic<br />

matter on disinfection demand – a tool to improve disinfection<br />

control, AWA Enviro, 2004, Sydney.<br />

[4] Bekbolet M., Ozkosemen G.: A preliminary investigation on <strong>the</strong> photocatalytic<br />

degradation <strong>of</strong> a model humic acid, Water Science Technology<br />

33 (6), (1996) 189–194.<br />

[5] Bekbolet M., Suphandag A. S., Uyguner C. S.: An investigation <strong>of</strong> <strong>the</strong><br />

photocatalytic efficiencies <strong>of</strong> TiO 2<br />

powders on <strong>the</strong> decolourisation <strong>of</strong><br />

humic acids, Journal <strong>of</strong> Photochemistry and Photobiology A: Chemistry<br />

148 (2002) 121–128.<br />

[6] Uyguner C. S., Bekbolet M.: A comparative study on <strong>the</strong> photocatalytic<br />

degradation <strong>of</strong> humic substances <strong>of</strong> various origins, Desalination<br />

176 (2005) 167–176.<br />

[7] Palmer F.L., Eggins B.R., Coleman H.M.: The effect <strong>of</strong> operational<br />

parameters on <strong>the</strong> photocatalytic degradation <strong>of</strong> humic acid, Journal<br />

<strong>of</strong> Photochemistry and Photobiology A: Chemistry 148 (2002)<br />

137–143.<br />

[8] Sobana N., Selvam K., Swaminathan M.: Optimization <strong>of</strong> photocatalytic<br />

degradation conditions <strong>of</strong> Direct Red 23 using nano-Ag doped<br />

TiO 2<br />

, Separation and Purification Technology 62 (2008) 648–653.<br />

[9] Dziedzic J., Wodka D., Nowak P., Warszyński P., Simon Ch., Kumariki<br />

I.: Photocatalytic degradation <strong>of</strong> <strong>the</strong> humic species as a method<br />

<strong>of</strong> <strong>the</strong>ir removal from water – comparison <strong>of</strong> UV and artificial sunlight<br />

irradiation, Physicochemical Problems <strong>of</strong> Mineral Processing 45<br />

(2010), 15–28.<br />

[10] Furman O.S., Teel A.L., Watts R.J.: Mechanism <strong>of</strong> base activation<br />

<strong>of</strong> persulfate, Environmental Science Technology 44 (2010),<br />

6423–6428.<br />

[11] Huie R.E., Clifton C.L., Kafafi SA: Rate constants for hydrogen<br />

abstraction reactions <strong>of</strong> <strong>the</strong> sulfate radical SO4 −• .Experimental<br />

and <strong>the</strong>oretical results for cyclic e<strong>the</strong>rs, J. Phys. Chem. 95 (1991)<br />

9336–9340.<br />

[12] Berlin, A.A.: Kinetics <strong>of</strong> radical-chain decomposition <strong>of</strong> persulfate in<br />

aqueous solutions <strong>of</strong> organic compounds, Kinetics and Catalysis 27<br />

(1986), 34–39.<br />

Elektronika 6/2012 117


Organic photovoltaics – chosen aspects<br />

Justyna Szostak, Ryszard Signerski, Kamila Żelechowska, Jan Godlewski<br />

Gdańsk University <strong>of</strong> Technology, Department <strong>of</strong> Physics <strong>of</strong> Electronic Phenomena, Faculty <strong>of</strong> Applied Physics and Ma<strong>the</strong>matics<br />

Preparation <strong>of</strong> inorganic solar cells usually requires very costly<br />

and complicated high temperature vacuum deposition techniques.<br />

Fur<strong>the</strong>rmore, materials used for fabrication <strong>of</strong> <strong>the</strong>se cells are<br />

expensive and <strong>of</strong>ten dangerous for <strong>the</strong> environment, not to mention<br />

<strong>the</strong> hazardous and toxic substances used during manufacturing<br />

process and its by-products [1]. Hence, organic solar cells<br />

(OSCs), that can be fabricated from relatively cheap materials,<br />

using simple solution coating techniques, i.e. ink-jet printing or<br />

spin and dip coating, which do not require high temperatures nor<br />

<strong>the</strong> usage <strong>of</strong> hazardous substances, seem to be a promising alternative<br />

for <strong>the</strong> inorganic photovoltaics. Moreover, <strong>the</strong> number<br />

<strong>of</strong> organic materials is much greater than <strong>the</strong> number <strong>of</strong> inorganic<br />

ones, so <strong>the</strong> potential <strong>of</strong> applications <strong>of</strong> organic materials in<br />

photovoltaics is greater as well. However, relatively low efficiency<br />

and fast degradation <strong>of</strong> organic solar cells [2, 3] are serious drawbacks<br />

that hinder commercialization <strong>of</strong> organic photovoltaic diodes.<br />

Designing organic solar cells with higher energy conversion<br />

efficiency and stability is impossible without extensive knowledge<br />

on all physical and chemical processes taking place inside organic<br />

photovoltaic devices in <strong>the</strong> dark and under illumination. Even<br />

though <strong>the</strong>re are many experimental works showing <strong>the</strong> performance<br />

<strong>of</strong> OSCs, <strong>the</strong>oretical model for <strong>the</strong> photovoltaic effect taking<br />

place in <strong>the</strong>se devices has not been developed yet. Significant<br />

differences in structure <strong>of</strong> organic and inorganic materials<br />

result in different types <strong>of</strong> interactions between molecules. Thus,<br />

even though <strong>the</strong> mechanisms <strong>of</strong> photovoltaic effect in inorganic<br />

systems are well known and described in literature, processes<br />

that affect photovoltaic phenomenon in organic systems still need<br />

fur<strong>the</strong>r understanding. Thus, research we run in our laboratory is<br />

focused on analysis <strong>of</strong> <strong>the</strong> above mentioned processes.<br />

Photovoltaic phenomenon in organic systems<br />

In organic semiconductors, absorption <strong>of</strong> light leads to creation <strong>of</strong><br />

columbically bound electron-hole pairs – excitons [4]. Low dielectric<br />

constants <strong>of</strong> organic materials result in high exciton binding energies<br />

(reaching 1.0 eV), so electron and hole cannot escape each<br />

o<strong>the</strong>r and <strong>the</strong>y remain bound. Such pairs may diffuse to dissociation<br />

sites (e.g. interfaces <strong>of</strong> two different materials) where charge<br />

transfer (CT) states are created and subsequently dissociation<br />

<strong>of</strong> excitons into free charge carriers may occur. To collect <strong>the</strong>se<br />

charge carriers a built in electric field, that will drive <strong>the</strong>m towards<br />

respective electrodes, is necessary. The performance <strong>of</strong> organic<br />

Fig. 1. Scheme <strong>of</strong> charge carrier photogeneration at <strong>the</strong> junction<br />

<strong>of</strong> two organic materials. E F<br />

– Fermi level, E g<br />

– energy gap; HOMOhighest<br />

occupied molecular orbital, LUMO – Lowest Unoccupied Molecular<br />

Orbital; I in<br />

– incident light intensity; 1 – excitation <strong>of</strong> material<br />

II, 2 – exciton diffusion towards <strong>the</strong> junction <strong>of</strong> two organic materials,<br />

3 – exciton dissociation into free charge carriers<br />

118<br />

photovoltaic cells is limited by <strong>the</strong> efficiency <strong>of</strong> many processes<br />

that constitute <strong>the</strong> photovoltaic phenomenon. Among o<strong>the</strong>rs, limitations<br />

are inefficient exciton dissociation and poor charge carrier<br />

transport resulting from high rates <strong>of</strong> exciton recombination (short<br />

exciton lifetimes) and low charge carrier mobilities. Also potential<br />

barriers created in <strong>the</strong> near-electrode regions stand a serious problem<br />

that limits <strong>the</strong> efficiency <strong>of</strong> charge carrier collection. Probability<br />

<strong>of</strong> exciton dissociation into free charge carriers is quite high when<br />

it reaches an interface with ano<strong>the</strong>r material <strong>of</strong> suitable energetic<br />

structure – <strong>the</strong>re must be an <strong>of</strong>fset between HOMO (<strong>the</strong> Highest<br />

Occupied Molecular Orbital) <strong>of</strong> one material and LUMO (<strong>the</strong> Lowest<br />

Unoccupied Molecular Orbital) <strong>of</strong> <strong>the</strong> o<strong>the</strong>r material (Fig. 1).<br />

This is why efficient organic solar cells comprise two layers <strong>of</strong> different<br />

materials and <strong>the</strong> larger <strong>the</strong>ir interface is <strong>the</strong> higher is <strong>the</strong><br />

number <strong>of</strong> photogenerated charge carriers. Molecule that donates<br />

an electron in such system is called a donor (D) while <strong>the</strong> one that<br />

accepts this charge carrier is called an acceptor (A). Energy difference<br />

between LUMO <strong>of</strong> <strong>the</strong> acceptor and HOMO <strong>of</strong> <strong>the</strong> donor<br />

determines <strong>the</strong> maximum open – circuit voltage (U oc<br />

) [5].<br />

Results and scope <strong>of</strong> research<br />

Due to high probability <strong>of</strong> exciton dissociation at <strong>the</strong> junctions <strong>of</strong><br />

two different materials future <strong>of</strong> organic solar cells lays in devices<br />

with active layer comprising two or more thin films, blends <strong>of</strong> organic<br />

materials or hybrid systems (based on organic and inorganic<br />

materials). Processes that play a significant role in <strong>the</strong> photovoltaic<br />

phenomenon strongly depend on <strong>the</strong> type <strong>of</strong> organic materials<br />

forming an active layer, this is why a thorough investigation <strong>of</strong><br />

performance <strong>of</strong> OSCs comprising various groups <strong>of</strong> organic materials<br />

is inevitable.<br />

A particularly interesting new generation <strong>of</strong> OPV systems utilizes<br />

semiconducting conjugated polymers. This type <strong>of</strong> cells is referred<br />

to as ‘polymer solar cells’ or ’plastic solar cells’. The principles<br />

<strong>of</strong> operation <strong>of</strong> <strong>the</strong>se solar cells have been a topic <strong>of</strong> various<br />

articles, because plastic solar cells are expected to <strong>of</strong>fer several<br />

advantages over silicon-based ones due to <strong>the</strong> fact, that polymers<br />

are mechanically flexible, semi-transparent and soluble in common<br />

organic solvents. The latter property is a crucial advantage<br />

<strong>of</strong> polymers since it allows deposition <strong>of</strong> <strong>the</strong>se materials by simple<br />

solution processing and printing technologies. Particular attention<br />

in <strong>the</strong> field <strong>of</strong> polymer solar cells is paid to bulk heterojunction<br />

solar cells (BHSC). In this type <strong>of</strong> cells donor and acceptor materials<br />

are mixed, so <strong>the</strong> active interface exist within <strong>the</strong> whole<br />

bulk <strong>of</strong> <strong>the</strong> active layer. However, it is much easier to determine<br />

<strong>the</strong> influence <strong>of</strong> different factors on <strong>the</strong> performance <strong>of</strong> a cell and<br />

to analyse <strong>the</strong> course <strong>of</strong> observed processes when structure <strong>of</strong><br />

a cell is less complicated. That is why in <strong>the</strong> course <strong>of</strong> our investigations<br />

double-layer devices instead <strong>of</strong> bulk ones were taken into<br />

consideration. We investigated <strong>the</strong> role <strong>of</strong> anodic and cathodic<br />

buffer layers on <strong>the</strong> performance <strong>of</strong> polymer solar cells. Buffer<br />

layers are thin films put between an anode or cathode and an<br />

active layer. They are introduced to organic solar cells to improve<br />

<strong>the</strong> efficiency, but <strong>the</strong> role <strong>the</strong>y play in various systems is not fully<br />

understood. The active layer <strong>of</strong> our cells comprised MEH-PPV<br />

(poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylen)])) – soluble<br />

polymer used commonly as a donor material and F 16<br />

ZnPc<br />

(hexadecafluorozincphthalocyanine) – small molecule that is an<br />

acceptor in this system. As buffer layers MoO 3<br />

and BCP (bathocuproine)<br />

were used. All <strong>the</strong> investigated systems were sandwiched<br />

between ITO (indium tin oxide) and Ag electrodes. There<br />

are many <strong>the</strong>ories on <strong>the</strong> effect <strong>of</strong> MoO 3<br />

and BCP interlayers, for<br />

example, thin films <strong>of</strong> MoO deposited on <strong>the</strong> top <strong>of</strong> ITO electro-<br />

3<br />

Elektronika 6/2012


de are believed to change <strong>the</strong> ionizing potential <strong>of</strong> ITO (causing<br />

a rise in <strong>the</strong> work function <strong>of</strong> <strong>the</strong> latter, and so an increase in<br />

<strong>the</strong> built in potential) and as a result an increase in <strong>the</strong> open-circuit<br />

voltage [6]. O<strong>the</strong>rs think that <strong>the</strong>se layers block an electron<br />

transfer to <strong>the</strong> anode which causes an increase in j sc<br />

(short-circuit<br />

current), U oc<br />

and η (energy conversion efficiency) [7]. BCP is <strong>of</strong>ten<br />

used due to its exciton blocking ability and its positive effect on<br />

j sc<br />

, U oc<br />

and η [8]. It is also regarded as a protective interlayer that<br />

reduces damage caused to <strong>the</strong> active layer during cathode deposition<br />

process [9]. Dark j-V characteristics obtained for our cells<br />

show that introduction <strong>of</strong> BCP buffer layer has a noticeable impact<br />

on forward and reverse currents causing a significant rise in <strong>the</strong><br />

rectification ratio (RR) and decrease in series resistance (R s<br />

) <strong>of</strong><br />

a cell. We did not observe effect <strong>of</strong> anodic buffer layer on RR<br />

in <strong>the</strong> absence <strong>of</strong> BCP layer. However, cells incorporating both<br />

MoO 3<br />

and BCP interlayers had much higher rectification ratios<br />

than <strong>the</strong> ones with BCP buffer layer only. Results <strong>of</strong> our research<br />

show, that presence <strong>of</strong> MoO 3<br />

and BCP layers affects injection <strong>of</strong><br />

charge carriers at electrode/organic material interface and <strong>the</strong>ir<br />

recombination. Fig. 2 shows <strong>the</strong> open-circuit voltage action spectra<br />

(U oc<br />

as a function <strong>of</strong> wavelength <strong>of</strong> incident light) obtained for<br />

cells with different set <strong>of</strong> buffer layers. It is noticeable that <strong>the</strong><br />

presence <strong>of</strong> BCP layer causes an increase in shunt resistance<br />

(R sh<br />

) improving U oc<br />

. Increase in R sh<br />

and simultaneous decrease in<br />

R s<br />

causes an increase in fill factor (FF). Inclusion <strong>of</strong> MoO 3<br />

buffer<br />

layer seems to cause an increase in j sc<br />

and FF, but it is not so<br />

evident. So far, <strong>the</strong> best photovoltaic performance was obtained<br />

for cells provided with both interlayers. The research on effects <strong>of</strong><br />

BCP and MoO 3<br />

layers on photovoltage, photocurrent and j-V characteristics<br />

under various illumination conditions in polymer/small<br />

molecule cells is still in progress. The fact, that we found evidence<br />

<strong>of</strong> beneficial influence <strong>of</strong> MoO 3<br />

anodic buffer layer ra<strong>the</strong>r only in<br />

<strong>the</strong> presence <strong>of</strong> cathodic buffer layer is very interesting and <strong>the</strong><br />

origin <strong>of</strong> such relation needs fur<strong>the</strong>r investigation. We think that<br />

<strong>the</strong> lack <strong>of</strong> significant improvement <strong>of</strong> cell’s performance in <strong>the</strong><br />

presence <strong>of</strong> anodic interlayer and <strong>the</strong> absence <strong>of</strong> BCP might be<br />

related to damage caused to <strong>the</strong> active layer by silver atoms during<br />

cathode deposition.<br />

We investigated <strong>the</strong> role <strong>of</strong> BCP and MoO 3<br />

buffer layers also<br />

in o<strong>the</strong>r photovoltaic devices based on copper phthalocyanine<br />

(CuPc) and perylene dye (MePTCDI) heterojunction provided<br />

with <strong>the</strong> same electrodes (i.e ITO and Ag) [10]. Strong rectification<br />

effect was observed in <strong>the</strong> presence <strong>of</strong> both buffer layers, just like<br />

in <strong>the</strong> case <strong>of</strong> MEH-PPV/F 16<br />

ZnPc cells. Similarly, we noticed that<br />

<strong>the</strong>se interlayers have beneficial effects on charge injection ability<br />

by limiting potential barriers existing at organic material/electrode<br />

interfaces. Reasonable photovoltaic parameters were obtained<br />

-U oc<br />

[mV]<br />

400<br />

300<br />

200<br />

ITO/MEH-PPV/F16 ZnPc/Ag<br />

ITO/MoO3 /MEH-PPV/F16 ZnPc/Ag<br />

ITO/MEH-PPV/F16 ZnPc/BCP/Ag<br />

ITO/MoO3/MEH-PPV/F16 ZnPc/BCP/Ag<br />

thin MEH-PPV layer, toluene+isopropanol<br />

0,8<br />

0,6<br />

0,4<br />

absorbance<br />

only in <strong>the</strong> presence <strong>of</strong> both buffer layers. Relation between<br />

