Cation-Controlled Interfacial Charge Injection in Sensitized Nanocrystalline TiO₂

The photophysical and photoelectrochemical properties of Ru(deeb)(bpy) 2 (PF 6 ) 2, where bpy is 2,2‘-bipyridine and deeb is 4,4‘-(COOEt) 2 -2,2‘-bipyridine, anchored to nanocrystalline TiO 2 (anatase) or ZrO 2 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 TiO 2 and ZrO 2 . Addition of LiClO 4 results in a red shift in the MLCT absorption and photoluminescence, PL, spectra on both TiO 2 and ZrO 2, and a concentration-dependent quenching of the PL intensity on TiO 2 . 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 TiO 2 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.