Gel polymer electrolytes for dye sensitised solar cells: a review

Dye sensitised solar cells (DSCs) based on mesoporous nanostructured TiO2 have attracted attention as an alternative to silicon photovoltaics. However, the use of a liquid electrolyte causes practical problems of leakages and its volatilisation, desorption and photodegradation of the dye, corrosion of the platinum secondary electrode and ineffective sealing of cells for long term applications in solar panels. Some of the attempts to solve this problem include the use of polymer electrolytes, gelling agents and both organic and inorganic hole conductors. A common feature of all such alternatives is the notable reduction in efficiency of the resultant solar cells primarily, due to the lower mobility of the iodide species through the solid or quasi-solid medium and imperfect wetting of pores with the electrolyte. The conversion efficiencies with the gel polymer electrolytes typically do not exceed 5%, but even at low efficiencies, these cells may become viable alternatives to the organic liquid containing Gratzel type cells due to improved stability and better sealing ability. The dyes are less liable to undergo photocorrosion and desorption, and the corrosion effects on the Pt electrode are lower. Some commonly used polymer electrolytes include poly(ethylene oxide), poly(propylene oxide), poly(acrylonitrile), poly(methyl methacrylate), poly(vinyl chloride) and poly(vinylidene fluoride). Plasticisers employed are ethylene carbonate, propylene carbonate, dimethyl carbonate, gamma-butyl lactone and diethylene carbonate. In addition, inert fillers, such as TiO2, SiO2, etc., are added to enhance ionic conductivity and stability at the interface with electrodes. These self-sealing gel polymer electrolytes have reported efficiencies of about 2-5%. Recently, the poly(acrylonitrile) based gel polymer electrolytes are reported to give an efficiency of 7.5% for the TiO2/Ru-N719 system under AM 1.5 illumination. Incorporation of TiO2 nanoparticles increases the efficiency of a DSC employing poly(vinylidene fluoride-co-hexafluoropropylene) from 5.72 to 7.18% where the standard liquid electrolyte gave an efficiency of 7.01%. The nanoparticles here are assumed to reduce recombination at the interface of dyed electrode/electrolyte. This review will cover only the application of gel polymer electrolytes for dye sensitised TiO2 systems in quasi-solid state solar cells and will not cover the closely related areas of gelated electrolytes where the standard liquid electrolyte is gelated with gelling agents and conducting polymers used as hole conductors.