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

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Multifunctional coatings for solar cells application Karolina Sieradzka, Michał Mazur, Danuta Kaczmarek, Jarosław Domaradzki, Damian Wojcieszak Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics In recent years, an increased interest of nanocrystalline oxide materials was been observed [1, 2]. Such oxides join a few selected, well-defined properties such as high transparency, good electrical conductivity, hydrophilic or hydrophobic properties, antireflective properties, etc. [3, 4]. In contrary to conventional semiconductors, such thin oxide films, prepared for transparent electronics or solar cells application, combine mainly two specific features [5]: high transparency in visible light and the ability of electrical conduction at room temperature. These oxides, depending on the level of electrical resistivity are divided into two groups of materials: Transparent Conducting Oxide (TCO) or Transparent Oxide Semiconductor. However, in the literature, there is increasing number of reports about trying to get the additional properties, but most showed examples of multilayers. For example, it might be a thin oxide films with additional antireflective properties, prepared as TiO 2 /SiO 2 multilayers [4, 6] in order to obtain the largest possible reduction of light reflectance. Pemble et al. described dual functionality self-cleaning thermochromic films prepared by APCVD method. The multilayers based on VO 2 and TiO 2 revealed good degradation of stearic acid under UV radiation and thermochromic properties with the switching temperature of 55 o C. In the first part of this paper, the possibilities of characterization of nanostructures used in the Laboratory of Optoelectrical Diagnostics of Nanomaterials located at Wroclaw Univeristy of Technology have been showed. Then antireflective, photoactive nanocrystalline Ti-V oxide as a multifunctional thin film has been presented. Directions of current research The scope of current work carried out in the Laboratory involves the research for new, nanocrystalline oxide materials with unique electrical and optical properties, which might be applied as multifunctional coatings for example solar cells, glasses for special application or optoelectronic elements. The subject of interest of staff from our Laboratory are closely related to preparation and analysis of mainly the optical, electrical, hydrophilic and hydrophobic and antistatic properties of thin oxide films. A modified magnetron sputtering method [7], developed and patented by the staff from our Division enabled to achieve transparent and semiconducting oxide based on TiO 2 doped with different metal ions with n- or p-type of electrical conduction. Review of measurements apparatus for thin oxide films characterization used in Laboratory Optical properties are analysed based on spectrophotometry measurements. Transmittance and reflectance spectra are studied in the spectral range from 200 nm to 2600 nm at normal incident. The spectra are recorded with the use of Ocean Optics spectrophotometers (QE65000 and NIR-256.2.1) and coupled DH-2000-BAL deuterium-halogen or XE-450 xenon lamps as an excitation light source. Additionally, the optical measurements performed with the aid of Janis ST-100H cryostat in a varied temperature conditions (from ~2K up to 700K) allow for the analysis of gasochromic and thermochromic properties. Electrical properties are examined in the wide temperature range in order to determine the basic electrical parameters, e.g. resistivity, type of electrical conductivity. The electrical temperature dependent measurements are performed in the range from 25 o C to 300 o C using air-cooled thermal chuck operated by ERS SP72 controller. In case of heterojunction structure the fundamental property is their current-to-voltage performance. The measurements 102 are performed using Keithley 4200-SCS semiconductor characterization system equipped with Cascade MicroTech probe station with thermal chuck heater. The system allows the measurements from 100 aA up to 100 mA within a voltage source-measure range from 1 