A Critical Review on Dye Sensitized Solar Cells - Nuicone.org

INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 1
A <strong>Critical</strong> <strong>Review</strong> on Dye Sensitized Solar Cells
Ayyan S. Karmakar, Jayesh P. Ruparelia
Abstract-- A Dye sensitized solar cell comprises of many
important components which include the electrolyte, sensitizer, a
light source, etc. Among all, the dye sensitizer is the particular
focus area of this particular paper. Metal complex dyes are
mostly used as a dye sensitizer. A lot of research has been done
using the platinum group metals like ruthenium, rhodium,
palladium, rhenium, osmium, etc. While other metal complexes
have shown promising efficiency but ruthenium has been the
most efficient of all. The polypyridyl complexes of ruthenium
have shown the best results in terms of solar efficiency which is
as high as 11%. Natural dyes such as fruit extracts like mulberry
and cabbage-palm fruit have also been worked upon as an
alternative to the conventional ruthenium and other metal dyes.
But their performance has not shown appreciable results
comparatively. Moreover, photochemical stability, which is the
other important characteristic apart from solar efficiency, is also
not achieved much in these dyes. Although, the first dye
sensitized solar cell (DSSC) was first made in 1991, its
commercialization has been a gradual process and till date very
scarce production has taken place. With overcoming the
shortcomings of the DSSC, it will prove to be an efficient
alternative to the commercial silicon based solar cells. The
present paper aims at bringing out the history of DSSC as well
as focus on the recent developments of the applications of dyes in
this specific area which focuses on solid state DSSCs.
Index Terms--DSSC, dye sensitizer, efficiency, metal complex ,
natural dyes, ruthenium.
D
I. INTRODUCTION
epletion of fossil fuels has led the world to shift from the
conventional energy sources to renewable energy to meet
the growing energy demand. Although the process is gradual
but the potential of renewable energy has been well talked
about in the past decade. Wind energy has been effective but
due to the inconsistency of the blowing wind, it has not been
quite efficient. Unlike wind energy, solar energy has a fair
amount of consistency. As a result, the conversion of solar
energy into different forms has been the core of the research
for the recent past years.
Photovoltaic devices have been found to convert the solar
energy into electrical energy. In these devices the charge
separates at an interface of two materials of different
conduction mechanism. Silicon based solar cells have been
the most effective under this technology. They are the solid
state junction devices. Dye sensitized solar cell (DSSC), a
third generation cell, has been the competitive technology for
the above. DSSC fabricated with the inclusion of
nanocrystalline materials has been effective in diverting from
the classical solid state junction devices. [1]
II. WORKING PRINCIPLE
The basic operating principle for any solar cell consists of
absorption, separation and collection. Different types
optimize these parameters accordingly to attain better
efficiency. Thus, absorption occurs in the first step of the
reactions occurring in DSSC. Under illumination, sensitizer
dye D absorbs a photon which leads to excited sensitizer state
D * . Photoexcitation of this sensitizer is then followed by the
electron injection into the conduction band of the
semiconductor (mesoporous). This takes the sensitizer to an
oxidized state D + . With the electron donation from the
electrolyte, containing a redox couple, the original state of the
dye restored. Iodide/triiodide couple is the preferred and
effective redox couple used. Iodide regenerates the sensitizer,
and itself gets regenerated by the reduction of triiodide at the
counter electrode. This way the circuit gets completed by
transfer of electron via the external load. The following
reactions summarize the working in a lucid manner: [1], [ 2]
D (absorbed) + hv D * (absorbed) (1)
D * (absorbed) D + (absorbed) + e - (injected) (2)
I - 3 + 2 . e - (cathode) 3 I - (cathode) (3)
D + (absorbed) + I - D (absorbed) + I - 3 (4)
The construction of a Dye Sensitized Solar Cell can be
categorized as follows: [2]
(1) A mechanical support coated with Transparent
Conductive Oxides
(2) The semiconductor film, usually TiO 2
(3) A sensitizer absorbed onto the surface of the
semiconductor
(4) An electrolyte containing a redox mediator
(5) A counter electrode capable of regenerating the redox
mediator
Due to the non-toxic, easily available and low cost
characteristics, TiO 2 has been the mostly preferred as the
semiconductor for the photoelectrode. ZnO and Nb 2 O 5 have
also been worked upon for the same. Sensitizer is in the form
of a dye, mostly metal complex dyes. Although a lot of dyes
have been tested and investigated including natural dyes,
Ruthenium complexes have proved to be the most effective
consistently.

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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, ‗NUiCONE – 2011‘
Fig1. Operating Principles and Energy level diagram of DSSC[2]
The present paper mainly deals with this section of the
DSSC comparing different metal complexes and natural dyes
which have been investigated for achieving better efficiency.
