Cation-controlled interfacial charge injection in sensitized nanocrystalline TiO2

The photophysical and photoelectrochemical properties of Ru(deeb)(bpy)(2)(P F6)(2), where bpy is 2,2'-bipyridine and deeb is 4,4'-(COOEt)(2)-2,2'-bipyr idine, anchored to nanocrystalline TiO2 (anatase) or ZrO2 films are reporte d. In neat acetonitrile (or 0.1 M tetrabutylammonium perchlorate) long-live d metal-to-ligand charge transfer (MLCT) excited states are observed on bot h TiO2 and ZrO2. Addition of LiClO4 results in a red shift in the MLCT abso rption and photoluminescence, PL, spectra on both TiO2 and ZrO2, and a conc entration-dependent quenching of the PL intensity on TiO2. The Li+-induced spectroscopic changes were found to be reversible by varying the electrolyt e composition. Time-resolved absorption measurements demonstrate that the p resence of lithium cations increases the quantum yield for interfacial char ge separation with no discernible influence on the rate of charge recombina tion. A second-order kinetic model quantified charge recombination transien ts. A model is proposed wherein Li+ ion adsorption stabilizes TiO2 acceptor states resulting in energetically more favorable interfacial electron tran sfer. The generality of this model was explored with different electrolytes and sensitizers. In regenerative solar cells, the addition of Li+ increase s both the efficiency and long wavelength sensitivity of the cell.