U oc<br />

and light intensity (I 0<br />

) obtained for CuPc/MePTCDI systems<br />

shows that BCP layer may hinder exciton dissociation at <strong>the</strong> Ag<br />

electrode (<strong>the</strong>re is no saturation <strong>of</strong> U oc<br />

even at high illumination<br />

intensities). MoO 3<br />

also acted favourably on <strong>the</strong> above mentioned<br />

characteristics and we suggested two possible explanations for<br />

this effect: MoO 3<br />

increases <strong>the</strong> rate <strong>of</strong> hole recombination at <strong>the</strong><br />

ITO/CuPc interface or in <strong>the</strong> absence <strong>of</strong> MoO 3<br />

voltage <strong>of</strong> polarity<br />

opposite to <strong>the</strong> one generated at CuPc/MePTCDI junction is<br />

developed at ITO/CuPc interface. On <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong>re was<br />

hardly any influence <strong>of</strong> regarded interlayers on <strong>the</strong> dependence <strong>of</strong><br />

short-circuit current on I 0<br />

.<br />

Apart from working on <strong>the</strong> role <strong>of</strong> buffer layers in various organic<br />

photovoltaic systems and o<strong>the</strong>r research topics, we have been<br />

interested in photovoltaic phenomenon taking place in hybrid systems,<br />

like cells based on and CdTe (cadmium telluride)/F 16<br />

ZnPc<br />

[11], TiO 2<br />

(titanium dioxide)/PdPc (palladium phthalocyanine) [12]<br />

or TiO 2<br />

/PBrPc (bromophosporus phthalocyanine) [13], that seem<br />

to be promising alternative to inorganic devices.<br />

Ano<strong>the</strong>r class <strong>of</strong> new materials for photovoltaic applications<br />

that need intensive research are carbon nanotubes (CNTs). They<br />

possess many advantageous properties – <strong>the</strong>y are optically transparent,<br />

flexible, light and environmentally resistant – that make<br />

<strong>the</strong>m attractive for improved photovoltaic devices. CNTs act as<br />

acceptors <strong>of</strong> electrons and due to <strong>the</strong>ir huge specific area <strong>the</strong>y are<br />

expected to be very efficient centres <strong>of</strong> exciton dissociation. Their<br />

high aspect ratio allows <strong>the</strong> establishment <strong>of</strong> percolation pathways<br />

at low doping levels, providing <strong>the</strong> means for high carrier mobility<br />

and efficient charge transfer to <strong>the</strong> appropriate electrodes. The<br />

CNTs are used as electrodes [14], layered at a desired location<br />

[15] or blended with a polymer [16]. The first report <strong>of</strong> using CNTs<br />

as electron acceptor in bulk-heterojunction solar cells concerned<br />

SWCNTs (single-walled carbon nanotubes) – polythiophene<br />

blends. Authors observed <strong>the</strong> increase in photocurrent <strong>of</strong> two orders<br />

<strong>of</strong> magnitude as compared to <strong>the</strong> polymer film without nanotubes<br />

[17]. Later, more reports appeared in <strong>the</strong> literature, giving<br />

examples <strong>of</strong> CNTs usage in different polymeric matrices [18–20].<br />

However, <strong>the</strong> advantages <strong>of</strong> CNTs usage in solar cells construction<br />

are hindered due to <strong>the</strong> difficulties in preparation <strong>of</strong> homogenous<br />

and stable suspensions <strong>of</strong> nanotubes in solvent or polymer matrices.<br />

Chemical functionalization <strong>of</strong> CNTs is frequently applied to<br />

enhance <strong>the</strong>ir stable suspension formation [21], however <strong>the</strong>re are<br />

only a few papers concerning functionalised CNTs as a material<br />

for photovoltaic devices. Functionalized CNTs were used in donor/<br />

acceptor type photovoltaic device by Pradhan et al. [16]. Nogueira<br />

and later Stylianakis reported covalent SWCNTs modification by<br />

thiophene for use in organic solar cells [22–23]. O<strong>the</strong>r functional<br />

groups were also bonded to CNTs. Convenient method for CNTs<br />

functionalization was presented by us [24–26]. CNTs were functionalized<br />

with different types <strong>of</strong> chemical residues, e.g. anthraquinone,<br />

phthalocyanine. Such modified soluble CNTs were deposited<br />

onto electrodes by simple, low cost techniques which can be<br />

used in PV manufacturing process. In <strong>the</strong> nearest future we plan<br />

to investigated <strong>the</strong> course <strong>of</strong> photovoltaic phenomenon in organic<br />

systems incorporating functionalized CNTs in <strong>the</strong> active layer <strong>of</strong><br />

a cell (Fig. 3 shows modified CNTs we plan to use in our study).<br />

100<br />

0,2<br />

0<br />

0,0<br />

400 500 600 700 800 900<br />

λ [nm]<br />

Fig. 2. Open-circuit voltage action spectra obtained for cells with<br />

different set <strong>of</strong> buffer layers; absorption spectrum <strong>of</strong> F 16<br />

ZnPc (green<br />

line) and MEH-PPV cast from toluene and isopropanol (pink line).<br />

MEH-PPV and F 16<br />

ZnPc thickness is ~30 nm and ~100nm respectively.<br />

Illumination through ITO electrode, incident light intensity 10 15<br />

photon/cm 2 s<br />

Fig. 3. Schematic representation <strong>of</strong> carbon nanotubes functionalized<br />

with anthracenyl residue and cobalt phthalocyanine derivative<br />

Elektronika 6/2012 119


Conclusions<br />

There are many types <strong>of</strong> organic and organic/inorganic (hybrid)<br />

photovoltaic devices that possess many beneficial properties over<br />

inorganic solar cells. That makes <strong>the</strong> former suitable for a broad<br />

range <strong>of</strong> applications and hence so much attention is paid to <strong>the</strong>ir<br />

development. In spite <strong>of</strong> intensive research carried on this field<br />

<strong>of</strong> science and technology <strong>the</strong>re are still many crucial problems<br />

that need to be solved. Extensive knowledge on physical basis<br />

<strong>of</strong> operation <strong>of</strong> organic and hybrid systems, fast development <strong>of</strong><br />

nanotechnology, great variety <strong>of</strong> organic materials and methods<br />

<strong>of</strong> <strong>the</strong>ir modification will pave <strong>the</strong> way for commercial applications<br />

<strong>of</strong> <strong>the</strong>se devices.<br />

References<br />

[1] Toward a Just and Sustainable Solar Energy Industry, A Silicon Valley<br />

Toxics Coalition White Paper, January 14, 2009.<br />

[2] Brabec Ch. J.: “Organic photovoltaics: technology and market”, Solar<br />

Energy Materials & Solar Cells, 83 (2004), 273-292.<br />

[3] Jorgensen M., K. Norrman, F. C. Krebs: “Stability/degradation <strong>of</strong><br />

polymer solar cells”, Solar Energy Materials & Solar Cells 92 (2008)<br />

686–714.<br />

[4] Clarke T. M., J. R. Durrant: “Charge photogeneration in organic solar<br />

cells”, Chemical Review 110 (2010) 6736–6767.<br />

[5] Scharber M. C., D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf, A. J.<br />

Heeger, C. J. Brabec: Adv. Mater. 2006, 18, 789–794.<br />

[6] Kinoshita Y., R. Takenaka, H. Murata: “Independent control <strong>of</strong> opencircuit<br />

voltage <strong>of</strong> organic solar cells by changing film thickness <strong>of</strong><br />

MoO3 buffer layer “, Appl. Phys. Lett. 92 (2008), 243309-1-3.<br />

[7] Kim D.Y., G. Sarasquerta, and F. So: “SnPc:C60 bulk hetero−junction<br />

organic photovoltaic cells with MoO3 interlayer”, Sol. Energ. Mat. Sol.<br />

C93 (2009), 1452–1456.<br />

[8] Vogel M., S. Doka, Ch. Breyer, M.Ch. Lux−Steiner, and K.Festiropoulos:<br />

“On <strong>the</strong> function <strong>of</strong> a bathocuproine buffer layer in organic photovoltaic<br />

cells”, Appl. Phys. Lett. 89 (2006), 163501−1−3.<br />

[9] Gommans H., B. Verreet, B.P. Rand, R. Muller, J. Poortmans, P. Heremans<br />

and J. Genoe: “On <strong>the</strong> Role <strong>of</strong> Bathocuproine in Organic Photovoltaic<br />

Cells”, Adv. Funct. Mater. 18 (2008), 3686–3691.<br />

[10] Signerski R., G. Jarosz: “Effect <strong>of</strong> buffer layers on performance <strong>of</strong> organic<br />

photovoltaic devices based on copper phthalocyanine – perylene<br />

dye heterojunction”, Opto−Electron. Rev. 19 (2011), 468–473.<br />

[11] Signerski R., G. Jarosz, B. Kościelska: “Photovoltaic effect in hybrid<br />

heterojunction formed from cadmium telluride and zinc perfluorophthalocyanine<br />

layers”, J. Non- Cryst. Sol. 356 (2010), 2053–2055.<br />

[12] Signerski R., G. Jarosz, B. Kościelska: “On photovoltaic effect in hybrid<br />

heterojunction formed from palladium phthalocyanine and titanium<br />

dioxide layers, J. Non-Cryst. Sol. 355 (2009), 1405–1407.<br />

[13] Signerski R., B. Kościelska: “Photovoltaic properties <strong>of</strong> a sandwich<br />

cell consisting <strong>of</strong> bromophosphorus phthalocyanine and titanium dioxide<br />

layers”, Opt. Mat. 27 (2005), 1480–1483.<br />

[14] Rowell M.W., M.A. Topinka, M.D. McGehee, H.J. Prall, G. Dennler,<br />

N.S. Sariciftici, L. Hu, G. Gruner: Appl. Phys. Lett. 88 (2006) 233506-<br />

1-233506-3.<br />

[15] Chaudhary S., H. Lu, A.M. Muller, C.J. Bardeen, M. Ozkan: Nano<br />

Lett. 7 (2007) 1973–1979.<br />

[16] Pradhan B., S.K. Batabyal, A.J. Pal.: Appl. Phys. Lett. 88 (2006)<br />

093106-1- 093106-3.<br />

[17] Kymakis E., P. Servati, P. Tzanetakis, E. Koudoumas, N. Kornilios, I.<br />

Rompogiannakis, Y. Franghiadakis, G.A.J. Amaratunga: Nanotechnology<br />

18 (2007) 435702/1–435702/6.<br />

[18] Reyes-Reyes M., R. Lopez-Sandoval, J. Liu, D.L. Carroll: Sol. Energy<br />

Mat. Sol. Cells 91 (2007) 1478–1482.<br />

[19] Previti F., S. Patane, M. Allegrini: Appl. Surf. Sci. 255 (2009) 9877–<br />

9879.<br />

[20] Singh I., P.K. Bhatnagar, P.C. Mathur, I. Kaur, L.M. Bharadwaj, R.<br />

Pandey: Carbon 46 (2008) 1141–1144.<br />

[21] Rastogi R., R. Kaushal, S.K. Tripathi, A. L. Sharma, I.Kaur, L. M.<br />

Bharadwaj: J. Coll. Interf. Sci. 328 (2008) 421–428.<br />

[22] Nogueira A.F., B. S. Lomba, M. A. Soto-Oviedo, C. R. D. Correia: J.<br />

Phys. Chem. C 49 (2007), 18431–18438.<br />

[23] Stylianakis M. M., J. A. Mikroyannidis, E. Kymakis: Sol. Energy Mat.<br />

Sol. Cells 94 (2010) 267–274.<br />

[24] Jubete E., K. Żelechowska, O. A. Loaiza, P. J. Lamas, E. Ochoteco,<br />

K. D. Farmer, K. P. Roberts, J. F. Biernat: Electrochim. Acta 56<br />

(2011) 3988–3995.<br />

[25] Sadowska K. (Żelechowska), J. F. Biernat, K. Stolarczyk, R. Bilewicz,<br />

K. Roberts, J. Rogalski: Bioelectrochem. 80(1) (2010) 73–80.<br />

[26] Sadowska K. (Żelechowska), K.P. Roberts, R. Wiser, J.F. Biernat, E.<br />

Jabłonowska and R. Bilewicz: Carbon 47 (2009) 1501–1510.<br />

Surface morphology and optical properties<br />

<strong>of</strong> polymer thin films<br />

JAN Weszka 1,2) , Magdalena Szindler 1,4) , Maria Bruma 3)<br />

1)<br />

Institute <strong>of</strong> Engineering Materials and Biomaterials, Silesian University <strong>of</strong> Technology,Gliwice, Poland<br />

2)<br />

Department <strong>of</strong> Physics, Center <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences,Zabrze, Poland<br />

3)<br />

Institute <strong>of</strong> Macromolecular Chemistry,Romania, 4) Corresponding author<br />

Industrial development has always been associated with <strong>the</strong> development<br />

<strong>of</strong> energy technologies, mainly consisted <strong>of</strong> <strong>the</strong> introduction<br />

<strong>of</strong> changes to <strong>the</strong> existing and implementing new types<br />

<strong>of</strong> energy sources. In <strong>the</strong> twentieth century, <strong>the</strong>se changes consisted<br />

mainly in <strong>the</strong> transition from coal as <strong>the</strong> primary energy<br />

fuel for petroleum and <strong>the</strong>n from oil to gas. Today, <strong>the</strong> economic<br />

and ecological reasons, looking for alternative sources <strong>of</strong> energy.<br />

Seems to be <strong>the</strong> most valuable comes from renewable sources<br />

and can be converted to any form <strong>of</strong> energy. The rapid development<br />

<strong>of</strong> electronics and materials science, and especially for semiconductor<br />

and chemistry <strong>of</strong> polymeric materials is related to <strong>the</strong><br />

introduction <strong>of</strong> modern engineering materials. Gained important<br />

conductive polymers [1, 2].<br />

The most famous <strong>of</strong> conductive polymer materials include<br />

polyacetylene, polythiophene, and polyphenylene. An important<br />

group <strong>of</strong> polymers whose main chains are composed<br />

<strong>of</strong> carbon atoms connected by alternating single and double<br />

bonds, called conjugated polymers. Conjugated polymers can<br />

be used in photovoltaic and optoelectronics. This group includes<br />

polyoxadiazoles. Conductive polymers <strong>of</strong>ten show a conductivity<br />

only slightly worse than <strong>the</strong> most conductive metals<br />

(Fig. 1) [3–6].<br />

120<br />

isolators semiconductors metals<br />

S/m<br />

10<br />

-16<br />

quartz<br />

Experiment<br />

double bonds conjugated polymers<br />

10 -12<br />

diamond 10-8<br />

glass<br />

10 -4 silicon 100<br />

germanium 104 10 6<br />

copper<br />

iron<br />

silver<br />

Fig. 1. Changes in <strong>the</strong> degree <strong>of</strong> doping will change <strong>the</strong> conductivity<br />

indicated by arrow<br />

To research was used a sample <strong>of</strong> experimental material from <strong>the</strong><br />

Institute <strong>of</strong> Macromolecular Chemistry, in Romania. The polymer,<br />

which construction is shown in Tab. 1, was used.<br />

The first stage <strong>of</strong> <strong>the</strong> study was to obtain <strong>the</strong> polymer thin films.<br />

For this purpose, <strong>the</strong> polymer was dissolved in NMP (1-methyl-2-<br />

Elektronika 6/2012


pyrrolidone). To 0.04 g <strong>of</strong> polymer was added 1 ml <strong>of</strong> NMP. Dissolution<br />

reaction was performed at 200 to 210°C in a time equal to<br />

10 hours. Then solution was applied to glass substrates by spincoating<br />

method. The thin films were coatedin room temperature<br />

with a different speed 1000, 2000 and 3000 rev/min. The resulting<br />

samples were gradually heated from room temperature to 180°C<br />

and cured for an 1 hour.<br />

In <strong>the</strong> next step <strong>the</strong> samples were examine under <strong>the</strong> microscopes:<br />

CLSM and AFM, and <strong>the</strong>n were tested with using<br />

a spectrometer UV/VIS. Studies using CLSM microscopy was<br />

performed to assess <strong>the</strong> state <strong>of</strong> <strong>the</strong> quality <strong>of</strong> <strong>the</strong> thin films.<br />

Images <strong>of</strong> polyoxadiazoles thin films were observed with a magnification<br />

<strong>of</strong> 5000× (<strong>the</strong> area was 120×120 microns). The study<br />

<strong>of</strong> surface topography with using a confocal microscope allowed<br />

us to assess <strong>the</strong> quality <strong>of</strong> thin films. Research with using<br />

atomic force microscopy AFM XE 100 were performed to<br />

characterize <strong>the</strong> surface morphology and to analysis surface<br />

roughness. Photos were taken in noncontact mode, watching<br />

an area <strong>of</strong> 10×10 microns. Studies with using a spectrometer<br />

UV/VIS were performed to determine <strong>the</strong> absorption coefficient<br />

<strong>of</strong> <strong>the</strong> thin films.<br />

Fig. 3. The topography image <strong>of</strong> <strong>the</strong> surface <strong>of</strong> <strong>the</strong> thin film <strong>of</strong> polymer<br />

DDE-Per-6F (2:1:1) coated with a 2000 rev/min spin speed obtained<br />

under <strong>the</strong> microscope: a) CLSM; b) AFM<br />

Tab. 1. Schematic structure <strong>of</strong> investigated polyoxadiazol<br />

Polymer<br />

Schematic structure<br />

y x z<br />

Molar<br />

ratio<br />

(x:y:z)<br />

DDE-Per-6F 2:1:1<br />

Fig. 4. The topography image <strong>of</strong> <strong>the</strong> surface <strong>of</strong> <strong>the</strong> thin film <strong>of</strong> polymer<br />

