Ultrafast Relaxation as a Possible Limiting Factor of Electron Injection Efficiency in Black Dye Sensitized Nanocrystalline TiO2 Films

The photoinduced electron injection process in black dye (BD, trithiocyanato (4,4',4 ''-tricarboxy-2,2':6',2 ''-terpyridine)ruthenium(II); Ru(tcterpy) (NCS)(3)) adsorbed on nanocrystalline TiO2 films was studied by means of transient absorption and luminescence spectroscopy with subnanosecond time resolution in air and in 0.5 M solution of 4-tert-butylpyridine in acetonitrile. The resulting intensities and temporal profiles differed substantially between these conditions, whereas the electron injection efficiencies were almost the same. Generation efficiency of triplet excited state can be also evaluated, and thus the branching ratio of photoexcited BD/TiO2 can be obtained. These results clearly indicate that ultrafast relaxation from the primary excited state to the ground state occurs efficiently, probably owing to ultrafast recombination at the BD/TiO2 interface, and non-electron-injecting dyes exist on the surface. Accordingly, we conclude that the electron injection efficiency in BD/TiO2 is not simply limited by competition between the rate of deactivation in excited state and the rate of electron injection.