Iodide/triiodide has been accepted as the redox mediator,
although electrochemical studies of Co(III)/Co(II) (dbbip) 2
redox couple as a mediator for DSSC have also been done.
[2]-[3]
Fig2. Schematic diagram of a Dye Sensitized Solar Cell[2]
III. SENSITIZERS
Mainly dye sensitizers can be classified into two: (1)
organic dyes (2) inorganic dyes. Inorganic dyes have given
better results of the two as the stability towards
photodegradation is less for organic dyes. Inorganic dyes used
for this purpose are mainly metal complex dyes such as
complexes of Ruthenium, Osmium, Iridium, etc. Organic dyes
mainly consist of fruit dyes and natural extract dyes. Since the
first DSSC was made 20 years back, a lot of research has
been carried out to find transition-metal complexes as well as
natural or organic dyes, but none has been able to match the
performance of the ruthenium complexes based on the
conversion yield and durability or long term stability.
Since sensitizer is a very critical part of the DSSC, it has
some essential characteristics. The ideal sensitizer for a single
junction photovoltaic cell converting standard global AM 1.5
sunlight to electricity should absorb all light below a
threshold wavelength of about 920 nm. Moreover, it should
also graft the semiconductor oxide surface with help of
attachment groups like carboxylate and phosphonate. The
attachment group of the dye ensures that it spontaneously
assembles as a molecular layer upon exposing the oxide film
to a dye solution. It will make a high probability that, once a
photon is absorbed, the excited state of the dye molecule will
relax by electron injection to the semiconductor conduction
band. It should inject electrons upon excitation and its energy
level should be well matched with the lower bound of the
conduction band to avoid energy transfer losses. It should be
rapidly regenerated by the mediator layer in order to avoid
electron recombination processes and be fairly stable, both in
the ground and excited states. Also, it should be stable
enough to sustain about 10 8 turnover cycles. Many different
compound have been investigated for semiconductor
sensitization, such as porphyrins, phtalocyanines, coumarin,
carboxylated derivatives of anthracene and polymeric films.
Among the photosensitizers investigated, transition metal
complexes have been the best so far. [4]-[ 6]
1.) Metal complex sensitizers
Metal complex sensitizers have two ligands specifically,
ancillary and anchoring. Anchoring ligands are required for
the complex adsorption on the semiconductor surface
whereas, ancillary are important for tuning of the overall
properties of the complex. Photovoltaic performances can be
analysed in terms of conversion yield and long term stability.
Fulfilling both the criterion in the best possible way have
been the polypyridyl complexes of Ruthenium and Osmium.
The general structure which is preferred and has proven good
for sensitizers is ML2(X)2, where M can be Ru or Os and L is
2,2‘-bipyridyl-4,4‘ –dicarboxylic acid and X presents a
halide, cyanide, thiocyanate, acetyl acetonate, thiacarbamate
or water substituent group. [1]
A. Ruthenium
Among all the Polypyridyl complexes of Ruthenium dyes
are the most efficient ones. They can be categorised under
carboxylate polypyridyl ruthenium dyes, phosponate
ruthenium dyes and polynuclear bipyridyl ruthenium dyes. [7]
N3 dye has been the most promising dye sensitizer (shown in
fig3). In 1993, Grätzel reported cis-[Ru(dcbH 2 ) 2 (NCS) 2 ],
known as N3 dye. Other efficient dyes been investigated are
N719, N749 and Z907. N3 dye having two NCS ligands
absorbs 800 nm radiation whereas N749, also known as Black
dye, absorbs 860 nm making it better in terms of
performance. However other factors like low adsorption
coefficients turns out to be a limitation for N749. [6]-[7]
Apart from the ruthenium complexes mentioned in Table
1, there have been a lot of other Ruthenium based dye
sensitizers investigated and reported to be efficient. The
panchromatic sensitizers have low molar extinction
coefficient in near IR region which turns out to be a drawback

INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 3
for the improvement in the performance of the DSSC. As a
consequence, there have been research carried out on near IR
dyes as sensitizer for DSSC which include ruthenium
complexes containing biquinoline or 1,8-naphthyridyl
moieties, cyclometallated ruthenium complexes, osmium
polypyridyl complexes, and phthalocyanine and perylene
dyes. A new ruthenium(II) – polypyridyl
complex was synthesized with 2,6-bis(quinolin-2-yl) pyridine
derivatives giving excellent results in terms of IPCE value of
