Preparation And Properties Of Polypyrrole Counter Electrode For Dye-Sensitized Solar Cells

The Dye-sensitized Solar Cells (DSSCs) is attracting more and more attentiondue to its high conversion efficiency, low cost, environment-friendliness, simplefabrication procedure and good plasticity. The DSSCs is usually composed of aworking electrode, electrolyte, and a counter electrode(CE). The CE is very importantin the DSSCs because the CE should exhibit high catalytic activity on the redox ofI-/I3-. The Pt, is often used as the CE owing to its high conductivity andelectrocatalytic activity for the redox of I-/I3-reduction. Because of the high cost of Pt,however, it is necessary to develop Pt-free materials as CE for DSSCs. So far，Pt-freeCE, such as carbon materials, sulfide, conducting polymer and some complexes havebeen employed as the CE. In this work, polypyrrole(PPy) has been synthesized onFTO substrate by an electrodeposition method and has been empolyed as the CE inDSSCs. Different doping anions, pH, and solvents have been used to adjust thepolymerization. The SEM, UV-visible-light absorption spectroscopy, cyclicvoltammetry, impedance spectroscopy, and I-V curve have been used to study theeffects of resultant PPy on the performance of the DSSCs. The results and conclusionsare summarized as follows:(1) The physical and chemical properties of PPy are greatly dependent on thenature of doping anions in the polymerization solution. The Cl-, SO42-, TsO-and LAS-have been used as the counter anions to prepare different polypyrrole films as the CE.It is found that PPy(LAS-) and PPy(TsO-) have a high doping density and goodcatalytic activity. When assembled into a DSSCs, they exhibit a low interfacecharge-transfer resistance and high photoelectric conversion efficiency(PCE). TheDSSCs with PPy(LAS-) as the CE exhibits the best performance, for which the PCEcan reach90.5%of the DSSCs with Pt as CE. The PPy(Cl-) and PPy(SO42-) have lowdoping density, poor catalytic activity, high interface charge-transfer resistance, andlow PCE. (2) The pH of the solution plays a very important role in the polymerization ofpyrrole. The pH of pyrrole-TsONa solution has been adjusted by adding TsOH so asto get different PPy films in electropolymerization. The pH2.0is the optimized valueas the resultant PPy has the highest doping density and the best performance ofcatalytic activity. When assembled into a DSSCs, it has the lowest interfacecharge-transfer resistance and the highest PCE. When the pH value is higher or lowerthan2.0, neither is the rusultant PPy nor the DSSCs exhibits good properties.(3) The PPy films can be prepared by using both organic and aqueous solutions,But while the properties of resultant films are dependent on the nucleophilic propertyof the solvent. By adjusting the ratio of water and ethanol in the solution, it is foundthat the addition of ethanol can lead to a difference in the morphology of PPy. Acauliflower-like structure has been observed in the obtained films, which mayincrease the surface area. When assembled into a DSSCs, it has a low interfacecharge-transfer resistance and high PCE, though with a relatively low conductivityand catalytic activity. Further study is undergoing to elucidate the mechanism.