1) Bogala graphite from Sri Lanka was tested as a low-cost alternative to platinum (Pt) as the counter electrode material in dye-sensitized solar cells (DSCs). 2) DSCs using ball-milled Bogala graphite as the counter electrode achieved a power conversion efficiency of 2.12%, lower than the 3.0% efficiency of Pt-based DSCs but still promising given the significantly lower cost of graphite. 3) Scanning electron microscopy and X-ray diffraction characterization showed that the Bogala graphite had a crystalline structure and surface morphology suitable for functioning as the catalytic counter electrode in DSCs.

1. BOGALA GRAPHITE AS A CONDUCTING AND CATALYTIC COUNTER
ELECTRODE FOR SnO2/ZnO BASED DSSCs.
A.L.Bandara1,2
, W.M.S.S.Wanigasekara1,2
, G. R. A. Kumara1,2
*, R. M. G. Rajapakse1,2
.
Introduction
To overcome the provisional problem of energy crises of rapid energy demands and depletion
of fossil fuels, the search for alternative energy sources was globally motivated. Hence, the
solar energy is widely interested in the renewable energy research area which is clean, safer
feasible and abundantly available. Dye-sensitized solar cells (DSCs) are highlighted in the
context due to production cost-effectiveness and acceptable high efficiency compared to
conventional silicon-based solar cells. A generic DSC is consist of transparent conducting
oxide (TCO) film on a glass substrate, interconnected nano-particulated layer of a wide band
gap semiconductor, an organic dye, a redox electrolyte solution and a platinum-coated TCO
counter electrode. The functionality behind DSCs involves ultrafast injection of electrons from
the photoexcited state of a dye to the conduction band of semiconductor. The counter
electrode (CE) is responsible for catalysis for the reduction of the oxidized form of the redox
couple. Most commonly used CE in DSCs is Pt which is coated as a catalyst on fluorine-doped
tin oxide (FTO) glass substrate [1-3]. Though Pt CE has its own high efficiency performance
due to good catalytic activity, it has the highest cost of material value, poor stability in
corrosive electrolytes and high processing temperatures. Therefore, Pt was replaced by the
graphite, aiming mainly in large scale applications due to its high availability, low-cost, good
catalytic activity, good conductance and high stability to most of the electrolytes. For recent
years, various carbon materials have been introduced and experimented as an efficient CE
catalyst [4]. The investigation is based on directly obtained graphite samples from Bogala area
in Sri Lanka, where they are cheaply available. In this research, we report the use of ball-
milled graphite, obtained in wide range as a raw material varied in large bandwidth of natural
graphite within the range of grain sizes below 43 µm, from flakes to finest powders without
considering the purity of Bogala graphite in DSC applications.
Materials and Methods
Preparation of SnO2/ZnO sensitized working electrode:
SnO2 colloidal suspension in water (Sigma,Aldrich) (3ml), acetic acid (Sigma,Aldrich) (5
drops) and ZnO (Breckland Scientific) (60 mg) were ground in a motar. Then Triton X-100 (3
drops) and ethanol (Hayman, England, 99.9%) (40.0 ml) were added to the mixture and the
resulting suspension was ultrasonicated for 10 min, and sprayed onto cleaned FTO (9Ω/□)
glass plates which were kept on a hot plate, at 150 °
C. Then, the as-prepared SnO2/ZnO
composite films were sintered, at 500 °
C, for 30 min, in air, and subsequently, they were
immersed in a 0.3 M solution of N719 dye in a mixture of tetrabutanol (volume ratio 1:1) for
24 h. Dye-soaked electrodes were removed from the dye solution, rinsed with acetonitrile and
dried with warm air.
Preparation of graphite counter electrode
Graphite counter electrodes were prepared by the doctor –blade method onto a FTO -coated
glass substrate (9 Ω/□). The graphite paste was prepared by ball-milled graphite powder, the
raw material obtained from Bogala Mines (size ranges from flakes to finer particles, up to <
125µm) (0.75 mg), Organic binder, morphoal (0.5 mg) and deionized water (39 ml) were

