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.