Cation-controlled interfacial charge injection in sensitized nanocrystalline TiO2

The photophysical and photoelectrochemical properties of Ru(deeb)(bpy)(2)(PF6)(2), where bpy is 2,2'-bipyridine and deeb is 4,4'-(COOEt)(2)-2,2'-bipyridine, anchored to nanocrystalline TiO2 (anatase) or ZrO2 films are reported. In neat acetonitrile (or 0.1 M tetrabutylammonium perchlorate) long-lived metal-to-ligand charge transfer (MLCT) excited states are observed on both TiO2 and ZrO2. Addition of LiClO4 results in a red shift in the MLCT absorption and photoluminescence, PL, spectra on both TiO2 and ZrO2, and a concentration-dependent quenching of the PL intensity on TiO2. The Li+-induced spectroscopic changes were found to be reversible by varying the electrolyte composition. Time-resolved absorption measurements demonstrate that the presence of lithium cations increases the quantum yield for interfacial charge separation with no discernible influence on the rate of charge recombination. A second-order kinetic model quantified charge recombination transients. A model is proposed wherein Li+ ion adsorption stabilizes TiO2 acceptor states resulting in energetically more favorable interfacial electron transfer. The generality of this model was explored with different electrolytes and sensitizers. In regenerative solar cells, the addition of Li+ increases both the efficiency and long wavelength sensitivity of the cell.