Near-IR transient absorption study on ultrafast electron-injection dynamics from a Ru-complex dye into nanocrystalline In2O3 thin films: Comparison with SnO2, ZnO, and TiO2 films

We investigated the dynamics of photoinduced ultrafast electron injection from the excited state of cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) (N3 dye) into the conduction band of nanocrystalline In2O3 films by measuring transient absorption of N3-sensitized In2O3 films in the wavelength region from 600 nm to 4 μm at 100–250-fs temporal resolutions, and the results were compared with those for N3-sensitized nanocrystalline ZnO, SnO2, and TiO2 films. Although the reaction kinetics could not be simply described by a single exponential function and the near-IR transient absorption spectral shapes were dependent on the environmental conditions, the predominant injection time for the In2O3 films was 5–10 ps. The SnO2 and In2O3 films showed similar injection dynamics, and the injection times for these films were much shorter than the 150-ps injection time reported for ZnO films. The predominant injection time for the TiO2 film was within 100 fs, which is in agreement with reported values. The differences in the injection times are qualitatively explained in terms of the density of acceptor states in the conduction bands at the LUMO level of N3 dye.