DDE-Per-6F (2:1:1) coated with a 3000 rev/min spin speed obtained<br />

under <strong>the</strong> microscope: a) CLSM; b) AFM<br />

a)<br />

Results<br />

Research on CLSM confocal microscopy was performed to assess<br />

<strong>the</strong> state <strong>of</strong> <strong>the</strong> quality <strong>of</strong> <strong>the</strong> thin films. Images <strong>of</strong> <strong>the</strong>m<br />

were observed at a magnification <strong>of</strong> 5000×. Research with<br />

using atomic force microscopy AFM XE 100 were performed<br />

to characterize <strong>the</strong> surface morphology and to analysis surface<br />

roughness.<br />

In Fig. 2 – 4 is a view surface topography <strong>of</strong> polymer DDE-Per-<br />

6F (2:1:1) coated properly with a 1000, 2000 and 3000 rev/min<br />

spin speed.<br />

In Figures 2a) – 4a) shows <strong>the</strong> CLSM polymer thin films images<br />

surface coated with different speeds. Fig. 2a) shows a numerous<br />

<strong>of</strong> agglomeration areas, exceeding even 10 microns, due<br />

to an early precipitation <strong>of</strong> <strong>the</strong> polymer. As speed increases <strong>the</strong><br />

accumulation rozwirowania become smaller and occur less frequently<br />

(Fig. 3a).<br />

In Figures 2b) – 4b) shows <strong>the</strong> view obtained with <strong>the</strong> AFM<br />

microscope. Looking at <strong>the</strong> topography <strong>of</strong> <strong>the</strong> thin film surface<br />

coated with a 1000 rev/min (Fig. 2b) can be seen a single precipitation<br />

<strong>of</strong> approximately 4×2.5 microns. The thin film coated with<br />

a 3000 rev/min spin speed (Fig. 4b) is characterized by numerous<br />

small irregularities do not exceed 0.5 microns.<br />

b)<br />

c)<br />

Fig. 5. The histogram <strong>of</strong> frequency <strong>of</strong> <strong>the</strong> occurred height for<br />

a polymer DDE-Per-6F (2:1:1) thin films coated with a spin speed at:<br />

a) 1000 rev/min, b) 2000 rev/min, c) 3000 rev/min<br />

Fig. 2. The topography image <strong>of</strong> <strong>the</strong> surface <strong>of</strong> <strong>the</strong> thin film <strong>of</strong> polymer<br />

DDE-Per-6F (2:1:1) coated with a 1000 rev/min spin speed obtained<br />

under <strong>the</strong> microscope: a) CLSM; b) AFM<br />

An analysis <strong>of</strong> <strong>the</strong> roughness <strong>of</strong> each thin film in <strong>the</strong> program<br />

carried out XEI’s Park System. Surface roughness was characterized<br />

by calculating <strong>the</strong> roughness parameters Rq, Ra, maximum<br />

height.<br />

In Fig. 5. shows histograms <strong>of</strong> <strong>the</strong> thin films DDE-Per-6F<br />

(2:1:1) – <strong>the</strong> distribution <strong>of</strong> surface irregularities. The shapes and<br />

histograms distribution indicate that <strong>the</strong> polymer DDE-Per-6F<br />

(2:1:1) thin film coated with a 3000 rev/min spin speed should be<br />

characterized by <strong>the</strong> lowest surface roughness (Fig. 5c).<br />

On <strong>the</strong> histograms <strong>the</strong> x axis represents <strong>the</strong> height <strong>of</strong> data<br />

points in <strong>the</strong> sample. The y axis represents <strong>the</strong> number <strong>of</strong> pixels<br />

in <strong>the</strong> selected region group.<br />

Elektronika 6/2012 121


Tab. 2. The roughness parameters <strong>of</strong> <strong>the</strong> polyoxadiazoles thin films in <strong>the</strong><br />

program carried out XEI’s Park System<br />

Spin speed [rev/min] Max height [nm] R a<br />

[nm] R q<br />

[nm]<br />

1000 33,728 10,634 14,447<br />

2000 46,776 5,276 10,999<br />

3000 43,010 4,773 8,393<br />

Where: Rq – <strong>the</strong> root-mean-squared roughness. It is <strong>the</strong> standard deviation<br />

<strong>of</strong> <strong>the</strong> height value, Ra – <strong>the</strong> roughness average, Max height – a maximum<br />

height.<br />

Fig. 6. shows <strong>the</strong> plot <strong>of</strong> absorbance on <strong>the</strong> wavelength <strong>of</strong> <strong>the</strong><br />

polymer DDE-Per-6F (2:1:1). The curves were determined for <strong>the</strong><br />

layers deposited at different spin speeds. The largest absorption<br />

coefficient <strong>of</strong> <strong>the</strong> polymer obtained with a spin speed 1000 rev/<br />

min and <strong>the</strong> absorbance reached a value <strong>of</strong> 1.3 at a wavelength <strong>of</strong><br />

about 380 nm. Spin coated thin film obtained with a 2000 rev/min<br />

is characterized by a much lower absorption coefficient equal to<br />

0.8 at a wavelength <strong>of</strong> 350 nm.<br />

Summary<br />

The spin coating method allows <strong>the</strong> deposition <strong>of</strong> <strong>the</strong> uniform thin<br />

films. The process is dependent primarily on <strong>the</strong> correct preparation<br />

<strong>of</strong> <strong>the</strong> solution. In view <strong>of</strong> consideration both <strong>the</strong> results <strong>of</strong><br />

surface topography and <strong>the</strong> absorbance <strong>of</strong> <strong>the</strong> best results were<br />

obtained for <strong>the</strong> thin film deposited at 3000 rev/min. Studies on<br />

<strong>the</strong> polymers conjugated are interesting because <strong>of</strong> <strong>the</strong>ir possible<br />

application in <strong>the</strong> structure <strong>of</strong> polymer solar cells and dye sensitized<br />

solar cells.<br />

Fig. 6. The dependence <strong>of</strong> absorbance on <strong>the</strong> wavelength <strong>of</strong> <strong>the</strong> polymer<br />

DDE-Per-6F (2:1:1)<br />

References<br />

[1] Drozdov N.: Ogniwa fotowoltaiczne dla energetyki słonecznej.<br />

Wydawnictwo Politechniki Lubelskiej, Lublin 2006.<br />

[2] Torbicz W., D. Pijanowska: Polimery elektroprzewodzące w elektronice<br />

i analityce biochemicznej. Elektronika 2009, nr 6, s. 36–43.<br />

[3] Shirakawa H.: Syn<strong>the</strong>tic Metals. 1995, 69, 3–8.<br />

[4] Gruin I.: Materiały polimerowe. PWN, Warszawa, 2003.<br />

[5] Schulz B., Y. Kaminorz; L. Brehmer: New Aromatic Poly(l,3,4-<br />

oxadiazole)s for Light Emitting Diodes. University Potsdam, Institute<br />

<strong>of</strong> Solid State Physics Kantstrasse 55, D – 14513 Teltow, Germany.<br />

[6] Poroń A.: Syntetyczne metale. Wiedza i Życie nr 2/2001.<br />

Photovoltaic bulk heterojunctions with interpenetrating<br />

network based on semiconducting polymers<br />

Michał Modzelewski, Ewa Klugmann-Radziemska<br />

Gdańsk University <strong>of</strong> Technology, Chemical Faculty<br />

Bulk heterojunctions with interpenetrating network architecture<br />

(BHS) is nowadays <strong>the</strong> most prospective one exploited for organic<br />

electronic devices manufacturing as it enables fabricating<br />

large and flexible areas <strong>of</strong> light-emitting diodes, transistors and<br />

solar cells. Bulk heterojunction with internal network <strong>of</strong> percolation<br />

paths is intermixed spatial blend <strong>of</strong> two physically stable phases<br />

(Fig. 1). One <strong>of</strong> <strong>the</strong> phases is composed <strong>of</strong> electron-donor<br />

compound which is characterized by low ionization potential and<br />

second with high electron affinity acts as an electron-acceptor.<br />

Exceptional position <strong>of</strong> BHS amongst all architectures <strong>of</strong> organic<br />

PV devices can be explained primarily by possibility <strong>of</strong> low-cost<br />

deposition <strong>of</strong> soluble organic photoactive compounds by means<br />

<strong>of</strong> novel techniques such as spin-coating, screen printing or doctor<br />

blading on <strong>the</strong> one side and by achieving internal network within<br />

an exciton diffusion length on <strong>the</strong> o<strong>the</strong>r side.<br />

Until that day <strong>the</strong> most effective bulk heterojunction based on<br />

semiconducting (conjugated) polymers is structure <strong>of</strong> Poly(3-hexyltiophene)<br />

(P3HT) acting as electron donor an Phenyl-C 60<br />

-butyric<br />

acid methyl ester as electron acceptor [1] (Fig. 2).<br />

There are different BHJ technologies referred to type <strong>of</strong> applied<br />

materials. The common part for all <strong>of</strong> <strong>the</strong>m is solubility <strong>of</strong> constituent<br />

compounds. As far as BHJs are charge transfer systems one may<br />

divide <strong>the</strong>m into polymer/fullerene derivatives, polymer/polymer,<br />

low-band gap polymers/fullerene derivatives, polymer/inorganic<br />

compound and polymer/oxide. All mentioned may be doped or not.<br />

To find our own place amongst organic photovoltaic we decided to<br />

122<br />

apply low-band gap polymer for organic solar cell. The main goal <strong>of</strong><br />

following paper was to construct an organic photovoltaic cell in BHJ<br />

architecture based on very low band gap polymer, soluble derivative<br />

<strong>of</strong> fullerene and organic dopant. To construct a junction which<br />

potentially would be able to harvest solar radiation from <strong>the</strong> spectrum<br />

<strong>of</strong> near infrared (Fig. 3) we selected three organic and soluble<br />

compounds with different type <strong>of</strong> induced conductivity: low-band<br />

gap polymer poly(indenefluorene) (PIF) (Fig. 4a) with low ionization<br />

potential acting as a base electron-donor, fullerene derivative<br />

<strong>of</strong> phenyl-C 60<br />

-butyric acid methyl ester (PCBM) acting as effective<br />

electron acceptor (Fig. 2b) and 7,7,8,8-tetracyanoquinodimethane<br />

(TCNQ) (Fig. 4b) as additional electron-acceptor <strong>of</strong> photoinduced<br />

electrons.<br />

Low iron glass<br />

PEDOT:PSS<br />

Light<br />

Donor:acceptor intermixed blend<br />

Indium Tin Oxide<br />

Metal cathode<br />

Fig. 1. Cross-section <strong>of</strong> organic photovoltaic cell containing blend<br />

as a photoactive layer and composed <strong>of</strong> PEDOT:PSS on substrate<br />

made from ITO (anode) with metal cathode deposited by vacuum<br />

evaporator<br />

Elektronika 6/2012


Fig. 2. Structure <strong>of</strong> a) Poly(3-hexyltiophene) and b) Phenyl-C 60<br />

-butyric<br />

acid methyl ester<br />

Absorbance, a.u.<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

300 400 500 600 700 800 900<br />

Wavelength, nm<br />

Fig. 3. Solar radiation spectrum relative absorbance for conjugated<br />

polymer Poly(indenefluorene) (PIF). First absorption peak is localized<br />

for ca. 300 nm (100% <strong>of</strong> maximum absorbance value) and second<br />

one for 783 nm which is about 40% <strong>of</strong> previous one<br />

Experiment<br />

Experiment was carried out during my internship in Laboratory<br />

<strong>of</strong> Photophysics <strong>of</strong> Organic Nanomaterials <strong>of</strong> Physics Department<br />

in Moscow State University <strong>of</strong> Russia. Experiment was<br />

divided into two parts. In first part we prepared two cells based<br />

on PIF and PCBM at two different weight ratio i.e. 1:1 and 2:1.<br />

In second part we added 7,7,8,8- tetracyanoquinodimethane to<br />

new blend <strong>of</strong> PIF:PCBM (1:1). P3HT:PCBM junctions was made<br />

for comparison purposes. Poly(indenefluorene) was syn<strong>the</strong>sized<br />

for us by U. Scherf from University <strong>of</strong> Wuppertal, Germany<br />

according to [2]. PCBM, TCNQ and P3HT were purchased from<br />

Sigma-Aldrich. The first part <strong>of</strong> experiment was to create <strong>the</strong><br />

base cells. PIF:PCBM (1:1) and PIF:PCBM (2:1) were dissolved<br />

in 10 microliters <strong>of</strong> chlorobenzene and spin-coated with<br />

angular speed <strong>of</strong> 1000 rpm on indium-tin oxide substrates <strong>of</strong><br />

surface resistivity less than 10 Ω per square. Aluminum cathode<br />

in first part was evaporated in Maxtek commercial evaporator<br />

(Fig. 5) under pressure <strong>of</strong> 4.9×10 -6 mbar and ytterbium<br />

cathode in second part under pressure <strong>of</strong> 5.2×10 -6 (Fig. 5).<br />

Subsequently cells were characterized by self made spectrometer<br />

and density current-voltage characteristic generator<br />

under STC conditions. In part one <strong>of</strong> experiment we obtained<br />

following characteristics <strong>of</strong> external quantum efficiency versus<br />

wavelength and photoinduced density <strong>of</strong> current versus voltage<br />

(Fig. 6, 7, 8). Multiple current-voltage curves for each type <strong>of</strong><br />

cell were collected for different points <strong>of</strong> cathodes (8 pixels) as<br />

presented in Fig. 5.<br />

In order to improve <strong>the</strong> charge transfer process in relation to<br />

<strong>the</strong> junction <strong>of</strong> PIF:PCBM we introduced 5 mg <strong>of</strong> TCNQ as a good<br />

electron-acceptor into new liquid blend <strong>of</strong> PIF:PCBM (1:1) in chlorobenzene.<br />

We obtained following results (Fig. 9, 10):<br />

n<br />

N<br />

N<br />

a)<br />

Fig. 4. Chemical formula <strong>of</strong> a) poly(indenefluorene), b) 7,7,8,8-tetracyanoquinodimethane<br />

b)<br />

N<br />

N<br />

Fig. 5. Left: Prepared organic solar cells with deposited layer <strong>of</strong> shaped<br />

aluminum, right: metal cathode deposition chamber with structural<br />

mask for selective deposition<br />

EQE (%)<br />

100<br />

10<br />

1<br />

0,1<br />

0,01<br />

1E-3<br />

PIF:PCBM 2:1<br />

PIF:PCBM 1:1<br />

P3HT:PCBM<br />

Current density (mA/cm 2 )<br />

10<br />

5<br />

0<br />

-5<br />

P3HT:PCBM (1:1)<br />

1E-4<br />

400 500 600 700 800 900<br />

Wavelength (nm)<br />

-10<br />

0,00 0,25 0,50 0,75 1,00<br />

Voltage (V)<br />

Fig. 6. Left: Absorption spectra <strong>of</strong> junctions in visible and near infrared part <strong>of</strong> spectrum: PIF:PCBM (1:1), PIF:PCBM (2:1) and for comparison<br />

purposes P3HT:PCBM (1:1), Right: Current-voltage characteristic <strong>of</strong> loaded junction <strong>of</strong> P3HT:PCBM (1:1)<br />

Elektronika 6/2012 123


PIF:PCBM (1:1)<br />

PIF:PCBM (2:1)<br />

Current density (mA/cm 2 )<br />

0,04<br />

0,02<br />

0,00<br />

Current density (mA/cm 2 )<br />

0,04<br />

0,02<br />

0,00<br />

-0,02<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

-0,02<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

Fig. 7. Current-voltage characteristics <strong>of</strong> loaded junctions <strong>of</strong> PIF:PCBM (1:1) (left) and PIF:PCBM (2:1) (right)<br />

Current density |J| (mA/cm 2 )<br />

10<br />

1<br />

0,1<br />

0,01<br />

1E-3<br />

P3HT:PCBM (1:1)<br />

PIF:PCBM (2:1)<br />

PIF:PCBM (1:1)<br />

1E-4<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

Fig. 8. Absolute value <strong>of</strong> current density against generated voltage<br />

for three junctions created in first part <strong>of</strong> experiment: PIF:PCBM<br />

(1:1), PIF:PCBM (2:1) and P3HT:PCBM (1:1), depicted for comparison<br />

purposes<br />

Results<br />

According to achieved characteristics we read out or calculated<br />

typical parameters which describe <strong>the</strong> performance <strong>of</strong> cells. The<br />

nominal parameters <strong>of</strong> solar cells manufactured in two stages<br />

were presented in Table.<br />

To define <strong>the</strong> base improvement we created two bulk heterojunctions<br />

i.e. PIF:PCBM (1:1) and PIF:PCBM (2:1). It turned<br />

out that increased amount <strong>of</strong> PIF in blend deteriorates <strong>the</strong> performance<br />

<strong>of</strong> <strong>the</strong> cell because <strong>of</strong> insufficient amount <strong>of</strong> electronacceptor<br />

compound. Taking that factors into account we came<br />

to a conclusion that <strong>the</strong>re is a particular need to apply additional<br />

compound with negative character <strong>of</strong> conductivity such as TCNQ.<br />

Photoinduced current density/intensity is supposed to be consider<br />

as <strong>the</strong> most crucial parameter <strong>of</strong> all solar cells. Insertion <strong>of</strong><br />

