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)-sensitized nanocrystalline TiO2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO2 film. For excitation intensities resulting in less than one excited dye molecule/TiO2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V-kin, a rapid acceleration of the charge recombination kinetics was again observed, for example 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 applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10(6)-fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V-kin. Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V-kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li+ ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.