Factors Controlling the Reduction Rate of Dye Cation in Dye-Sensitized Solar Cells

In dye-sensitized solar cells, dye-cations are formed by the injection of photo-excited electron from dyes to metal oxide semiconductor. To obtain the electrons for external circuit, the dye cations must be reduced by oxidized species of redox couple in electrolyte solutions. However, the dye cations can be also reduced by the injected electrons in the semiconductor. To reduce the dye cations by the species but not by the injected electrons, typically dyes are designed to have more than 0.4 eV energy difference between the dye’s HOMO level and electrolyte’s redox potential. However, to obtain high open circuit voltage, the difference needs to be decreased. In order to reduce the difference, we need dyes that can be reduced with small energy difference. To understand the factors controlling the reduction kinetics, we measured the reduction rate for several dyes whose structures are systematically varied. For the dye having alkyl chains, we found that the alkyl chains slow the reduction kinetics for both the injected electrons in semiconductor and oxidized species in electrolyte. We also found the structure of donor parts affected on the reduction rate. We will discuss how these findings can be applied to design the dyes for highly efficient dye-sensitized solar cells.