mV up to 210 V. Additonally, measurements performed at different temperature with and without light illumination provide to study of their influence on electrical performance of solar cells. A special software Keithley Interactive Test Environment enables for the rapid determination of solar cells parameters such as: shortcircuit current, open circuit voltage, maximum power, maximum current, maximum voltage, fill factor and cell efficiency. The analysis of electrical conduction mechanisms are done using the impedance spectroscopy (IS) method. Capacitance and conductivity data measured in a wide frequency range from 40 Hz to 110 MHz are recorded with a help of an Agilent Technologies 4294A precision impedance analyser. Electrical investigations are completed with stationary and transient photoelectrical measurements. The measurements performed with a monochromatic light on the range of 200 nm up to 1400 nm allow to investigate photoelectrical effect at the prepared heterostructures and to study theirs spectral sensitivity. Investigation of photoelectrical properties performed by a scanned light beam allows building two-dimensional maps of photocurrent distribution at the active area of the junction. These optical beam induce current (OBIC) maps usually allow to study generation-recombination centres in junctions, inter-level shorts and a latch-up mechanism. The surface wettability of the thin films are evaluated by contact angle measurement of deionized water under room temperature and daylight illumination. The measurement are carried out with a computer controlled Theta Lite goniometer system manufactured by Attension. The antistatic properties of thin films are investigated using a modern measurement station – JCI 155v5 Charge Decay Test Unit operated in the chamber with controlled humidity and temperature. The ability for static charge dissipation from samples surface vs. time were measured. Results of the measurments Thin oxide films were prepared mainly based on TiO 2 , which is in particular non-toxic, transparent to light in wide spectral range and thermally, chemically, mechanically stable material. The thin films were deposited in pure oxygen atmosphere by developed, modified high energy reactive magnetron sputtering (HE RMS) method. As a result the nanocrystalline oxides with thermodynamically stable rutile form with average crystallites size under 10 nm (Fig. 1) has been achieved. Typically, in a conventional magnetron sputtering process, directly after deposition of TiO 2 thin films, have the anatase phase, which is less thermodynamically stable. Then, the crystalline rutile form of TiO 2 is received only when additional thermal heating at 750 o C has been used. These achievements have contributed to a number of unique and desirable properties. These films, in addition to high transparency for light in the visible range, revealed among others increased hardness. The results of hardness measurements using nanoindenter (Fig. 2) showed that the nanocrystalline TiO 2 thin films deposited in HE RMS process has a high hardness of 14.3 GPa, what is comparable to DLC materials. While in case of TiO 2 films sputtered in the conventional process the hardness was 3.5 GPa and 7.9 GPa respectively for thin films after deposition and after annealing at 800 o C. Elektronika 6/2012
Optical and electrical parameters of thin oxide films based on TiO 2 Thin film [% at.] T 550 [%] λ cutoff [nm] ρ 300K [Ω cm] S [mV/K] TiO 2 83 338 1·10 11 – – TiO 2 :(5.5 Pd) 50 370 9.6·10 2 -11 Ti-V oxides (3.0 V) 73 381 2.7·10 5 -200 Ti-V oxides (23.0 V) 73 430 5,6·10 2 -20 n type TiO 2 :(0.9 Eu, 5.8 Pd) 35 450 2.2·10 -1 -90 TiO 2 :(10.2 V, 6.9 Pd) 40 400 1.5 -410 Ti-V oxides (19.0 V) 73 380 8·10 4 +685 TiO 2 :(15.8 Co, 6.9 Pd) 29 * 560 3.5·10 3 +77.5 p type TiO 2 :(0.6 Tb, 9.0 Pd) 22 * 510 1·10 5 +120 * average transmission determined at the wavelength λ=650 nm Fig. 1. AFM image of the nanocrystalline TiO 2 thin films Fig. 2. Nanoindenter images of TiO 2 thin films surface after hardness measurement From the point of view of oxide materials prepared for application in transparent electronics very important is a possibility to produce the transparent