35% at 900 nm. [9],[10]
A heteroleptic Ru dye (RC730) containing crown-ether
moities on 4,4‘ positions of the bipyridine ligand was
successfully synthesized yielding overall efficiency of 2% and
IPCE of 31% at 530 nm. [11] A novel heteroleptic ruthenium
complex of the type [Ru(bpin)(dcbpyH2)Cl]Cl (where bpin is
2,6-bis(pyrazol-1-yl)isonicotinic acid and dcbpyH2 is 4,4_-
dicarboxy-2,2_-bipyridine) was synthesized and characterized
for tuning the LUMO level of the ruthenium sensitizer to
achieve greater stabilization in the excited state which keeps
the excess energy to maintain high driving force for electron
injection. The photovoltaic performance of this complex as
photosensitizer in a nanocrystalline TiO2-based solar cell was
studied and its overall energy conversion efficiency was
determined (1.9%). [12]
Another heteroleptic sensitizer, Ru((4,4-dicarboxylic acid-
2,2¢-bipyridine)(4,4¢-bis(p-hexyloxystyryl)-2,2-
bipyridine)(NCS) 2 , gave a conversion yield of 7.1% in
conjunction with binary ionic liquid electrolyte and excellent
stability when soaked under light at 60 o C.[13] methods are
now available for the preparation of Ru(II)bipyridine
analogous complex that is coordinated by 1-(2,4,6-
trimethylbenzyl)-2-(20-pyridyl)benzimidazole ligand. nc-
DSC, sensitized with CS23 exhibits 3.40% electrical
conversion efficiency which is nearly the same with reference
ruthenium complex Z-907 under the same conditions. [14]
TABLE I
RUTHENIUM COMPLEX BASED DYE SENSITIZERS AND THEIR PROPERTIES [5],
[8]
Fig4. Structure of N3 dye
Fig3. Structures of N719 and Z907
Fig5. Structure of near IR synthesized dye

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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, ‗NUiCONE – 2011‘
B. Osmium
Osmium sensitizers were found to 50% less efficient than
Ru complexes, but they have greater photochemical stability
compared to Black dye. The complex ion
[OsII(H3tcterpy)(CN) 3 ]− (H3tcterpy = 4,4‘,4‘‘-tricarboxy-
2,2‘:6‘,2‘‘-terpyridine) and osmium sensitizers containing
2,2‘-bipyridine-4,4‘-bisphosphonic acid ligand were
synthesized and characterized and it was found that IPCE
values were lower than the Ru Complex for the former, but
the values above 900 nm were slightly higher than Ru. The
complex in CH3OH showed a reversible OsII → OsIII
oxidation process and allow at the same time to extend the
spectral response of the TiO 2 photoanodes. [7], [15], [16]
C. Platinum
A square-planar platinum(II) based dye containing 4,4’-
dicarboxy-2,2‘- bipyridine and quinoxaline-2,3-dithiolate
ligands achieves efficient sensitization of nanocrystalline
TiO 2 solar cells over a wide visible range, generating a shortcircuit
photocurrent of 6.14 mA cm -2 and an open-circuit
potential of 600 mV under simulated AM 1.5 solar
irradiation, with a solar energy conversion efficiency of 2.6
%. [7],[17]
D. Rhenium
Rhenium (I) complexes based benzathiazole derivatives
have been reported to exhibit solar energy efficiency of
around 1.43-1.76%. One of the chlorotricarbonyl rhenium (I)
complex is shown in Fig 8. [7], [18]
E. Iridium
A novel type of efficient iridium (III) sensitizers with
carboxyl pyridine ligands was synthesized, yielding a
maximum of 66% IPCE and 2.16% power conversion
efficiency under simulated AM 1.5 sunlight. The energy
conversion efficiency can be improved by fine tuning of the
spectral overlap between the Ir (III) dye and the solar
spectrum. Cyclometalated Ir(III) complexes may have two
advantages. First, the high stability found in chelate ring
systems of cyclometalated Ir(III) complexes and second,
because the excited-state lifetime of cyclometalated Ir(III)
complex is longer than that of N3, the higheroverall solar
energy conversion efficiency may be anticipated.[19]
Fig7. Structure of Platinum based dyes containing 4,4’-dicarboxy-2,2‘-
bipyridine (1) and quinoxaline-2,3-dithiolate (2) ligands
Fig8. Structure of
rhenium complex
Fig6. Structure of an Osmium complex OsII (H3tcterpy)(CN) 3]
(TBA)(H3tcterpy = 4,4‘,4‘‘-tricarboxy-2,2‘:6‘,2‘‘-terpyridine)
Fig9. Structure of Iridium complex
F. Other Metal Complexes
Other metal complexes that have been researched are
copper and iron. While copper (I) complex has shown

INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 5
surprisingly higher IPCE(incident photon to current
efficiency) values with solar efficiency of 1.9 to 2.3%, Iron
(II) ligands have shown higher stability towards
photochemical degradation with solar energy efficiency of
0.29%. [7], [20], [21]
Apart from these mentioned metal complexes, a new type
of ruthenium complex containing a 2-quinolinecarboxylate
ligand was designed and synthesized for the same purpose.
