High-Performance Dye-Sensitized Solar Cells with Nanostructured Platinum Counter Electrode Deposited by Chemical Reduction at Low Temperature

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.

Recently, Professor 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 structure.

Figure 1. 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

Acknowledgement

This research work was supported by the Ministry of Education, Singapore (Project No. RG-284-001-096)

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