oxide semiconductors both with electron and hole type of electrical conduction. In Tabl. 1 the basic optical and electrical parameters of thin films based on TiO 2 with different material compositions, determined from optical and electrical measurements were summarized. On the basis of electrical results it can be stated that the electrical conductivity of prepared films is primarily activated by the presence of Pd dopant [8]. Unfortunately, simultaneously with increasing concentration of Pd in the TiO 2 matrix (from 5.5 to 9.0 at. %) there was a significant decrease in transparency, up to 22% for TiO 2 : (Tb,Pd) thin films (Tabl.). In addition, doping of TiO 2 matrix with Pd element resulted in the shift of optical absorption edge towards longer wavelength. The authors in previous work [8] have found that improved electrical conductivity in TiO 2 :Pd thin films is the results of the presence of nanocrystalline islands of mettalic Pd. Another example are TiO 2 :(Co,Pd) thin films, in which increase conductivity was due to the substitution of Ti 4+ ions located in the crystal lattice of TiO 2 by dopant ions Co 3+ and Co 2+ . Thus, within the TiO 2 energy band gap appeared the acceptor levels, which in consequence resulted decrease of resistivity [8]. Average transparency in the visible wavelength for TiO 2 : (Co,Pd) thin films is 29% and it is much lower compared to the undoped TiO 2 (Tabl.). As it can be seen, good electrical conductivity of TiO 2 : Pd and TiO 2 : (Co,Pd) thin films were obtained at the cost of their transparency level to light. Type of electrical conductivity was determined on the basis of Seebeck coeficient (S). A positive value of S indicates p-type conductivity in oxide. When the sign is negative the conductivity is dominated by n-type conductivity. As it can be observed from the characteristics of S (1000/T) shown in Fig. 3, the doping of thin films based on TiO 2 makes it possible to receive oxide semiconductors, both the n-type (Fig. 3a) and the p-type (Fig. 3b). N-type of electrical conductivity was obtained for TiO 2 :Pd, TiO 2 :V (3 at. % of V), Ti-V oxides (23 at. % of V), TiO 2 :(Eu,Pd) oraz TiO 2 :(V,Pd). While the opposite type of electrical conductivity revealed in case of Ti-V oxides (19 at. % of V), TiO 2 :(Tb,Pd) and TiO 2 :(Co,Pd) thin films. In Fig. 3 also interesting property of Ti-V oxides it has been observed, in which by different V concentration the type of electrical conductivity has been controlled. However, the analysis of thermoelectic properties in case of vanadium oxides are very complex, because of multivalence of vanadium oxides, ranging from 2 + to 5 + and their structural transformation in specified temperature. For example, n-type conduction is possible by replacing Ti 4+ ions in their site position in TiO 2 with reduced to +4 valence state vanadium ions (V4+). It is well known that titanium and vanadium dioxides may form V 1−x Ti x O 2 solid solution in a broad range of x values [9]. Therefore, in such films there are available extra electrons able to move in external electric field, giving n-type conduction. N-type conduction is also observed a) b) S [ µV/K] S [µV/K] 550 500 450 400 350 100 N-type Ti:V oxides (23 at. % of V) 0 -100 -200 -300 -400 500 400 300 200 100 0 TiO 2 :V (3 at. % of V) TiO 2 :Pd TiO 2 :(V, Pd) T [K] TiO 2 :(Eu,Pd) 2,0 2,5 3,0 3,5 1000/T [K -1 ] 550 500 450 400 350 P-type Ti:V oxides (19 at. % of V) TiO 2 :(Co,Pd) T [K] TiO 2 :(Tb,Pd) 2,0 2,5 3,0 3,5 1000/T [K -1 ] Fig. 3. Characteristics of Seebeck coefficient of semiconducting thin-film based on TiO 2 : a) with electron and b) hole type of electrical conduction Elektronika 6/2012 103
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Page 3 and 4: Spis treści str. 1. Analysis of se
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Page 9 and 10: Summary The silver nanopowder paste
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Page 13 and 14: Laser texturing and microtreatment
Page 15 and 16: Tab. 2. Wyniki średnie, odchylenia
Page 17 and 18: Tab. 1. Elementy składowe komercyj
Page 19 and 20: Rys. 2. Analizowane kształty tekst
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