by Professor Jianyong Ouyang
Dye-sensitized solar cells (DSCs) are regarded as the next-generation solar
cells owing to the low fabrication cost and high photovoltaic efficiency.1,2
A DSC usually has a mesoporous TiO2 work electrode, a monolayer of
dye chemically attached to TiO2, an electrolyte and a counter electrode.
The light-to-electricity conversion includes the following steps:
- Light absorption by dye. A photon stimulates an electron
transition from the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) of the dye.
- Exciton dissociation and charge generation. The electron
on the LUMO of the dye transfers to the conduction band of TiO2.
Then, it transports along the mesoporous TiO2 to the external circuit.
- Dye regeneration. The dye returns to the original neutral
state by taking an electron from iodide in electrolyte. As the result of this
regeneration, iodide is oxidized into triiodide.
- Regeneration of iodide/triiodide on the counter electrode. Triiodide
diffuses from the interface between dye and electrolyte to the counter electrode
and is reduced into iodide there.
Thus, the counter electrode plays a key role in the light-to-electricity conversion
of DSCs. The requirements on the counter electrode include high electrical conductivity
and excellent catalysis on the reduction of triiodide to iodide.
Since platnium has good electrical conductivity and excellent electrochemical
catalysis, it is frequently used as the counter electrode of DSCs. Various techniques
have been developed for the deposition of platnium, such as pyrolysis, magnetron
sputtering, e-beam evaporation, electrochemical deposition and chemical vapor
deposition.3-6 The performance of Pt is strongly
affected by deposition techniques.
Among various deposition techniques described above, platnium fabricated by
pyrolysis is the best approach for producing counter electrode in terms of photovoltaic
efficiency. However, the pyrolysis takes place at a temperature higher than
380°C, which makes it unsuitable for the Pt deposition on materials of poor
stability at high temperature, such as flexible materials like polymers. Flexible
counter electrode is needed for flexible DSCs, which becomes more and more important
in practical application.
One possible way to deposit Pt films at low temperature is through the chemical
reduction of Pt salts. For example, Pt precursors can be reduced into metallic
Pt by polyols, such as ethylene glycol (EG). The polyol reduction has been extensively
studied in preparation of Pt nanoparticles. But it is rarely used on the deposition
of Pt films.7 The polyol reduction of Pt precursors
has been regarded as an interesting and simple deposition technique to produce
Pt films on substrates.8,9 However,
no following up work was reported for the deposition of Pt film by polyol reduction,
and there has been no report on the practical application of Pt films deposited
by polyol reduction.
Jianyong Ouyang and his research team at the National
University of Singapore discovered that nanostructured Pt films could be
deposited on conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, chemical structure shown in the inset of Figure 1) or polyethylene
terephthalate (PET) coated with indium tin oxide (ITO) by polyol reduction of
H2PtCl6 at low temperature.10
The Pt deposition was made by dropping EG solution of H2PtCl6
on a substrate at 180°C. The yellowish solution first turned to black, and
then a smooth film distributed with black spots was formed on the substrate
after about 5 min. Two structures were observed on the Pt film: porous and dense
Pt structures. The dense Pt structure is made of grains of about 50 nm in diameter.
It has metallic luster with high reflection and good adhesion to the substrate.
It could sustain a sonication process in an ultrasonic bath and the adhesive
tape peel test. In contrast, the porous Pt is formed from the precipitation
of the Pt nanoparticles in solution. It had a poor adhesion to the dense Pt
Current density-voltage curve of a DSC with nanostrutured Pt counter
electrode fabricated by EG reduction of H2PtCl6
at 180°C. The inset is the chemical structure of PEDOT:PSS.
The nanostructured Pt films deposited by polyol reduction of H2PtCl6
were used as the counter electrode of high-efficiency DSCs. The DSCs exhibited
high photovoltaic performance. Figure 1 shows the current density-voltage curve
of a DSC tested under AM1.5 solar illumination. The photovoltaic efficiency
is 8.4%, which is almost the same as that of control DSCs with conventional
Pt counter electrode fabricated by pyrolysis. In addition, the Pt deposited
by EG reduction exhibited good stability as the counter electrode of DSCs.
The Pt deposition by polyol reduction of H2PtCl6 at low
temperature enables the Pt deposition on flexible substrates like polymers.
Pt films were deposited on flexible ITO/PET substrates by the EG reduction of
H2PtCl6. They were used as the flexible counter electrode
of DSCs. The DSCs exhibited photovoltaic efficiency of 5.8 %. This efficiency
is higher than that of DSCs with other flexible counter electrodes.11
This research work was supported by the Ministry of Education, Singapore (Project
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