The DSSC based on this sensitizer proved effective in the
visible region as well as near IR region with the overall
conversion efficiency being 8.2% and fill factor of 0.72. [22]
A solar efficiency of 7.2% has been observed in DSSC
working on Mordant dyes as sensitizers. A group of 6 best
performing mordant dyes were investigated and it was found
that they produced photocurrents > 0.2mA which is
comparable to the conventional N3 dye. One of the mordant
dyes has been shown in fig 9. [23]
2.) Natural Dyes
The second type of dye sensitizers used is the organic or
the natural dyes. Coumarin derivatives, merocyanine
derivatives and polyene dyes have been designed successfully
as organic-dye photosensitizers in DSSCs, and high solar
energy to electricity conversion efficiencies of up to 8%
under AM 1.5 irradiation have been attained. The lower
performance of DSSCs based on organic dyes compared to
those based on Ru complexes is probably due to the lower
open-circuit voltage (Voc) that is generated in the DSSCs
based on organic dyes, rather than the performance of the
short-circuit photocurrent density (Jsc), which is almost the
same. [24] Novel iminocoumarin dyes having carboxyl and
hydroxyl anchoring groups have been investigated.The IPCE
value for iminocoumarin dye sensitized solar cell was
21.38%. The overall low efficiency of the dyes is ascribed to
the lack of light harvesting ability at longer wavelength
region. [25]
Fruit dyes like mulberry and others have also been tested
for DSSC. Fill factor values of 0.40 to 0.61 have been
achieved on dye sensitization of dye extracts from mulberry,
chaste tree fruit and cabbage palm fruit. [26]
IV. CHARACTERIZATION OF DSSC
Once the DSSC is fabricated, it is now important to
evaluate its performance. The two main criterions to do the
same is (1) Overall Efficiency (2) Photochemical stability.
Other required parameters are IPCE (Incident Photon to
Current Efficiency also known as Quantum efficiency), I sc
(short-circuit current), V oc (open circuit voltage), FF (fill
factor).
The short-circuit current is the current through the solar cell
when the voltage across the solar cell is zero (i.e., when the
solar cell is short circuited). Open Voltage Current is the
maximum voltage available from a solar cell and this occurs
at zero current. Fill factor is defined as the ratio of the
maximum power from the actual solar cell to the maximum
power from a ideal solar cell. Efficiency is defined as the
ratio of energy output from the solar cell to input energy
from the sun.
Any photovoltaic device should have a serviceable life of
about 20 years without significant loss of performance.
Efficient dyes like N3 sustained 10 8 cycles after long time
illumination. Regeneration is an important factor here, and it
should occur fast to maintain the long term stability of the
cell. Common tests are based on1000h stability tests at 80 o C
for evaluating the photochemical stability of the DSSC.
V. SOLID STATE DSSC
As observed, the liquid state DSSCs i.e. the electrolyte
being in liquid form have shown efficiency as high as 11 %
(AM 1.5 light). But, due to the liquid state of the electrolyte
there are some limitations to the cell. The liquid can get
evaporated if the cell is not sealed properly. Moreover, there
could be a reaction between the electrolyte and compounds
like water and oxygen molecules due to this lapse. Also, the
production of multi-cell modules is a tougher process for
liquid electrolyte. The Solid state DSSC have been the
ultimate solution to all the above mentioned problems. With
the replacement of the liquid electrolytes with solid state or
quasi solid state hole conductor. The types of hole conductors
used can be p-type semi conductor, ionic electrolyte and
polymer electrolyte. [27]
Fig10. Structure of Mordant Black 5 (bis–azo)
VI. CONCLUDING REMARKS
A lot of study and research has been carried out on
DSSC. Different aspects of Dye sensitized solar cell have
been focused and investigations have been thus carried on.
Many sensitizers including inorganic and organic dyes have
been used. Of all the sensitizers reported the Ruthenium
complexes have been the most preferred because of their high
conversion efficiency which now have reached to 11%.
Although Organic dyes have higher molar extinction
coefficients, their lower stability limits them to be preferred.
On the other hand, Ruthenium complex have higher costs but
polypyridine complexes of Ruthenium have intense charge
transfer absorption across the whole visible range and have
easy tuneable redox properties making them gain an upper
hand on other metal sensitizers. Lately, a modified DSSC has
been reported known as the e DSSC. This has come into

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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, ‗NUiCONE – 2011‘
existence because of the encapsulation problem posed by the
use of liquid in the conventional wet type DSSC.
With the growing research in the different sections of
DSSC including sensitizers, thin films and other
semiconductors, redox couples, the efficiency has already
risen from 7%, of the initial cells two decades back, to 11%
leading to the invasion of Dye sensitized solar cells
commercially over the conventional Si based solar cells in the
coming future.
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