2. mixed and the mixture was placed on stirrer at 80° C until excess distilled water was removed
by evaporation. Then, the resulting paste was deposited on a cleaned FTO glass substrate by
doctor–blading to obtain a thin film of thickness of 500 µm. The plates were sinter at 350 °C
for 30 min.
Construction of DSCs
The fabricated working electrode was sandwiched with a graphite deposited FTO counter
electrode and the intervening space was filled with the liquid electrolyte 0.1 M LiI, 0.05 M I2,
0.6 M dimethylpropylimidazolium iodide, tertiarybutylpiridine in methoxypropionitrile.
Characterization of DSCs
Current–Voltage (I-V) characteristics of the DSCs were measured using solar simulator
(1.5AM, 100 mWcm-2
). The optimum composition for Graphite counter electrode and
sintering temperature were identified for optimum, highest power conversion efficiency ƞ.
The SEM imaging was conducted to investigate surface morphology of graphite film and ball
milled graphite powder.
The crystalline structures and composition of powder samples were characterized by X-ray
Diffraction spectroscopy.
Results and Discussion
Figure 01: The SEM Surface image of graphite
Counter electrode
The I-V characterization of the DSCs prepared using Bogala graphite CE and Pt CE is
compared in the Figure 02, and Table 01 summarizes the solar cell parameters for each CE.
Table 01: Solar cell parameters for Pt CE and graphite CE, treated with equal illumination.
Sample Isc / mA Jsc / mA cm−2
Voc /V FF %
Pt CE 1.52 5.19 0.727 0.704 3.01
Graphite CE 1.13 4.54 0.709 0.530 2.12
The surface morphology of
the graphite films are presented in
figure 01 which reveals that the
Bogala graphite used in film has
edge planes, i.e., average number
of binding sites.

3. Figure 02: I-V characteristics of DSCs prepared using Bogala graphite CE and Pt CE
10 20 30 40 50 60 70 80 90
0
500
1000
1500
2000
2500
counts
2 theta
(002)
(101)
(004)
(110)
Figure 03: XRD pattern for powdered Bogala graphite sample.
Conclusion
The investigation of novel CE material from Bogala graphite in order to replace Pt CE in large
–scale applications in SnO2/ZnO-based solar cells was highly successful. Optimized counter
electrode from Bogala graphite is found to have low charge transfer resistance, high catalytic
effect with an efficiency closer similar DSC made using Pt counter elecrode. The developed
graphite CE shows an energy conversion efficiency of 2.12%.
References
1 B. O’Regan, M. Gratzel, Nature, 1991, 353, 737.
2. Lee, Y. L., Chang, C. H., Journal of Power Sources 2008, 185, 584-588.
3. Nozik, A. J., Beard, M. C., Luther, J. M., Law, M., Ellingson, R. J., and Johnson, C. J.,
Chemical Reviews 2010, 110, 6863-6890.
4. Veerappan, G., Bojan, K., and Rhee, S. W.,appl. Mater. and interfaces, 2011, 3,857-862.
0,0
1,0
2,0
3,0
4,0
5,0
0 0,2 0,4 0,6 0,8
Photocurrentdensity/mAcm-2
Voltage/ V
Pt
grahite
The highest power
conversion efficiency, ,
for graphite counter
electrode based DSC is
2.12% and that for Pt-
based DSC is 3.0%.
The resistivity of
Graphite CE is in the range
10-20 Ω/□ and that of Pt
CE is in the range 8-15
Ω/□.
The crystallographic structure of
electrode material can be obtained from
the XRD depicted in the figure 03
where the diffraction peak centered at
2 = 26.5° is originated from the
crystalline nature of graphite.
Interlayer distance d(002) obtained is
approximately 0.327 nm according
to calculations based on the Bragg’s
equation.