TCNQ to <strong>the</strong> polymer/fullerene matrix produces <strong>the</strong> short circuit<br />

current improvement. Although <strong>the</strong> degree <strong>of</strong> that improvement is<br />

ra<strong>the</strong>r poor we can conclude that TCNQ increases <strong>the</strong> efficiency<br />

<strong>of</strong> charge transfer between constituent phases and causes significant<br />

extending <strong>of</strong> absorption spectrum up to 900 nm. Despite<br />

<strong>the</strong> current growth process it is not possible that TCQN improve<br />

<strong>the</strong> conductivity <strong>of</strong> percolation paths. External quantum and power<br />

conversion efficiencies growth for PIF:PCBM:TCNQ (2:2:1)<br />

are mainly due to <strong>the</strong> extended absorption spectrum but deterioration<br />

<strong>of</strong> open circuit voltage is a result <strong>of</strong> application ytterbium<br />

as a cathode which has lower work function than aluminum.<br />

10<br />

P3HT:PCBM<br />

PIF:PCBM:TCNQ<br />

1<br />

0<br />

P3HT:PCBM<br />

EQE (%)<br />

1<br />

0,1<br />

0,01<br />

Current density (mA/cm 2 )<br />

-1<br />

-2<br />

-3<br />

-4<br />

1E-3<br />

400 500 600 700 800 900<br />

Wavelength (nm)<br />

-5<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

Fig. 9. Left: Absorption spectra <strong>of</strong> junctions in visible and near infrared part <strong>of</strong> spectrum: PIF:PCBM:TCNQ (2:2:1) and for comparison purposes<br />

P3HT:PCBM (1:0.8), Right: Current-voltage characteristic <strong>of</strong> loaded junction <strong>of</strong> P3HT:PCBM (1:0.8)<br />

124<br />

Elektronika 6/2012


0,03<br />

0,02<br />

10<br />

Current density (mA/cm 2 )<br />

0,01<br />

0,00<br />

-0,01<br />

-0,02<br />

-0,03<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

Current density (mA/cm 2 )<br />

1<br />

0,1<br />

0,01<br />

1E-3<br />

PIF:PCBM:TCNQ<br />

P3HT:PCBM (1:0.8)<br />

1E-4<br />

-0,2 0,0 0,2 0,4 0,6 0,8 1,0<br />

Voltage (V)<br />

Fig. 10. Left: Current-voltage characteristics <strong>of</strong> loaded junctions <strong>of</strong> PIF:PCBM:TCNQ (2:2:1), Right: absolute value <strong>of</strong> current density against<br />

generated voltage for two junctions created in second part <strong>of</strong> experiment: PIF:PCBM:TCNQ (2:2:1) and P3HT:PCBM (1:1), depicted for comparison<br />

purposes<br />

Tabl. Performance <strong>of</strong> solar cells created in both steps <strong>of</strong> experiment (↑ – highest value, ↓ – lowest value, i) – improvement)<br />

Active blend layer (weight ratio)<br />

PIF:PCBM (1:1) PIF:PCBM (2:1) P3HT:PCBM (1:1) PIF:PCBM:TCNQ (2:2:1) P3HT:PCBM (1:0.8)<br />

Concentration 10:10 mg 10:5 mg 10:10 mg 10:10:5 mg 10:8 mg<br />

Solvent cb cb odcb cb odcb<br />

Jsc, max [µA/cm2 ] 15.1 11↓ 5496 21↑ i) 4372<br />

J [µA/cm 2 ] 13.7 10.5 19.5<br />

sc, ave<br />

V oc, max<br />

[V] 0.496↑ 0.431 0.397 0.299↓ 0.625<br />

V oc, ave<br />

[V] 0.472 0.389 0.286<br />

FF max<br />

[%] 29.5 32↑ 52.7 26.6↓ 66.7<br />

FF ave<br />

[%] 29.2 29.8 26.45<br />

PCE max<br />

[%] 0.00215↑ 0.00125↓ 1.16 0.00156 i) 1.69<br />

PCE ave<br />

[%] 0.00189↑ 0.00113↓ 0.00147 i)<br />

EQE max<br />

[%] 0.2↓ 0.2↓ 41.1 0.56↑ i) 36.6<br />

I heat<br />

(A) 40-100 60-80<br />

v growth<br />

[Ĺ/sec] 5-15 7<br />

d [nm] 45 (aluminum) 95 (ytterbium)<br />

Where: J sc, max<br />

, J sc,ave<br />

– maximum and average short circuit current density, V oc, max<br />

, V oc, ave<br />

– maximum and average open circuit voltage, FF max<br />

, FF ave<br />

– maximum<br />

and average fill factor, PCE max<br />

, PCE ave<br />

– maximum and average power conversion efficiency, I heat<br />

– intensity <strong>of</strong> evaporator’s current, v growth<br />

– speed <strong>of</strong> cathode<br />

material deposition, d – thickness <strong>of</strong> cathode, cb – chlorobenzene, odcb – o-dichlorobenzene.<br />

Summary<br />

Poly(indenefluorene) belongs to <strong>the</strong> exotic and rare group <strong>of</strong><br />

syn<strong>the</strong>tic, photoactive, macromolecular compounds which may<br />

replace crystalline silicon in electronics in <strong>the</strong> future. Although <strong>the</strong><br />

attempt <strong>of</strong> performance improvement it is highly probable that PIF<br />

itself is characterized by low exciton diffusion length and <strong>the</strong>refore<br />

(apart from PCBM and TCNQ) application <strong>of</strong> o<strong>the</strong>r electronacceptor<br />

in blends with that polymer may be pointless. There is<br />

a particular need <strong>of</strong> application o<strong>the</strong>r conjugated polymers with<br />

optimized band gap around 1.5 eV. Polymers with lower values<br />

<strong>of</strong> band gap may not deliver satisfying voltages even though <strong>the</strong>y<br />

have well absorption properties. The main obstacle in commercial<br />

applications <strong>of</strong> low-band gap polymer based devices is that <strong>the</strong>y<br />

have low values <strong>of</strong> operating currents and external quantum efficiencies.<br />

Few available examples <strong>of</strong> photovoltaic devices based<br />

<strong>of</strong> that kind <strong>of</strong> polymers manufactured in [3, 4] have EQEs less<br />

than 0.01%. New bulks based on o<strong>the</strong>r macromolecular chromophores<br />

are needed as presented devices based on PIF may be<br />

used only as a photonic detector <strong>of</strong> radiation.<br />

References<br />

[1] Koster L. J. A., V. D. Mihailetchi, P. W. M. Blom: Ultimate efficiency<br />

<strong>of</strong> polymer/fullerene bulk heterojunction solar cells”, Appl. Phys. Lett.<br />

88, 093511 (2006).<br />

[2] Reisch H., Wiesler U., Scherf U., Tuytuylkov N.: Poly(inden<strong>of</strong>luorene)<br />

(PIF), a Novel Low Band Gap Polyhydrocarbon, Macromolecules, 29<br />

(1996).<br />

[3] Bundgaard E., Krebs F.C.: Low band gap polymers for organic photovoltaics,<br />

Solar Energy Materials and Solar Cells, 91 (2007).<br />

[4] Petritsch K.: Organic Solar Cell Architectures, PhD Thesis, Cambridge,<br />

Graz (2000).<br />

Elektronika 6/2012 125


Experimental Validation <strong>of</strong> New Classes <strong>of</strong> Dyes<br />

for Dye-Sensitised Solar Cells<br />

Michal G. Zubel, Jacqueline M. Cole, Paul G. Waddell<br />

Cavendish Laboratory, Department <strong>of</strong> Physics, University <strong>of</strong> Cambridge, Cambridge, United Kingdom<br />

Solar power is numbered, along with wind, water (waves, tides,<br />

dams, run-on-<strong>the</strong>-river systems), biomass growth (traditional<br />

and advanced) and geo<strong>the</strong>rmal heat among renewable energy<br />

sources. Although <strong>the</strong> accuracy <strong>of</strong> any long-term forecast can be<br />

considered questionable, especially in <strong>the</strong> times when <strong>the</strong> spectacular<br />

failure <strong>of</strong> <strong>the</strong> predictions <strong>of</strong> <strong>the</strong> Club <strong>of</strong> Rome, expressed<br />

in <strong>the</strong>ir 1972 book “Limits to Growth”, has become evident, it is recognised<br />

that <strong>the</strong> photovoltaic energy harvesting seems to be <strong>the</strong><br />

only branch able to provide cheap and clean energy in <strong>the</strong> future<br />

[1]. This is because <strong>the</strong> o<strong>the</strong>r above-mentioned “green” technologies<br />

have not enough growth potential and area <strong>of</strong> availability to<br />

be regarded major sources <strong>of</strong> cheap energy for <strong>the</strong> future.<br />

Energy production from renewable sources encompass still<br />

emerging technologies. They constitute only 6% <strong>of</strong> <strong>the</strong> global final<br />

energy consumption (2009), excluding traditional biomass [2].<br />

Their share <strong>of</strong> <strong>the</strong> global electricity consumption (2010) is higher<br />

but it is still smaller than 20%, whereas about 16% is attributed<br />

to hydroelectricity [2], <strong>the</strong> most established renewable energy<br />

source. All <strong>the</strong> o<strong>the</strong>r sources contribute around 3.3% <strong>of</strong> <strong>the</strong> global<br />

share. Obviously, fossil fuels dominate both rankings.<br />

The main parameter to characterise energy sources is <strong>the</strong><br />

average cost <strong>of</strong> production per kilowatt‐hour or megawatt-hour.<br />

Looking at <strong>the</strong> comparison <strong>of</strong> <strong>the</strong> cost [2] it is no wonder that photovoltaics<br />

are not in widespread use. The cost <strong>of</strong> electric energy<br />

from this source is 5–10 times higher than from <strong>the</strong> cheapest renewable<br />

one, i.e. hydroelectricity. It is also estimated to be around<br />

3 times more expensive than electric energy from coal [3]. Various<br />

reports give different figures on <strong>the</strong> average costs <strong>of</strong> energy, but<br />

in each <strong>of</strong> <strong>the</strong>m <strong>the</strong> tendency for photovoltaic energy to be much<br />

more expensive is clear.<br />

Solar cells will not be competitive with o<strong>the</strong>r energy sources as<br />

long as <strong>the</strong>y have not reached grid parity. This has been achieved<br />

in some locations (e.g. mountain areas), in which <strong>the</strong> cost <strong>of</strong> energy<br />

transport is substantial. Use <strong>of</strong> solar cells can appear to be<br />

<strong>the</strong> only solution in some areas without access to <strong>the</strong> grid. They<br />

can also be used to power <strong>of</strong>f-grid, self-standing devices (road<br />

and water signs, meteorological stations etc.). Despite <strong>the</strong>se niche<br />

applications a real breakthrough can only be achieved by<br />

reaching gird parity for regardless <strong>of</strong> location.<br />

Nowadays, silicon dominates <strong>the</strong> photovoltaics market as it<br />

pr<strong>of</strong>its from <strong>the</strong> microelectronic technology that has been optimised<br />

over many years. However, <strong>the</strong> high price <strong>of</strong> silicon solar<br />

cells, which is unlikely to be lowered fur<strong>the</strong>r (due to <strong>the</strong> substantial<br />

cost <strong>of</strong> <strong>the</strong> material itself, <strong>the</strong> high quality and purity required<br />

and <strong>the</strong> high energy processes involved in <strong>the</strong> production) limits<br />

<strong>the</strong>ir application and drives research towards alternative methods<br />

that would be more cost-effective [1]. The prospective solutions<br />

encompass organic, nanocrystalline and dye-sensitised solar<br />

cells (DSSC). The latter branch is <strong>the</strong> focus <strong>of</strong> this work.<br />

In silicon solar cells silicon itself serves to absorb photons as<br />

well as to separate and transport charge carriers. This means that<br />

one type <strong>of</strong> material has to be optimised with regards to three different<br />

processes. As <strong>the</strong> electron-hole pair can only be separated<br />

in <strong>the</strong> p-n junction region, a high value <strong>of</strong> diffusion length <strong>of</strong> electrons<br />

and holes is necessary for <strong>the</strong> charge carriers to reach <strong>the</strong><br />

junction. This can only be achieved by providing crystalline material<br />

<strong>of</strong> very high quality, which is expensive to produce. Silicon has<br />

a relatively small value <strong>of</strong> absorption coefficient compared to dye<br />

molecules which means that <strong>the</strong> Si wafers used to produce photovoltaic<br />

cells cannot be thinner than 300–500 µm. This makes<br />

126<br />

substantial reduction <strong>of</strong> <strong>the</strong> material amount per wafer impossible<br />

and thus maintains <strong>the</strong> high price <strong>of</strong> <strong>the</strong> device. Moreover, charge<br />

carriers generated far from <strong>the</strong> junction region can recombine at<br />

defects <strong>of</strong> crystalline lattice and at <strong>the</strong> surface. These remarks<br />

show contradicting requirements to be met by silicon that are limiting<br />

<strong>the</strong> performance <strong>of</strong> Si-based PV cells.<br />

Overview <strong>of</strong> Dye-Sensitised Solar Cells<br />

Structure and Principles <strong>of</strong> Operation<br />

Dye-sensitised solar cell (DSSC) are based on <strong>the</strong> concept <strong>of</strong><br />

separating <strong>the</strong> elements responsible for absorption <strong>of</strong> photons,<br />

separation and transport <strong>of</strong> charge carriers and thus let <strong>the</strong>m be<br />

optimised separately. For this various types <strong>of</strong> material must be<br />

used. The principles <strong>of</strong> operation are similar to <strong>the</strong> process <strong>of</strong><br />

photosyn<strong>the</strong>sis taking place in green plants. The main parts <strong>of</strong> a<br />

dye-sensitised solar cell in <strong>the</strong> configuration proposed by Grätzel<br />

[4] (Fig.1) include a substrate plate (transparent to visible light)<br />

covered with a thin conductive oxide (TCO) layer (usually indium<br />

tin oxide), a wide-band-gap semiconductor film (usually titanium<br />

dioxide), to which dyes attach, a liquid electrolyte layer (typically<br />

an iodide/triiodide redox couple) and a counter electrode. A photon<br />

<strong>of</strong> energy corresponding to <strong>the</strong> visible range passes through<br />

both <strong>the</strong> upper substrate and <strong>the</strong> semiconductor to be absorbed<br />

by <strong>the</strong> dye (Fig. 1, (1)). The electron <strong>of</strong> <strong>the</strong> chromophore that has<br />

been excited (S S* transition) is subsequently injected into <strong>the</strong><br />

conduction band <strong>of</strong> <strong>the</strong> semiconductor (e cb<br />

) (2) and fur<strong>the</strong>r to <strong>the</strong><br />

outer circuit. This process is only possible if <strong>the</strong> LUMO level <strong>of</strong> <strong>the</strong><br />

dye lies above or at <strong>the</strong> same energy as <strong>the</strong> conduction band <strong>of</strong><br />

<strong>the</strong> semiconductor. The ionised dye (S + state) is <strong>the</strong>n regenerated<br />

from <strong>the</strong> electrolyte via a redox reaction between <strong>the</strong> dye molecule<br />

and <strong>the</strong> counter electrode. Iodide ions (I - ) transmit electrons to <strong>the</strong><br />

sensitiser transforming into triiodide (I 3-<br />

) (3). By means <strong>of</strong> diffusion<br />

in <strong>the</strong> electrolyte triiodide eventually reaches <strong>the</strong> counter electrode<br />

where it is reduced back into iodide (4). The electrons reducing<br />

<strong>the</strong> triiodide (e Pt<br />

) come from <strong>the</strong> external circuit after losing energy<br />

through resistance. As in <strong>the</strong> case <strong>of</strong> (2), <strong>the</strong> processes (3)<br />

and (4) occur only when <strong>the</strong> energy level <strong>of</strong> electrolyte is equal or<br />

higher than <strong>the</strong> ground state <strong>of</strong> <strong>the</strong> dye. The maximum energy per<br />

electron that can be obtained from <strong>the</strong> cell (or open circuit voltage<br />

V oc<br />

in voltage scale) is <strong>the</strong> difference between Fermi level <strong>of</strong> <strong>the</strong><br />

semiconductor and <strong>the</strong> redox potential <strong>of</strong> <strong>the</strong> electrolyte.<br />

In <strong>the</strong> Grätzel design separate elements associated with <strong>the</strong><br />

fundamental processes in <strong>the</strong> cell operation can be differentiated.<br />

The dye molecule is responsible for <strong>the</strong> light absorption. Charge<br />

separation takes place at <strong>the</strong> dye-semiconductor and dye-electrolyte<br />

interfaces for electrons and holes, respectively. Electrons are<br />

transported in <strong>the</strong> conduction band <strong>of</strong> <strong>the</strong> semiconductor, whereas<br />

holes are carried by <strong>the</strong> electrolyte by means <strong>of</strong> redox reaction.<br />

The basic Grätzel setup can be fur<strong>the</strong>r investigated and improved.<br />

The materials utilised can be changed as long as <strong>the</strong>y<br />

are able to play <strong>the</strong>ir given roles. O<strong>the</strong>r nanocrystalline semiconducting<br />

compounds (e.g. ZnO and SnO 2<br />

) have already been tried<br />

as electron conductors but <strong>the</strong>y have turned out to <strong>of</strong>fer reduced<br />

performance [5]. There has recently been much investigation<br />

into alternative liquid electrolytes (e.g. because <strong>of</strong> sealing issues)<br />

such as ionic liquids [6] as well as gel electrolytes [7] and solid<br />

polymeric hole conductors [7]. The latter in particular seems to<br />

be promising as polymeric hole conductors have superior performance<br />

to <strong>the</strong>ir electron‐conducting counterparts [8]. None<strong>the</strong>less,<br />

