Parameters influencing charge recombination kinetics in dye-sensitized nanocrystalline titanium dioxide films

Optical excitation of Ru-II(2,2'-bipyridyl-4,4'dicarboxylate)(2)(NCS)(2)-se nsitized nanocrystalline TiO2 films results in injection of an electron int o the semiconductor. This paper addresses the kinetics of charge recombinat ion which follows this charge separation reaction. These charge recombinati on kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the T iO2 film. For excitation intensities resulting in less than one excited dye molecule/TiO2 particle, the recombination kinetics were independent of exc itation intensity. Increasing the excitation intensity above this level res ulted in a rapid acceleration in the charge recombination kinetics. Similar ly, for positive electrical potentials applied to the TiO2 electrode, the r ecombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V-kin, a rapid acce leration of the charge recombination kinetics was again observed, for examp le from similar to 1 ms at +0.1 V vs Ag/AgCl to similar to 3 ps at -0.8 V ( similar to 10(8) fold increase in the rate). Moreover, at a constant applie d potential the charge recombination kinetics were found to be strongly dep endent upon electrolyte composition (up to 10(6)-fold change in rate). This strong dependence upon the electrolyte composition was found to be associa ted with shifts in the threshold potential V-kin. Spectroelectrochemical me asurements were used to monitor the shift in the trap/conduction band densi ty of states induced by the electrolyte composition. A direct correlation w as observed between the threshold voltage V-kin observed from kinetic measu rements, and the threshold voltage for electron occupation of conduction ba nd/trap states of the TiO2 observed from spectroelectrochemical measurement s. This direct correlation was observed for a wide range of electrolyte com positions including protic and aprotic solvents and the addition of Li+ ion s and 4-tert-butylpyridine. We conclude that the charge recombination kinet ics in such dye-sensitized films are strongly dependent upon the electron o ccupation in trap/conduction band states of the TiO2 film. This occupation may be modulated by variations in light intensity, applied electrical poten tial, and electrolyte composition. These results are discussed with relevan ce to the function of dye-sensitized photoelectrochemical devices.