Ionic Liquid-Induced Local Charge Compensation: Effects on Back Electron-Transfer Rates in Dye-Sensitized TiO2 Thin Films

The effect of ionic liquid electrolytes on back electron-transfer rates for dye-sensitized TiO2 thin films was investigated using microsecond millisecond transient absorption. For D35/TiO2 and [Ru(dcb)(3)](2+)/TiO2 in electrolytes based on 1-alkyl-3-methyl-imidazolium hexafluorophosphate, significantly slower back electron-transfer rates, compared to those observed in neat acetonitrile (CH3CN) and LiClO4/CH3CN, were found. Surprisingly, no such trends were observed for N3/TiO2 under the same conditions. This inconsistency points to the need for mechanistic understanding of how the structure and properties of dyes affect the electrolyte dye interface interactions in ionic liquid (IL) based dye-sensitized solar cells (DSSCs). To explain the observed behavior we propose an electrostatic effect at the TiO2 electrolyte interface, where the bulky IL cations rearrange at the TiO2 surface, locally compensating the charge. This would be consistent with N3 behaving differently because of its negatively charged SCN-ligands. This accumulation of cations at the interface affects the interaction between conduction band TiO2 electrons and the oxidized dye. As a result, slower back electron-transfer rates are observed when charge is effectively compensated. Therefore, here, the study of back electron-transfer kinetics was used as an indirect probe of local charge compensation at the dye semiconductor electrolyte interfaces. The results show that the mechanism of local charge compensation is dependent on dye structure.