<strong>the</strong> liquid iodide electrolyte remains <strong>the</strong> most efficient for <strong>the</strong> time<br />

Elektronika 6/2012


Fig. 1. (A) Schematic representation <strong>of</strong> a dye-sensitised solar cell and<br />

(B) <strong>the</strong> chain <strong>of</strong> reactions (transmission <strong>of</strong> an electron) leading to<br />

energy conversion (see text for details) [9]<br />

being. Eventually, various dyes can be tried out in <strong>the</strong> setup to<br />

improve <strong>the</strong> efficiency <strong>of</strong> <strong>the</strong> photon absorption and charge separation<br />

steps.<br />

The Concept <strong>of</strong> Bulk Heterojunction<br />

Chromophores in DSSCs have much greater absorption coefficient<br />

than crystalline silicon. However, a single dye layer on <strong>the</strong><br />

semiconductor surface has proven not to be sufficient. This is because<br />

full coverage <strong>of</strong> <strong>the</strong> surface is difficult to achieve and, what<br />

is more, <strong>the</strong> optically-active area <strong>of</strong> <strong>the</strong> molecule is smaller than<br />

<strong>the</strong> molecule itself. Thus a thicker layer <strong>of</strong> <strong>the</strong> chromophores is<br />

needed. As a junction in planar configuration is not able to meet<br />

this requirement, <strong>the</strong> concept <strong>of</strong> a “bulk (hetero)junction” [1] has<br />

been introduced (also used in all-organic solar cells [10]) (Fig. 2).<br />

As <strong>the</strong> molecules need to have contact with both <strong>the</strong> semiconductor<br />

and <strong>the</strong> electrolyte in order to be able to play <strong>the</strong>ir role,<br />

<strong>the</strong> bulk junction effect is achieved by using mesoscopic titanium<br />

dioxide nanocrystals. This leads to a tremendous increase in <strong>the</strong><br />

interface surface (approximately by a factor <strong>of</strong> 1000). Because <strong>of</strong><br />

this, light rays propagating perpendicularly to <strong>the</strong> surface cross<br />

many molecules increase <strong>the</strong> chance <strong>of</strong> incident photons being<br />

absorbed. Experimental results show that 97.5% <strong>of</strong> <strong>the</strong> 500 nm<br />

light is absorbed by a 12 μm dyed mesoporous titania film [11].<br />

However, too thick a layer forces electrons to make a longer way<br />

to be withdrawn from <strong>the</strong> cell, increasing <strong>the</strong> probability <strong>of</strong> recombination.<br />

Moreover, it slows down diffusion inside <strong>the</strong> pores<br />

– experimental results show that <strong>the</strong> diffusion coefficient <strong>of</strong> <strong>the</strong> I − 3<br />

ion in acetonitrile in a 55% porous TiO 2<br />

membrane decreases by<br />

an order <strong>of</strong> magnitude with respect to free diffusion in <strong>the</strong> same<br />

solvent [5]. Because <strong>of</strong> this trade-<strong>of</strong>f, precise optimisation <strong>of</strong> <strong>the</strong><br />

thickness <strong>of</strong> <strong>the</strong> titania layer is necessary.<br />

Fig. 2. Schematic representation <strong>of</strong> <strong>the</strong> concept <strong>of</strong> a bulk (hetero‐)<br />

junction [13]<br />

Processes at <strong>the</strong> Semiconductor-Chromophore-<br />

Electrolyte Interface<br />

The previously described working cycle <strong>of</strong> a DSSC is <strong>the</strong> ideal<br />

mechanism <strong>of</strong> energy generation. There are also many sources<br />

<strong>of</strong> losses that have to be taken into account. One <strong>of</strong> <strong>the</strong>m is associated<br />

with <strong>the</strong> semiconductor-dye-electrolyte interface. There<br />

are some more “parasitic” pathways that electrons can take, thus<br />

decreasing <strong>the</strong> cell performance (Fig. 3) [12]. After having excited<br />

<strong>the</strong> chromophore, electrons are injected to <strong>the</strong> conduction band <strong>of</strong><br />

<strong>the</strong> semiconductor (Fig. 3, τ inject<br />

). The process is typically very competitive<br />

with <strong>the</strong> o<strong>the</strong>r paths <strong>of</strong> electron relaxation. Afterwards <strong>the</strong><br />

electron delocalises into <strong>the</strong> bulk (τ bulk<br />

), relaxing simultaneously via<br />

vibration (τ relax<br />

). If <strong>the</strong> electron <strong>the</strong>rmalises faster than it delocalises<br />

in <strong>the</strong> bulk, it may remain on <strong>the</strong> surface, which <strong>of</strong>ten supports<br />

surface states due to unsaturated surface bonds or lattice defects.<br />

The electron can also return from <strong>the</strong> bulk as <strong>the</strong> surface area is <strong>of</strong><br />

<strong>the</strong> semiconductor is large by design (see Fig. 2) and because it<br />

is attracted by <strong>the</strong> positively charged chromophore. There are two<br />

additional pathways <strong>of</strong> relaxation from <strong>the</strong> surface states: <strong>the</strong> electron<br />

can ei<strong>the</strong>r regenerate a chromophore (τ chrom.<br />

) or be accepted<br />

by a triiodide molecule in <strong>the</strong> electrolyte (τ electr.<br />

). In both cases <strong>the</strong><br />

cell is short-circuited and electron energy is lost. The focus <strong>of</strong> this<br />

work is highly related to <strong>the</strong> subtle interplay between <strong>the</strong> abovementioned<br />

elements because this is <strong>the</strong> key to <strong>the</strong> kinetics <strong>of</strong> <strong>the</strong><br />

cell operation.<br />

Fig. 3. (A) Processes taking place at <strong>the</strong> semiconductor-dye-electrolyte interface (numbers are <strong>the</strong> continuation after Fig. 1) and <strong>the</strong> names <strong>of</strong><br />

corresponding time constants τ (see text) along with (B) simulated values <strong>of</strong> <strong>the</strong> time constants [12]<br />

Elektronika 6/2012 127


Dyes in Dye-Sensitised Solar Cells<br />

There are certain requirements that dye molecules have to meet<br />

in order to be applicable in DSSCs. Obviously, <strong>the</strong>y must have<br />

strong absorption over <strong>the</strong> visible spectrum and preferably also in<br />

near IR, that carries around 47% <strong>of</strong> energy <strong>of</strong> <strong>the</strong> Sun. Moreover,<br />

<strong>the</strong> dyes have to be stable both in <strong>the</strong>ir ground and excited state<br />

as <strong>the</strong>y have to withstand millions <strong>of</strong> redox turnovers. Fur<strong>the</strong>rmore,<br />

<strong>the</strong>y must have suitable energy levels, both excited and<br />

ground, in relation to <strong>the</strong> semiconductor conduction band edge<br />

and redox charge mediator, respectively.<br />

The first compounds that were used as sensitisers were organometallics.<br />

They were followed later on by purely organic chromophores.<br />

The latter are preferred because <strong>the</strong>y do not contain<br />

transition metals ions and thus can be potentially cheaper to<br />

mass produce. The mechanism <strong>of</strong> sensitisations differs between<br />

<strong>the</strong> two families <strong>of</strong> compounds: purely organic conjugated molecules<br />

exhibit π π* excitation whereas transition-metal/ligand<br />

complexes transfer an electron from a non-bonding lone pair d-<br />

orbital <strong>of</strong> metal to a π* orbital <strong>of</strong> ligand. There are some minor differences<br />

in <strong>the</strong> mechanism <strong>of</strong> sensitisation between <strong>the</strong> two chromophore<br />

families [12] but both <strong>of</strong> <strong>the</strong>m have been experimentally<br />

proven to be useful in DSSC. For <strong>the</strong> time being organometallics<br />

bring slightly better results.<br />

In order to minimise losses inside <strong>the</strong> cell, electron transfer<br />

from <strong>the</strong> dye to <strong>the</strong> semiconductor and fur<strong>the</strong>r to <strong>the</strong> outer circuit<br />

must be rendered much more competitive than <strong>the</strong> o<strong>the</strong>r “parasitic”<br />

electron pathways. One approach to this challenge is to investigate<br />

<strong>the</strong> link between <strong>the</strong> chromophore and <strong>the</strong> surface <strong>of</strong><br />

<strong>the</strong> titania. Proper anchoring conditions have to provide an ultra<br />

fast electron path. The carboxylic (–COOH) group that is responsible<br />

for this process has been included in all <strong>the</strong> chromophores<br />

for DSSC applications up to now and it has shown to bring good<br />

results. Ano<strong>the</strong>r solution to achieve more competitive sensitisation<br />

and electron transfer is to slow down <strong>the</strong> “parasitic” electron<br />

pathways, i.e. backtransfer to <strong>the</strong> electrolyte and to <strong>the</strong> ground<br />

state <strong>of</strong> <strong>the</strong> ionised dye molecule (Fig. 3A). The former in particular<br />

has turned out to be harmful to <strong>the</strong> operation <strong>of</strong> <strong>the</strong> cell (Fig.<br />

3B). An approach proposed to overcome this problem includes<br />

engineering <strong>the</strong> molecules to contain additional parts responsible<br />

for blocking access <strong>of</strong> <strong>the</strong> electrolyte to <strong>the</strong> surface [14, 15].<br />

Recently a new molecular design for a chromophore has<br />

been proposed [15, 16], according to which molecules should<br />

involve electron donating group(s), π conjugated bridge, electron<br />

withdrawing group(s) and anchoring group (D-π-A, in <strong>the</strong><br />

given order). In <strong>the</strong> ground state <strong>the</strong> electron density is higher<br />

at <strong>the</strong> electron donating region whereas upon excitation it is<br />

moved to <strong>the</strong> acceptor group and thus towards <strong>the</strong> semiconductor<br />

surface. In this way electron approaches <strong>the</strong> surface when<br />

entering <strong>the</strong> excited state what is believed to facilitate its transfer<br />

to <strong>the</strong> semiconductor. The D-π-A design also has ano<strong>the</strong>r<br />

advantage: slowing down <strong>the</strong> “parasitic” electron backtransfer<br />

from <strong>the</strong> conduction band <strong>of</strong> <strong>the</strong> semiconductor to <strong>the</strong> ionised<br />

chromophore. Namely, when <strong>the</strong> electron has been withdrawn<br />

from <strong>the</strong> dye, <strong>the</strong> electron density in <strong>the</strong> region <strong>of</strong> <strong>the</strong> electron<br />

donating group, that is far away from <strong>the</strong> semiconductor surface,<br />

decreases. Thus it is more probable for <strong>the</strong> chromophore to<br />

be regenerated by <strong>the</strong> electrolyte than by <strong>the</strong> electron trapped<br />

at <strong>the</strong> semiconductor surface.<br />

Experimental section<br />

The authors will investigate o<strong>the</strong>r possible anchoring groups, inter<br />

alia sulfonate group (‐SO ‐ ), to check whe<strong>the</strong>r <strong>the</strong>y can render<br />

3<br />

electron transfer to <strong>the</strong> outer circuit more competitive, as this topic<br />

has not been studied extensively. Besides that we will focus<br />

on <strong>the</strong> structure-properties relations <strong>of</strong> <strong>the</strong> chromophore. We will<br />

investigate a few families <strong>of</strong> molecules with a relatively simple<br />

common backbone. They are highly conjugated and <strong>the</strong>y involve<br />

groups donating or withdrawing electrons via resonance. Examp-<br />

D r<br />

Fig. 4. The backbones <strong>of</strong> <strong>the</strong> examples <strong>of</strong> families <strong>of</strong> <strong>the</strong> compounds<br />

to be investigated experimentally. A r<br />

= EWG r<br />

– electron withdrawing<br />

group; D r<br />

= EDG r<br />

= electron donating group; <strong>the</strong> subscript<br />

r stands for resonance; R is substituted by benzene or naphthalene<br />

derivatives<br />

les <strong>of</strong> families that have been chosen are shown in Fig. 4. Various<br />

substituents will be tried out to check <strong>the</strong>ir influence on <strong>the</strong> cell<br />

performance. This, in conjunction with <strong>the</strong> relative simplicity <strong>of</strong><br />

<strong>the</strong> molecules is believed to let <strong>the</strong> relation between structure and<br />

properties <strong>of</strong> <strong>the</strong> dyes be scrutinised. Basing on <strong>the</strong> results hopefully<br />

some hints to <strong>the</strong> chromophore design will be formulated.<br />

The authors start by crystallising <strong>the</strong> chosen compounds in<br />

order to perform X-ray diffraction experiments on <strong>the</strong>m, which<br />

allows a precise description <strong>of</strong> <strong>the</strong> structure <strong>of</strong> <strong>the</strong> compound.<br />

Absorption spectra <strong>of</strong> <strong>the</strong> investigated chromophores both in solution<br />

as well as anchored to <strong>the</strong> mesoporous titania surface will<br />

also be collected. Then a complete solar cell will be fabricated<br />

and characterised. All <strong>the</strong>se result will allow us to see whe<strong>the</strong>r <strong>the</strong><br />

chosen chromophores are suited for use in PV cells.<br />

Of course, besides <strong>the</strong> processes at <strong>the</strong> semiconductor-choromophore-electrolyte<br />

interface, <strong>the</strong>re are many more factors influencing<br />

<strong>the</strong> cell performance. They involve working temperature,<br />

electrolyte conductivity, substrate conductivity, cell width and cell<br />

contacts, counter electrode, iodine concentration, mass transport<br />

conditions as well as titania layer thickness, to name <strong>the</strong> most<br />

important [11], which are beyond <strong>the</strong> scope <strong>of</strong> this work.<br />

References<br />

SO 3<br />

-<br />

A r<br />

[1] Grätzel M.: Photovoltaic and photoelectrochemical conversion <strong>of</strong> solar<br />

energy, Phil. Trans. R. Soc. A 365, 993 (2007).<br />

[2] REN21, Renewable Energy Policy Network for <strong>the</strong> 21 st century, Renewables<br />

2011 – Global Status Report, www.ren21.net<br />

[3] United States Energy Information Administration, Annual Energy Outlook<br />

2011, www.eia.gov<br />

[4] Oregan, B.; Grätzel, M.: A low-cost, high-efficienty solar-cell based<br />

on dye-sensitized colloidal TiO 2<br />

films, Nature 1991, 353, 737–740.<br />

[5] Low K. S.: PhD progress report, University <strong>of</strong> Cambridge, Cavendish<br />

Laboratory, Department <strong>of</strong> Physics, Structure and Dynamics Group<br />

(2009).<br />

[6] Desilvestro H.: Ionic Liquids – The Panacea for Dye Solar Cells?,<br />

Dyesol Ltd (2007), www.dyesol.com<br />

[7] Nogueira A.F., C. Longo, M.-A. De Paoli: Polymers in dye sensitized<br />

solar cells: overview and perspectives, Coordination Chemistry Reviews<br />

248 (2004) 1455–1468.<br />

[8] Weiss D. S., M. Abkowitz: Advances in Organic Photoconductor<br />

Technology, Chem. Rev. 110 (2010), 479<br />

[9] Longo C. et al.: DSSCs: A Successful Combination <strong>of</strong> Materials, J.<br />

Braz. Chem. Soc., 14, 889-901, (2003).<br />

[10] Roncali J.: Molecular Bulk Heterojunctions: An Emerging Approach<br />

to Organic Solar Cells, Acc. <strong>of</strong> Chemical Research, vol. 42, No. 11<br />

(2009) 1719-1730.<br />

[11] Desilvestro H.: What Physical Factors Affect Current-Voltage Characteristics<br />

<strong>of</strong> Dye Solar Cells?, Dyesol Ltd (2008), www.dyesol.com<br />

[12] Prezhdo O. V., W. R. Duncan, V. V. Prezhdo: Dynamics <strong>of</strong> <strong>the</strong> Photoexcited<br />

Electron at <strong>the</strong> Chromophore–Semiconductor Interface, Acc.<br />

Chem. Res. 41, 339 (2008).<br />

[13] G. Smestad, Sol. Energy Mater. Sol. Cells 55, 157 (1998).<br />

[14] Justin Thomas K. R. et al.: 2,3-Disubstituted Thiophene-Based Organic<br />

Dyes for Solar Cells, Chem. Mater. (2008) 20, 1830–1840.<br />

[15] Mingfei Xu et al.: Energy-Level and Molecular Engineering <strong>of</strong> Organic<br />

D-π-A Sensitizers in Dye‐Sensitized Solar Cells, J. Phys. Chem. C<br />

(2008), 112, 19770–19776.<br />

[16] Masataka Katono et al.: D-π-A Dye System Containing Cyano-Benzoic<br />

Acid as Anchoring Group for Dye-Sensitized Solar Cells, Langmuir<br />

(2011) 27, 14248–14252.<br />

Dr<br />

N<br />

-<br />

O 3 S SO 3<br />

-<br />

N<br />

R<br />

128<br />

Elektronika 6/2012


Azaheterocyclic materials for organic photovoltaic cells<br />

Natalia Nosidlak 1) , Monika Pokladko-Kowar 1) , Ewa Gondek 1) , Andrzej Danel 2) ,<br />

Jerzy Sanetra 1<br />

1)<br />

Institute <strong>of</strong> Physics, Cracow University <strong>of</strong> Technology, Krakow, Poland<br />

