Anions in Electrolyte Solutions Retarding Electron Transfers at TiO ₂ Electrode/Dye Molecule/Electrolyte Interfaces

In dye-sensitized solar cells, dye molecules are anchored to a mesoporous TiO2 electrode immersed in an electrolyte solution containing redox species and supporting electrolytes. For high energy conversion efficiency, photoinduced oxidized dyes must be reduced by redox species, a process termed the regeneration reaction. However, under certain conditions, some oxidized dyes undergo recombination, which is a reduction process by injected electrons in TiO2. While various factors influencing the reduction kinetics have been elucidated, no systematic study on the effect of anions has been conducted. We prepared the cells with combinations of three anions and three metal-free organic dyes and measured the transient absorption of oxidized dyes induced by a pulsed laser. For a dye having a long π-conjugation bridge, employing I3– results in slower recombination compared to employing ClO4– or TFSI–. This retardation effect diminishes with decreasing π-conjugation length of the dyes. We interpret these observations as the concentration of I3– in the vicinity of dyes, which scales with the polarizability of the dyes, and the repulsive force to the injected electrons by the I3– near the dyes, resulting in a lower chance of meeting the electrons and oxidized dyes.