2)<br />

Department <strong>of</strong> Chemistry, University <strong>of</strong> Agriculture, Krakow, Poland<br />

Recently we can observe growing interest in organic photovoltaic<br />

cells. Nowadays, when <strong>the</strong> development <strong>of</strong> renewable sources <strong>of</strong><br />

energy is necessary, photovoltaic energy becomes an opportunity<br />

to reduce consumption <strong>of</strong> fossil fuels. Organic photovoltaic cells<br />

are easy built by deposition techniques like spin coating, moreover<br />

PV devices have lower weight, flexible shape and low production<br />

cost in comparison with inorganic equivalents [1]. These<br />

features have fuelled <strong>the</strong> interest <strong>of</strong> both science and industry.<br />

The conversion <strong>of</strong> solar light into electric power requires <strong>the</strong><br />

generation <strong>of</strong> both negative and positive charges. The operation<br />

<strong>of</strong> a photovoltaic cell can be generally divided into three basic<br />

steps: light absorption, charge separation and charge collection<br />

<strong>of</strong> <strong>the</strong> appropriate carriers to anode and metallic cathode.<br />

(2)<br />

where P light<br />

– power irradiated at cell surface unit [3].<br />

Experiment<br />

For <strong>the</strong> purpose <strong>of</strong> this article <strong>the</strong> regioregular P3OT (poly<br />

3-octylthiophene) (Fig. 2) with HOMO level <strong>of</strong> about – 5,2 eV and<br />

LUMO level about – 2,85 eV, as polymer matrice were choosen.<br />

conversion step<br />

light absorption<br />

exciton creation<br />

exciton diffusion<br />

charge separation<br />

charge transport<br />

charge collection<br />

INCYDENT PHOTONS<br />

reflection and transmission<br />

recombination <strong>of</strong> charges<br />

limited mobility <strong>of</strong> charges<br />

recombination near electrodes<br />

barriers at electrodes<br />

SEPARATED CHARGE AT ELECTRODES<br />

loss mechanism<br />

recombination <strong>of</strong> excitons<br />

exciton transfer<br />

with subsequent recombination <strong>of</strong> excitons<br />

no charge separation<br />

and subsequent recombination <strong>of</strong> excitons<br />

Fig. 1. The specific conversion steps and loss mechanisms in an organic<br />

solar cell<br />

When photons are absorbed, only small part <strong>of</strong> incident light<br />

is absorbed because <strong>of</strong> size <strong>of</strong> band gap. Required band gap is<br />

about 1,1 eV, when most <strong>of</strong> organic semiconductors has 2.0 eV.<br />

This fact reduce possibility <strong>of</strong> absorption to 30%. Also some part<br />

<strong>of</strong> incident light is lost because <strong>of</strong> reflection and transmission.<br />

In case <strong>of</strong> exciton creation and diffusion, loss mechanism includes<br />

recombination <strong>of</strong> excitons. Length <strong>of</strong> diffusion should be<br />

equal to thickness <strong>of</strong> film, but is about 10nm when thickness <strong>of</strong><br />

film is about 100 nm.<br />

In <strong>the</strong> next step <strong>of</strong> conversion loss mechanism includes recombination<br />

<strong>of</strong> excitons but also cases when charges are no<br />

separated. This situation have place when difference between<br />

ionization potential and electron affinity is insufficient and excitons<br />

are able to jump into <strong>the</strong> material with a lower band gap<br />

without charge separation. Charge transport efficiency is limited<br />

by recombination especially when applied material used as transporting<br />

medium is same for electrons and holes. The last step <strong>of</strong><br />

conversion is charge collection on electrodes. Losses here are<br />

caused by recombination near electrodes and by potential barrier<br />

<strong>of</strong> electrodes [2].<br />

Parameters describing fulfilled solar cells: V OC<br />

– open circuit<br />

voltage, I SC<br />

– short circuit current, I mpp<br />

, V mpp<br />

– voltage and current<br />

at <strong>the</strong> maximum power point, FF – fill factor determined by:<br />

(1)<br />

η – energy conversion efficiency calculated by <strong>the</strong> following formula:<br />

Fig. 2. The principal molecular chemical formula for poly(3-octylthiophene)<br />

The active layer <strong>of</strong> solar cells contain P3OT and three kind<br />

<strong>of</strong> 1H- pyrazolo[3,4-b]quinoxalines: PAQX1, PAQX2, PAQX3,<br />

PAQX4 (Fig. 3).<br />

a) b)<br />

Ph<br />

Ph<br />

N<br />

N<br />

N<br />

N<br />

N<br />

CH 3<br />

c) d)<br />

H 3<br />

C<br />

N C H 3<br />

F N CH 3<br />

M eO N C H 3<br />

N<br />

N<br />

N<br />

Fig. 3. Investigated chromophore molecules <strong>of</strong>: a) PAQX1, b) PAQX2,<br />

c) PAQX4, d) PAQX3<br />

Photovoltaic cells were fabricated on ITO (Indium Tin Oxide)<br />

covered glass slides (15×15 mm) which were cleaned in an ultrasonic<br />

bath using organic solvents. Next <strong>the</strong> ITO was covered<br />

with PEDOT:PSS thin film by spin-coating and left for 30 minutes<br />

in vacuum heater at 70 o C. After 30 minutes <strong>the</strong> active layer<br />

(P3OT + PQXx) was spinned over. At <strong>the</strong> end photovoltaic cells<br />

were defined by aluminum electrodes <strong>the</strong>rmally evaporated in<br />

vacuum. The device has following architecture: ITO/PEDOT:PSS<br />

(poly(3,4-ethylene dioxythiophene)-poly-(styrene sulphonate)/<br />

active layer/Al. Aluminum electrode acts as cathode (collecting<br />

N<br />

N<br />

N<br />

N<br />

N<br />

N<br />

Elektronika 6/2012 129


I[µA/cm 2 ]<br />

15<br />

10<br />

5<br />

0<br />

-5<br />

-10<br />

I[µA/cm 2 ]<br />

15<br />

10<br />

5<br />

0<br />

-5<br />

-10<br />

Fig. 4. Typical architecture <strong>of</strong> <strong>the</strong> photovoltaic device<br />

electrons) while ITO is applied as <strong>the</strong> anode for <strong>the</strong> purposes<br />

<strong>of</strong> collecting holes. Active layer thickness is about 150 nm. The<br />

overall architecture <strong>of</strong> <strong>the</strong> device is presented in Fig. 4.<br />

Results<br />

Current-voltage (I-V) characteristics <strong>of</strong> <strong>the</strong> device were measured<br />

using <strong>the</strong> Keithley 2400 source-meter. Device was illuminated<br />

with maximum intensity <strong>of</strong> 1.3 mW/cm 2 . I-V characteristics after<br />

illumination are given in <strong>the</strong> Fig. 5, 6, 7 and 8.<br />

-15<br />

-20<br />

-0,5 0,0 0,5 1,0 1,5<br />

V[V]<br />

a) b)<br />

Fig. 7. I–V characteristic <strong>of</strong>: a) ITO/PAQX3+P3OT/Al, b) ITO/PEDOT:<br />

PSS/PAQX3+P3OT/Al<br />

30<br />

20<br />

-15<br />

-20<br />

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0<br />

15<br />

10<br />

5<br />

V[V]<br />

10<br />

5<br />

15<br />

10<br />

I[µA/cm 2 ]<br />

10<br />

0<br />

-10<br />

I [µΑ/cm 2 ]<br />

0<br />

-5<br />

-10<br />

I[µA/cm 2 ]<br />

0<br />

-5<br />

-10<br />

-15<br />

-20<br />

-0,5 0,0 0,5 1,0 1,5<br />

V[V]<br />

a) b)<br />

-25<br />

-0,5 0,0 0,5 1,0 1,5<br />

V[V]<br />

Fig. 5. I–V characteristic <strong>of</strong>: a) ITO/PAQX1+P3OT/Al, b) ITO/PEDOT:<br />

PSS/PAQX1+P3OT/Al<br />

I[ µ A/cm2]<br />

5<br />

0<br />

-5<br />

-10<br />

-15<br />

-20<br />

-20<br />

-30<br />

-0,5 0,0 0,5 1,0 1,5<br />

V[V]<br />

a) b)<br />

Fig. 8. I–V characteristic <strong>of</strong>: a) ITO/PAQX4+P3OT/Al, b) ITO/PEDOT:<br />

PSS/PAQX4+P3OT/Al<br />

Solar cells parameters extracted from I–V characteristics in <strong>the</strong><br />

Fig. 5, 6, 7 and 8.<br />

-15<br />

-20<br />

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0<br />

U[V]<br />

I[µA/cm 2 ]<br />

15<br />

10<br />

5<br />

0<br />

-5<br />

-10<br />

-15<br />

IµA/cm 2 ]<br />

15<br />

10<br />

5<br />

0<br />

-5<br />

-10<br />

-15<br />

Cell V oc<br />

[V] I SC<br />

[μA/cm 2 ] FF η (%)<br />

ITO/PAQX1+P3OT/Al 0,8 12,24 0,27 0,2<br />

ITO/PEDOT:PSS/PAQX1+P3OT/Al 1,13 18,2 0,17 0,27<br />

ITO/PAQX2+P3OT/Al 0,58 8,45 0,23 0,086<br />

ITO/PEDOT:PSS/PAQX2+P3OT/Al 0,9 10,36 0,24 0,17<br />

ITO/Z1+P3OT/Al 0,59 5,39 0,22 0,053<br />

-20<br />

-0,5 0,0 0,5 1,0 1,5<br />

V[V]<br />

a) b)<br />

-20<br />

-0,5 0,0 0,5 1,0 1,5 2,0<br />

Fig. 6. I–V characteristic <strong>of</strong>: a) ITO/PAQX2+P3OT/Al, b) ITO/PEDOT:<br />

PSS/PAQX2+P3OT/Al<br />

V[V]<br />

ITO/PEDOT:PSS/Z1+P3OT/Al 1,22 5,12 0,16 0,076<br />

ITO/PAQX4+P3OT/Al 0,86 4,29 0,19 0,053<br />

ITO/PEDOT:PSS/PAQX4+P3OT/Al 0,56 10,06 0,25 0,11<br />

Summary<br />

Conjugated PEDOT:PSS polymers have much lower mobility<br />

than inorganic semiconductors and are traditionally better for<br />

conducting holes than electrons. Typical hole mobilities range<br />

are about 0.001 cm 2 V -1 s -1 [1]. Additional layer <strong>of</strong> PEDOT:PSS<br />

facilitates transport <strong>of</strong> charge carriers to electrode. As one can<br />

see in cells with PEDOT:PSS layer energy conversion efficiency<br />

increased in comparison to <strong>the</strong> cell with <strong>the</strong> same active layer.<br />

We obtained <strong>the</strong> best energy conversion efficiency for <strong>the</strong> architecture<br />

ITO/PEDOT:PSS/PAQX1+P3OT/Al. Results <strong>of</strong> this work<br />

proved that photovoltaic cells need fur<strong>the</strong>r modification in order<br />

to increase energy conversion efficiency.<br />

References<br />

[1] Gondek E., A. Danel, I. V. Kityk, J Mater Sci: Mater Elctron (2009)<br />

20:461-468.<br />

[2] Sanetra J.: Efekt fotowoltaiczny w organicznych ogniwach<br />

słonecznych – wybrane zagadnienia. PK, Krakow, 2006.<br />

[3] Gondek E., I. V. Kityk, A. Danel, J. Sanetra: SpectrochImica Acta Part<br />

A 70 (2008) 117–121.<br />

130<br />

Elektronika 6/2012


The influence <strong>of</strong> inclination and azimuth angle<br />

<strong>of</strong> PV modules on <strong>the</strong> energetic gain<br />

Artur Bugała, Grażyna Frydrychowicz-Jastrzębska<br />

Poznań University <strong>of</strong> Technology<br />

The angle <strong>of</strong> incidence <strong>of</strong> solar radiation on <strong>the</strong> PV panel or plane<br />

is a function on many factors: <strong>the</strong> angle <strong>of</strong> solar declination, <strong>the</strong><br />

angle <strong>of</strong> latitude, <strong>the</strong> hour angle, <strong>the</strong> azimuth angle and <strong>the</strong> angle<br />

<strong>of</strong> receiver inclination to <strong>the</strong> ground.<br />

The declination angle depends on <strong>the</strong> day <strong>of</strong> <strong>the</strong> year, <strong>the</strong> hour<br />

angle taking <strong>the</strong> value in accordance to <strong>the</strong> time <strong>of</strong> <strong>the</strong> day, <strong>the</strong><br />

angle <strong>of</strong> <strong>the</strong> latitude depends on <strong>the</strong> location. The next parameter,<br />

that is <strong>the</strong> azimuth angle <strong>of</strong> <strong>the</strong> receiver is <strong>the</strong> deflection accounted<br />

from <strong>the</strong> local meridian to <strong>the</strong> south direction [4]. Dependence<br />

<strong>of</strong> <strong>the</strong> energy possible from Sun on <strong>the</strong> above mentioned parameters<br />

may be reduced by optimal orientation <strong>of</strong> <strong>the</strong> PV panel. Many<br />

authors dealt with this problem [1, 2, 3, 4, 6, 7].<br />

Density <strong>of</strong> solar radiation flux<br />

Total solar radiation is a hemispherical radiation, achieving <strong>the</strong><br />

receiver surface <strong>of</strong> any spatial orientation, from <strong>the</strong> solid angle<br />

in <strong>the</strong> range 2π [sr], supplemented by <strong>the</strong> component reflected<br />

from <strong>the</strong> ground and from <strong>the</strong> objects surrounding <strong>the</strong> receiver. In<br />

particular case <strong>of</strong> a receiver located in parallel to <strong>the</strong> ground <strong>the</strong><br />

reflected component does not exist. Direction <strong>of</strong> incidence <strong>of</strong> <strong>the</strong><br />

direct radiation at an inclined surface is evident – it is identical to<br />

<strong>the</strong> direction <strong>of</strong> <strong>the</strong> radiation beam.<br />

In case <strong>of</strong> <strong>the</strong> receiver that is inclined with respect to <strong>the</strong> ground<br />

proper definition <strong>of</strong> <strong>the</strong> diffused radiation is <strong>the</strong> most difficult.<br />

Three components should be specified in this radiation:<br />

– isotropic radiation;<br />

– circumsolar radiation;<br />

– <strong>the</strong> radiation coming from <strong>the</strong> bright horizon.<br />

The first <strong>of</strong> <strong>the</strong>se components is distinguished by <strong>the</strong> fact that it<br />

comes uniformly from <strong>the</strong> whole hemisphere. On <strong>the</strong> o<strong>the</strong>r hand,<br />

<strong>the</strong> circumsolar component is a result <strong>of</strong> diffusion <strong>of</strong> <strong>the</strong> solar radiation.<br />

Never<strong>the</strong>less, it is specifically related to <strong>the</strong> direct component.<br />

The third component is focused near to <strong>the</strong> horizon and<br />

observable in case <strong>of</strong> clear sky.<br />

Additionally, in case <strong>of</strong> <strong>the</strong> receiver inclined at a certain angle<br />

β≠0 to <strong>the</strong> ground <strong>the</strong> radiation reflected from <strong>the</strong> ground and surrounding<br />

objects should be considered to, apart from <strong>the</strong> direct<br />

and diffused components <strong>of</strong> radiation, all being <strong>the</strong> components<br />

<strong>of</strong> <strong>the</strong> so-called total radiation.<br />

The Authors <strong>of</strong> early computational models focused chiefly at<br />

determining <strong>the</strong> direct component, devoting considerably less attention<br />

to <strong>the</strong> o<strong>the</strong>r ones.<br />

The diffused radiation was considered as an isotropic one.<br />

Such an approach is appropriate, but only in case <strong>of</strong> <strong>the</strong> surfaces<br />

located in parallel to <strong>the</strong> surface <strong>of</strong> Earth.<br />

One <strong>of</strong> <strong>the</strong> first methods, that partially took into account <strong>the</strong><br />

effect <strong>of</strong> receiver inclination is <strong>the</strong> Liu-Jordan method [8]. The authors<br />

introduce correction factors into <strong>the</strong> calculation. Never<strong>the</strong>less,<br />

<strong>the</strong> method does not fully reflect <strong>the</strong> complex character <strong>of</strong><br />

<strong>the</strong> diffused component, as in <strong>the</strong> proposed model it is considered<br />

as an isotropic one.<br />

Since <strong>the</strong> time <strong>of</strong> development <strong>of</strong> <strong>the</strong> Liu-Jordan method, i.e.<br />

about 50 years ago, many ma<strong>the</strong>matical models have been developed,<br />

that allowed to determine <strong>the</strong> total density <strong>of</strong> <strong>the</strong> solar radiation<br />

incident on <strong>the</strong> arbitrarily oriented receiver surface. However,<br />

<strong>the</strong>y are more complex from <strong>the</strong> ma<strong>the</strong>matical point <strong>of</strong> view.<br />

The HDKR model seems to be particularly appropriate. It is<br />

a result <strong>of</strong> <strong>the</strong> methods developed by several authors, i.e. <strong>the</strong> models<br />

<strong>of</strong> J.E. Hay, J.A. Davies, T.M. Kluchera, and D.T. Reindl [5].<br />

In <strong>the</strong> first <strong>of</strong> <strong>the</strong>m <strong>the</strong> authors assume isotropic distribution<br />

<strong>of</strong> <strong>the</strong> diffused radiation, considering <strong>the</strong> diffused, isotropic, and<br />

circumsolar components. The effect <strong>of</strong> anisotropy was here taken<br />

into account only as a function <strong>of</strong> atmospheric transparency for<br />

<strong>the</strong> direct radiation. D.T. Reindl supplemented <strong>the</strong> ma<strong>the</strong>matical<br />

apparatus by <strong>the</strong> component resulting from <strong>the</strong> bright horizon.<br />

T.M Klucher inserted <strong>the</strong> effect <strong>of</strong> clouding factor to <strong>the</strong> correction<br />

factor <strong>of</strong> <strong>the</strong> bright horizon <strong>of</strong> <strong>the</strong> equation derived by Reindl. The<br />

relationship modified by him explains <strong>the</strong> considered components<br />

with sufficient accuracy, even in case <strong>of</strong> <strong>the</strong> receivers inclined at<br />

large angles with respect to <strong>the</strong> ground [5].<br />

The Liu – Jordan Method<br />

Authors <strong>of</strong> <strong>the</strong> present paper used <strong>the</strong> Liu-Jordan method. Its<br />

advantage lies in ra<strong>the</strong>r simple ma<strong>the</strong>matical apparatus [8].<br />

Moreover, comparison <strong>of</strong> <strong>the</strong> results obtained by o<strong>the</strong>r authors [3]<br />

for <strong>the</strong> calculation carried out both with <strong>the</strong> method considering<br />

anisotropy <strong>of</strong> <strong>the</strong> diffusion radiation and ignoring its effect, has<br />

shown that <strong>the</strong> results <strong>of</strong> <strong>the</strong> radiation power density in yearly<br />

scale differ only slightly. In case <strong>of</strong> <strong>the</strong> Liu-Jordan method <strong>the</strong><br />

results are a little lower as compared to <strong>the</strong> ones obtained with<br />

consideration <strong>of</strong> anisotropy.<br />

Specification <strong>of</strong> <strong>the</strong> example results obtained with <strong>the</strong> isotropic<br />

and anisotropic methods is presented in Table [3].<br />

According to <strong>the</strong> example data <strong>of</strong> Table, maximum <strong>of</strong> <strong>the</strong> function<br />

<strong>of</strong> solar radiation power density in yearly scale incident at<br />

<strong>the</strong> plane inclined and directed at appropriate azimuth angle occurs<br />

for <strong>the</strong> angles β = 30°, γ = 15° for <strong>the</strong> isotropic model, or for<br />

β = 40° and γ = 15° in case <strong>of</strong> consideration <strong>of</strong> anisotropy.<br />

Differences in <strong>the</strong> calculation results obtained with both methods<br />

usually are <strong>of</strong> <strong>the</strong> order <strong>of</strong> several percent, reaching even 1<br />

percent in case <strong>of</strong> small inclination <strong>of</strong> <strong>the</strong> receiver with respect to<br />

<strong>the</strong> ground. Maximum difference occurs in case <strong>of</strong> vertical orientation,<br />

i.e. for photovoltaic applications existing in case <strong>of</strong> façade<br />

BIPV solutions.<br />

Under our geographical and wea<strong>the</strong>r conditions <strong>the</strong> Liu-Jordan<br />

method is well justified. In summer months, when higher factor<br />

<strong>of</strong> anisotropy may be expected, <strong>the</strong> optimal inclination angle is<br />

not large, due to power gain, reaching <strong>the</strong> values <strong>of</strong> 25–35°. On<br />

<strong>the</strong> o<strong>the</strong>r hand, in winter conditions <strong>the</strong> optimal inclination angle<br />

amounts to 55–60°. Therefore, <strong>the</strong> isotropic Liu-Jordan model<br />

may be used with sufficient accuracy, particularly in case <strong>of</strong> lower<br />

power systems [4, 8].<br />

Tabl. Maximum power density in yearly scale falling at an inclined plane directed at appropriate azimuth angle<br />

Model<br />

The angles β<br />

and azimuth<br />

angle γ<br />

β = 20° γ = 15° β = 30° γ = 15° β = 40° γ = 15° β = 45° γ = 15° β = 60° γ = 20° β = 80° γ = 25° β = 90° γ = 30°<br />

Isotropic computation model 3870 MJ/m 2 3900 MJ/m 2 3870 MJ/m 2 3850 MJ/m 2 3600 MJ/m 2 3100 MJ/m 2 2740 MJ/m 2<br />

Anisotropic computational<br />

model<br />

4000 MJ/m 2 4100 MJ/m 2 4140 MJ/m 2 4120 MJ/m 2 3900 MJ/m 2 3450 MJ/m 2 3100 MJ/m 2<br />

Elektronika 6/2012 131


Results <strong>of</strong> computer simulation<br />

The above considerations and <strong>the</strong> relationship (1) to (6) served<br />

as a basis for a program developed with a view to making <strong>the</strong><br />

calculation and computer simulation. The hourly distribution <strong>of</strong><br />

radiation power density for recommended months, days and day<br />

hours 4–20 in Warsaw was determined.<br />

Figure 3 shows results <strong>of</strong> hourly distribution in all months<br />

<strong>of</strong> year for direct and diffused components <strong>of</strong> solar radiation in<br />

Warsaw.<br />

Fig. 1. PV installation on Frosta ro<strong>of</strong> in Bydgoszcz [foto from<br />

Authors]<br />

Fig. 3. Results <strong>of</strong> computer simulation <strong>of</strong> hourly distribution in all<br />

months <strong>of</strong> year for direct and diffused components <strong>of</strong> solar radiation<br />

in Warsaw<br />

Fig. 2. PV tracker installation Alarm System PHU communication industry<br />

Leżajsk, [photography by permission Stanisław Krupa]<br />

Figures 1 and 2 show <strong>the</strong> example <strong>of</strong> <strong>the</strong> systems in case <strong>of</strong><br />

<strong>the</strong> Polish conditions.<br />

Total density G β<br />

<strong>of</strong> solar radiation flow is a sum <strong>of</strong> components<br />

[4, 8]:<br />

G = G ⋅ R + G ⋅ R + G + G ⋅ ρ ⋅ R (1)<br />

β<br />

b<br />

b<br />

d<br />

d<br />

(<br />

b d) o o<br />

where: G b<br />

, G d<br />

– direct and diffusive component <strong>of</strong> density <strong>of</strong> solar<br />

radiation, and ρ o<br />

– coefficient <strong>of</strong> <strong>the</strong> bed reflectivity assumed from<br />

0,07 (for dry asphalt) to 0,095 (for fresh snow), R b<br />

, R d<br />

, R o<br />

– <strong>the</strong><br />

correction efficients defined below, related to direct, diffusive, and<br />

reflected components, respectively. Therefore:<br />

<br />

cos θ<br />

β<br />

Rb<br />

=<br />

cos θ<br />

Z<br />

(2)<br />

<br />

1+<br />

cos β<br />

R d<br />

=<br />

2<br />

(3)<br />

<br />

1−<br />

cos β<br />

R o<br />

=<br />

2<br />

(4)<br />

Where: Θ Z<br />

– is an angle <strong>of</strong> incidence <strong>of</strong> <strong>the</strong> radiation on a horizontal<br />

surface [3,6][fryd,jorda]:<br />

cos θ = cos φ cos δ cos ω + sin φ sin δ (5)<br />

Z<br />

Fig. 4. Hourly distribution <strong>of</strong> radiation power density falling on a horizontal<br />

plane, for γ = 0°, on July 20<br />

and Θ β<br />

– is an angle <strong>of</strong> incidence <strong>of</strong> <strong>the</strong> radiation on a plane inclined<br />

at <strong>the</strong> angle β to <strong>the</strong> ground, being a function <strong>of</strong> many variables:<br />

cos θ β = sinφ sin δ cos β − cos φ sin δ sin β sin γ +<br />

+ cos δ cosφ<br />

cos β cos ω + cos δ sinφ<br />

sin β cos γ cos ω + (6)<br />

+ cos δ sin β sin ωsin<br />

γ<br />

ϕ – <strong>the</strong> angle <strong>of</strong> latitude, δ – <strong>the</strong> declination angle, ω – <strong>the</strong> hour<br />

angle, β – <strong>the</strong> angle <strong>of</strong> receiver inclination to <strong>the</strong> ground. The G b<br />

and G d<br />

values are assessed on <strong>the</strong> grounds <strong>of</strong> many years data<br />

obtained from wea<strong>the</strong>r stations [1, 7].<br />

132<br />

Fig. 5. Hourly distribution <strong>of</strong> radiation power density falling on a horizontal<br />

plane, for γ = 0°, on January 20<br />

Elektronika 6/2012


Fig. 6. Hourly distribution <strong>of</strong> radiation power density falling on a plane<br />

inclined at <strong>the</strong> angle β = 30°, for γ = 0°, on July 20<br />

Fig. 7. Hourly distribution <strong>of</strong> radiation power density falling on a plane<br />

inclined at <strong>the</strong> angle β = 30°, for γ = 15°, on July 20<br />

Fig. 10. Power gain at different spatial settings <strong>of</strong> <strong>the</strong> receiver on<br />

August 8<br />

Figures 4, 5 show example results for particular months, considerably<br />

differing with regard to possible solar power gain, provided<br />

<strong>the</strong> receiver is set-up horizontally.<br />

The next Figures 6, 7 present <strong>the</strong> plots <strong>of</strong> solar radiation power<br />

density falling <strong>of</strong> <strong>the</strong> surface <strong>of</strong> <strong>the</strong> power receiver for its varying<br />

spatial orientation (<strong>the</strong> angles β and γ) on July 20, and <strong>the</strong> Figures<br />

8, 9 – on January 20.<br />

The Figure 10 presents <strong>the</strong> power gain in <strong>the</strong> hours from 4<br />

a.m. to 8 p.m. at different spatial settings (<strong>the</strong> angles β and γ) <strong>of</strong><br />

<strong>the</strong> receiver on August 8.<br />

Summary<br />

The above consideration and computer simulations allow to state<br />

<strong>the</strong> following:<br />

– The hourly distribution <strong>of</strong> radiation power density is significantly<br />

affected by <strong>the</strong> declination and hour angles that is clearly<br />

visible by comparing Figs. 4 and 5.<br />

– Figures 4 and 6 enable comparing values <strong>of</strong> power density<br />

<strong>of</strong> solar radiation reaching <strong>the</strong> receiver arranged horizontally<br />

and at an angle optimal with regard to energetic gain. Radiation<br />

power density possible to be gained for optimal panel<br />

angle β = 30° is 1,13 (12 a.m.) times bigger than for its horizontal<br />

set-up.<br />

– For January, power density possible to be gained for optimal<br />

angle β = 65° is nearly 1,6 (12 a.m.) times bigger than for its<br />

horizontal set-up, Figs. 5 and 8.<br />

– The effect <strong>of</strong> azimuth angle <strong>of</strong> <strong>the</strong> energetic gain is not so important<br />

like <strong>the</strong> one <strong>of</strong> <strong>the</strong> receiver set-up angle, which is visible<br />

in Figs. 6 and 7 and – 8 and 9. Never<strong>the</strong>less is should not<br />

be neglected in <strong>the</strong> calculation.<br />

References<br />

Fig. 8. Hourly distribution <strong>of</strong> radiation power density falling on a plane<br />

inclined at <strong>the</strong> angle β = 65°, for γ = 0°, on January 20<br />

Fig. 9. Hourly distribution <strong>of</strong> radiation power density falling on a plane<br />

inclined at <strong>the</strong> angle β = 65°, for γ = 15°, on January 20<br />

[1] Bzowska D., E. Kossecka:” Analiza promieniowania słonecznego<br />

w Warszawie w aspekcie energetyki słonecznej” IPPT PAN,<br />

Warszawa 4, 1993.<br />

[2] Chmielniak T.J.:” Technologie energetyczne”, Wydawnictwo Politechniki<br />

Śląskiej, Gliwice, 419–426, 2004.<br />

[3] Chwieduk D.:” Energetyka słoneczna budynku”, Arkady, 2011.<br />

[4] Frydrychowicz-Jastrzębska G.:” The effect <strong>of</strong> spatial orientation <strong>of</strong><br />

solar energy receiver on <strong>the</strong> energetic gain”, International Conference<br />

on Renewable Energies and Power Quality ICREPQ’11,<br />

Canary Island, CD, 2011.<br />

[5] Klucher T.M.: Evaluating Models to Predict Insolation to Tilted Surfaces,<br />

Solar Energy, 23, 111, 1979.<br />

[6] Kudish A.I., A. Ianetz:” Analysis <strong>of</strong> <strong>the</strong> solar radiation data for Beer<br />

Sheva Israel and its environs, Solar Energy, vol.48, Nr 2, 97–106.<br />

[7] Labouret A., M.Villoz: Energie solaire photovoltaique, Editions le<br />

Moniteur, Serie Environment et securite 3° Edition, Dunod, Paris<br />

2006.<br />

[8] Liu B.Y.H., R.C.Jordan:” The interrelationship and characteristic distribution<br />

<strong>of</strong> direct, diffuse and total solar radiation”, Solar Energy, 4, 3,<br />

1960.<br />

[9] Perez R., P. Seals, P. Ineichen:” A New Simplified Version <strong>of</strong> <strong>the</strong> Perez<br />

Diffuse Irradiance Model for Tilted Surfaces, 39, 221, 1987.<br />

[10] http://www.transport.gov.pl<br />

Elektronika 6/2012 133


Research achievements <strong>of</strong> <strong>the</strong> DSOD Lodz in <strong>the</strong> field<br />

<strong>of</strong> photovoltaics<br />

Katarzyna Znajdek, Maciej Sibiński<br />

Technical University <strong>of</strong> Lodz, Department <strong>of</strong> Semiconductor and Optoelectronic Devices<br />

The Department <strong>of</strong> Semiconductor and Optoelectronics Devices<br />

was established in 2008 as a part <strong>of</strong> <strong>the</strong> Faculty <strong>of</strong> Electrical,<br />

Electronic, Computer and Control Engineering at <strong>the</strong> Technical<br />

University <strong>of</strong> Lodz in Poland. The team <strong>of</strong> <strong>the</strong> DSOD Lodz consists<br />

<strong>of</strong> two pr<strong>of</strong>essors, two associate pr<strong>of</strong>essors, nine doctors,<br />

five PhD students and seven technical and administration employees.<br />

The Department’s research activities cover optoelectronic,<br />

micro- and nanoelectronic technologies, modeling and analysis <strong>of</strong><br />

electrical and <strong>the</strong>rmal phenomena in semiconductor devices, as<br />

well as designing and manufacturing <strong>of</strong> semiconductor devices,<br />

including solar cells. The main fields <strong>of</strong> <strong>the</strong> research interest are<br />

power semiconductor devices, high temperature electronics with<br />

emphasis on SiC technology, integrated intelligent systems, photonic<br />

systems and photovoltaic cells [1].<br />

Resources and technological base<br />

Both teaching and research activities are conducted in <strong>the</strong><br />

Department’s laboratories which enable investigation and experiments<br />

covering designing, manufacturing and testing <strong>of</strong> photovoltaic<br />

devices. Laboratories’ equipment base <strong>of</strong> <strong>the</strong> DSOD at TUL<br />

covers among o<strong>the</strong>rs:<br />

• Technological Centre <strong>of</strong> Microtechnology comprising Clean-<br />

Room laboratory, equipped with <strong>the</strong> facilities for fabrication<br />

and characterization <strong>of</strong> semiconductor or metal thin films, as<br />

well as for mapping <strong>of</strong> designed semiconductor structures.<br />

• Optoelectronics under <strong>the</strong> patronage <strong>of</strong> Corning Cable Systems<br />

Poland<br />

• Hybrid Systems Laboratory, equipped, inter alia, with <strong>the</strong><br />

pneumatic screen printer, <strong>the</strong> ball mill and <strong>the</strong> station for screen<br />

printing pastes preparation.<br />

• Photovoltaic Laboratory, fully equipped with an integrated<br />

measurement system that simulates standard test conditions<br />

STC, in accordance to <strong>the</strong> requirements <strong>of</strong> IEC 60904-1 and<br />

IEC 60904-3. The system includes C class solar simulator (in<br />

accordance to IEC 60904-9) with a silicon photodetector that<br />

controls <strong>the</strong> stability <strong>of</strong> <strong>the</strong> light source.<br />

• Specialized program SCAPS (Solar Cell Capacitance Simulator)<br />

for comprehensive simulations <strong>of</strong> solar cells dedicated to<br />

develop physical models and simulate new photovoltaic device<br />

structures.<br />

DSOD’s Laboratory <strong>of</strong> <strong>Photovoltaics</strong>, apart from technological<br />

and measuring equipment, possesses its own micro power solar<br />

plant <strong>of</strong> 2 kW p<br />

, consisted <strong>of</strong> four various types <strong>of</strong> photovoltaic arrays:<br />

– multicrystalline silicon: M-Si (6 modules <strong>of</strong> 115 W p<br />

each),<br />

– ribbon silicon: R-Si (6 modules <strong>of</strong> 110 W p<br />

each),<br />

– amorphous silicon: α-Si (6 modules <strong>of</strong> 64 W p<br />

each),<br />

– cooper indium diselenide: CIS (6 modules <strong>of</strong> 40 W p<br />

each).<br />

The installation is connected to <strong>the</strong> internal grid through Sunny<br />

Boy inverters, independently for each array type, which are<br />

<strong>the</strong>n attached toge<strong>the</strong>r by Sunny Boy Control Plus for data collection.<br />

All parameters <strong>of</strong> <strong>the</strong> PV plant are monitored and archived<br />

in <strong>the</strong> computer server located in <strong>the</strong> Laboratory. Moreover,<br />

<strong>the</strong>y can be observed in <strong>the</strong> real time conditions, by anyone (with<br />

no password or special access permission needed), via <strong>the</strong> PV<br />

Laboratory’s web site: http://www.fotowoltaika.dsod.pl [2]. Electricity<br />

produced by solar panels is used for <strong>the</strong> University internal<br />

power demands.<br />

134<br />

Research projects currently being realized<br />

Among seven currently running scientific projects in <strong>the</strong> Department<br />

<strong>of</strong> Semiconductor and Optoelectronics Devices, two <strong>of</strong> <strong>the</strong>m<br />

are being realized in <strong>the</strong> field <strong>of</strong> photovoltaics. One <strong>of</strong> <strong>the</strong>m is<br />

Polish research project granted by <strong>the</strong> National Science Centre<br />

within <strong>the</strong> program Opus (general grant). The o<strong>the</strong>r one is a project<br />

supporting international cooperation granted by <strong>the</strong> Ministry <strong>of</strong><br />

Science and Higher Education under <strong>the</strong> Polonium program.<br />

Opus: Transparent emitter electrode for PV<br />

applications based on carbon nanotubes<br />

This project is being realized by DSOD Lodz as a leader, in cooperation<br />

with two o<strong>the</strong>r Polish research centers, which are: Institute<br />

<strong>of</strong> Electronic Materials Technology <strong>of</strong> <strong>the</strong> Warsaw University<br />

<strong>of</strong> Technology and Institute <strong>of</strong> Fluid-Flow Machinery <strong>of</strong> <strong>the</strong> Polish<br />

Academy <strong>of</strong> Sciences. It has been granted by <strong>the</strong> National Science<br />

Centre for two years, from January 2012 to December 2013.<br />

The aim <strong>of</strong> <strong>the</strong> project is to manufacture transparent conductive<br />

layers, based on carbon nanotubes (CNT), and apply <strong>the</strong>m<br />

as front electrodes <strong>of</strong> various types solar cells. Obtaining thin,<br />

flexible transparent layers is to be achieved by using low-cost manufacturing<br />

techniques, such as screen printing, and by utilizing<br />

crude, easily available nanotube material with organic media.<br />

Created layers would replace currently used conductive oxides,<br />

such as ITO or ZnO: Al. Their application as transparent contacts<br />

in photovoltaic structures would eliminate certain disadvantages<br />

<strong>of</strong> previously used electrodes, such as fracturing in flexible photovoltaic<br />

structures. Additionally, exploration possibilities <strong>of</strong> new<br />

devices, manufacturing in roll-to-roll process, with no use <strong>of</strong> expensive<br />

vacuum technology, is planned [3].<br />

The following research work are scheduled in <strong>the</strong> project:<br />

– Preparation <strong>of</strong> carbon nanotube based suspensions <strong>of</strong> various<br />

composition and percentage, testing <strong>the</strong>ir properties and developing<br />

techniques, allowing to deposit controlled layer on test<br />

substrates. Simultaneously, a number <strong>of</strong> selected conductive<br />

oxide samples, including indium tin oxide ITO, ZnO 2<br />

and ZnO:<br />

Al, is expected to be manufactured, in order to compare optoelectronic<br />

and mechanical properties <strong>of</strong> new and traditional<br />

Fig. 1. Composition containing 0.25% carbon nanotubes printed on<br />

glass substrate<br />

Elektronika 6/2012


Fig. 2. CNT layer resistivity changes during dynamic bending [4]<br />

electrode design. Additionally, <strong>the</strong> development <strong>of</strong> simulation<br />

models for certain types <strong>of</strong> solar cells with carbon nanotube<br />

contact would be implemented.<br />

Some preliminary results shown in Fig. 1 and 2 have already<br />

been achieved. Obtained layers showed sufficient optical transparency<br />

(Fig. 1) and insensitivity <strong>of</strong> its electrical resistivity for dynamic<br />

bending (Fig. 2).<br />

– As a second stage <strong>of</strong> research work, complete thick and thin<br />

film photovoltaic structures with transparent carbon nanotube<br />

based electrode, will be manufactured. Scheduled domain <strong>of</strong><br />

study includes production <strong>of</strong> traditional silicon PV cells, as well<br />

as inorganic thin film structures, CIS solar cells. Constructed<br />

solar cells would enable a comprehensive examination <strong>of</strong> new<br />

carbon nanotube contacts, in elastic and rigid structures, as well<br />

as comparison to traditional electrodes. The structure <strong>of</strong> manufactured<br />

devices will be examined by using advanced measurement<br />

techniques. Optoelectronic parameters will be measured<br />

with calibrated testing equipment in standard test conditions.<br />

– On <strong>the</strong> basis <strong>of</strong> obtained results, indispensable corrections <strong>of</strong><br />

composition and/or deposition technique will be applied, as<br />

well as <strong>the</strong>oretical models describing phenomena in test solar<br />

cells will be verified.<br />

As a result <strong>of</strong> <strong>the</strong> project a description <strong>of</strong> production technology<br />

for using CNT in each <strong>of</strong> examined solar cell structures will<br />

be created.<br />

Polonium: New solutions for solar cells down<br />

converters<br />

Polonium is an international Polish-French program granted by<br />

<strong>the</strong> Polish Ministry <strong>of</strong> Science and Higher Education, French Ministry<br />

<strong>of</strong> Foreign Affairs and French Ministry <strong>of</strong> Higher Education<br />

and Research. However, <strong>the</strong> financial support is only given for<br />

<strong>the</strong> scientists cooperation and personal exchange, but not for<br />

research.<br />

This project is currently realized by <strong>the</strong> team <strong>of</strong> DSOD Lodz<br />

in <strong>the</strong> cooperation with <strong>the</strong> Lyon Institute <strong>of</strong> Nanotechnology <strong>of</strong><br />

INSA Lyon in France. It has been granted for years 2012 and<br />

2013.<br />

The aim <strong>of</strong> <strong>the</strong> project entitled: “New solutions for solar cells<br />

down converters” is to improve solar cell performance by <strong>the</strong><br />

use <strong>of</strong> novel solution for light capture. We proposed <strong>the</strong> application<br />

<strong>of</strong> a film, composed <strong>of</strong> ZnO nanoparticles (NPs), as a down<br />

converting layer, for enhancing external quantum efficiency and<br />

conversion efficiency <strong>of</strong> a photovoltaic device. Preliminary research<br />

showed that ZnO nanoaggregates have a great potential <strong>of</strong><br />

application as down converting layers. Investigations performed<br />

on a zinc oxide nanoparticle layer, proved that it generates several<br />

low-energy photons out <strong>of</strong> every high-energy incident photon.<br />

Optical down conversion, by ZnO nanoparticles should enhance<br />

solar cell efficiency by converting ultraviolet sunlight into visible<br />

light, which is more effectively used by <strong>the</strong> device (solar cell quantum<br />

efficiency is lower in <strong>the</strong> UV region than in visible zone, due to<br />

front surface recombination <strong>of</strong> hot photocarriers). Basic research<br />

<strong>of</strong> ZnO nanoparticle converters on silicon test structures are currently<br />

being realized [5].<br />

Fig. 3a presents <strong>the</strong> simplified band diagram <strong>of</strong> a down converting<br />

material placed above a solar cell. In this system, absorbed<br />

photons are reemitted as two lower energy via a midgap splitting<br />

level in <strong>the</strong> down-conversion process [6]. The idea <strong>of</strong> using this in<br />

single junction solar cell is schematically presented in Fig. 3b.<br />

Fur<strong>the</strong>r research aiming <strong>the</strong> increase <strong>of</strong> <strong>the</strong> solar cell efficiency<br />

will be concentrated on <strong>the</strong> development <strong>of</strong> novel photovoltaic<br />

devices using ZnO nanoparticle down converters in tandem<br />

solar cell. We will investigate new type <strong>of</strong> tandem devices<br />

including zinc oxide nanoparticles. ZnO NPs layer is supposed<br />

to be located under <strong>the</strong> top junction in order to efficiently capture<br />

and convert solar light, which is than partially forwarded to<br />

<strong>the</strong> bottom junction and partially reflected back and thus can be<br />

used by <strong>the</strong> top cell.<br />

Ano<strong>the</strong>r idea concerning <strong>the</strong> use <strong>of</strong> ZnO nanoparticles as<br />

down converters is <strong>the</strong>ir introduction into metal-semiconductor<br />

solar cells. Forecasted studies include <strong>the</strong>oretical investigations<br />

(computer simulations) and practical realization <strong>of</strong> <strong>the</strong> final device<br />

ZnO NPs-metal-semiconductor solar cells.<br />

Projects proposals submitted for financing<br />

The DSOD’s team <strong>of</strong> photovoltaics applied for two ano<strong>the</strong>r<br />

projects financing in programs funded by <strong>the</strong> National Science<br />

Centre. One <strong>of</strong> <strong>the</strong>m is national Preludium program, which<br />

is dedicated for pre-doctoral grants. Second is an international<br />

project for non co-financed international grants, called Harmonia.<br />

It is going to be realized in cooperation with <strong>the</strong> University<br />

<strong>of</strong> INSA Lyon in France. Decisions on financing in both National<br />

Science Centre programs are planned to be announced in <strong>the</strong><br />

end <strong>of</strong> July 2012.<br />

Fig. 3. Down-conversion system: a) band diagram, b) schematic <strong>of</strong> a down-converting layer consisting <strong>of</strong> nanoparticles, placed on top <strong>of</strong><br />

a pre-existing solar cell [6]<br />

Elektronika 6/2012 135


Preludium: Flexible layers <strong>of</strong> cadmium telluride<br />

based photovoltaic cells<br />

This project is going to be realized by <strong>the</strong> team <strong>of</strong> DSOD Lodz<br />

and obtained results will be an important part in <strong>the</strong> Ph.D. <strong>the</strong>sis<br />

preparing by one <strong>of</strong> <strong>the</strong>m.<br />

The scientific aim <strong>of</strong> <strong>the</strong> project is research concerning fabrication<br />

methods <strong>of</strong> flexible layers as components <strong>of</strong> polycrystalline<br />

cadmium telluride based solar cells. Experiments planned to be<br />

taken during <strong>the</strong> realization <strong>of</strong> <strong>the</strong> project will be focused on <strong>the</strong><br />

investigation <strong>of</strong> most optimal materials for component layers <strong>of</strong><br />

considered flexible photovoltaic (PV) structure and methods <strong>of</strong><br />

<strong>the</strong>ir fabrication.<br />

As a part <strong>of</strong> <strong>the</strong> project designing and realization <strong>of</strong> thin film<br />

solar cell test structure is expected. Proposed photovoltaic construction<br />

will be realized on <strong>the</strong> flexible substrate covered with<br />

back contact, CdTe base layer, CdS emitter layer and transparent<br />

front electrode made <strong>of</strong> thin ZnO:Al film, carbon nanotube (CNT)<br />

based polymer composite layer or graphene layer. Investigation<br />

and analysis <strong>of</strong> those structures will be an important element in<br />

verification <strong>of</strong> <strong>the</strong> hypo<strong>the</strong>sis about <strong>the</strong> suitability <strong>of</strong> selected layers<br />

as a solution in flexible photovoltaic cells applications.<br />

In order to realize flexible, cadmium telluride based, photovoltaic<br />

test structure, a number <strong>of</strong> experimental research will be<br />

conducted. They include both technological processes <strong>of</strong> each<br />

layer deposition as well as verification <strong>of</strong> <strong>the</strong>ir suitability in PV cell<br />

construction.<br />

As a first step, <strong>the</strong> possibility <strong>of</strong> employing various flexible materials,<br />

as substrate layers for fabrication CdTe/CdS solar cells,<br />

will be investigated. An essential parameter, which will be examined,<br />

is high temperature resistance. This property is crucial in<br />

substrate selection because <strong>of</strong> high-temperature recrystallization<br />

process <strong>of</strong> semiconductor CdTe layer, which will be than deposited<br />

on it.<br />

Next step will be developing and testing <strong>of</strong> available technologies<br />

and materials due to base contact layers deposition on<br />

selected substrate. Contact layers will be than tested in terms <strong>of</strong><br />

electrical and mechanical properties. Directly on those layers cadmium<br />

telluride and cadmium sulfur will be deposited, respectively<br />

as a base and an emitter <strong>of</strong> <strong>the</strong> cell.<br />

Final technological step planned in <strong>the</strong> project is selection <strong>of</strong><br />

<strong>the</strong> optimum conductive layer as a front cell electrode. Tests and<br />

measurements in terms <strong>of</strong> electrical parameters (low resistance),<br />

optical (high transmittance) and mechanical (flexibility) properties<br />

<strong>of</strong> proposed transparent conductive layers (metal oxides, CNT<br />

composites or grapheme layers) will be conducted using equipment<br />

and laboratory base <strong>of</strong> <strong>the</strong> applicant.<br />

Harmonia: New solutions for solar cells down<br />

converters<br />

Project which is going to be realized within <strong>the</strong> international program<br />

Harmonia covers extended research described in Polonium<br />

program in cooperation with <strong>the</strong> Lyon Institute <strong>of</strong> Nanotechnology<br />

<strong>of</strong> INSA Lyon France. Because Polonium program is supporting<br />

only personal exchange, we applied to <strong>the</strong> National Science Centre<br />

for financing research.<br />

The experimental work in <strong>the</strong> frame <strong>of</strong> this project is aimed<br />

at designing, fabrication and characterization <strong>of</strong> several types<br />

<strong>of</strong> silicon solar cells with thin-film ZnO nanoparticle layers incorporated<br />

into <strong>the</strong>ir structure as down-converts. The purpose <strong>of</strong><br />

<strong>the</strong>se ZnO thin films is to convert high energy incident photons<br />

into low energy ones which are better matched to <strong>the</strong> silicon<br />

bandgap. The implementation <strong>of</strong> luminescent down converting<br />

layers containing ZnO nanoparticles to <strong>the</strong> photovoltaic structure<br />

is expected to enhance <strong>the</strong> final PV conversion efficiency<br />

<strong>of</strong> <strong>the</strong> device. Planned experimental research will permit to develop<br />

new types <strong>of</strong> silicon solar cell structures using <strong>the</strong> ZnO<br />

nanoaggregate coatings. Optical, electrical and mechanical<br />

properties <strong>of</strong> obtained layers and final PV architectures, will be<br />

investigated and compared with <strong>the</strong> conventional silicon solar<br />

cells manufactured using <strong>the</strong> same fabrication technology, as<br />

well as with <strong>the</strong> commercially available Si photovoltaic devices.<br />

Numerical models for various types <strong>of</strong> described Si solar cells<br />

containing ZnO NPs down-converting layers will be proposed<br />

and on <strong>the</strong>ir basis computer simulations <strong>of</strong> <strong>the</strong> new solar cells<br />

performances will be carried out. Practical verification <strong>of</strong> <strong>the</strong><br />

<strong>the</strong>oretical simulations will be performed on different solar cells<br />

architectures containing a new type <strong>of</strong> down-converting layers.<br />

Summary<br />

Photovoltaic team <strong>of</strong> <strong>the</strong> Department <strong>of</strong> Semiconductor and<br />

Optoelectronic devices is a dynamic, young research group<br />

focused on technology <strong>of</strong> thin-film solar cells. Apart from educational<br />

activity many research projects and investigation were<br />

undertaken within last decade. The overall effort may be summarized<br />

by two finished, two running and two proposed national<br />

projects, twelve defended master <strong>the</strong>sis and one doctoral<br />

<strong>the</strong>sis in <strong>the</strong> field <strong>of</strong> photovoltaics. Additionally, one PhD dissertation<br />

and one DSc dissertation are to be finished in <strong>the</strong><br />

near future. The group is actively cooperating with many national<br />

and some international partners in <strong>the</strong> field <strong>of</strong> photovoltaic<br />

technology and measurements. It is also worth to mention that<br />

<strong>the</strong> team is as well engaged in many PV educational projects,<br />

namely multi-university education program <strong>of</strong> EFS founds<br />

“Photovoltaic application training”. Moreover voluntary student<br />

team, connected with DSOD photovoltaic group, is maintaining<br />

laboratory web site and is realizing numerous interesting interdisciplinary<br />

projects.<br />

References<br />

[1] http://dsod.pl/<br />

[2] http://www.fotowoltaika.dsod.pl/<br />

[3] Sibiński M.: Transparent emitter electrode for PV applications based<br />

on carbon nanotubes. OPUS grant application, 9 June 2011.<br />

[4] Sibiński M., K. Znajdek, S. Walczak, M. Słoma, M. Górski, A. Cenian:<br />

Comparison <strong>of</strong> ZnO:Al, ITO and carbon nanotube transparent conductive<br />

layers in flexible solar cells applications. Materials Science and Engineering:<br />

B, In Press: Apr. 2012, doi:10.1016/j.mseb.2012.03.037.<br />

[5] Lisik Z.: New solutions for solar cells down converters. POLONIUM<br />

program application, 29 Sept. 2011.<br />

[6] Abrams Z. R., A. Niv, and X. Zhang: J. Appl. Phys. 109, 114905,<br />

2011.<br />

100<br />

95<br />

75<br />

25<br />

5<br />

0<br />

100<br />

95<br />

75<br />

25<br />

136<br />

Elektronika 6/2012<br />

5<br />

0


IV<br />

Elektronika 6/2012

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!