Ultrafast interfacial electron transfer in dye-sensitized ZnO nanocrystalline films: comparison with other metal oxides (Invited Paper)

Abstract

Dye sensitized nanocrystalline semiconductor films are used as a photoactive part in dye-sensitized solar cells, which are recently attracting much interest both in basic and applied studies. Electron transfer reaction from a photoexcited dye molecule, which is chemically adsorbed on the surface of semiconductor, into the semiconductor conduction band is the primary step to generate photocurrent. Ultrafast pump-probe spectroscopy with a <100 fs time resolution and in a visible-to-IR wavelength range was used to elucidate the interfacial electron transfer mechanism in dye-sensitized nanocrystalline metal oxide films of ZnO, TiO2, and others. We found two types of reaction paths; one is direct electron transfer from the excited molecule to the conduction band and the other is stepwise transfer through an intermediate, which was assigned to a charge transfer complex formed by the excited molecule and a surface state on the semiconductor. The order of the observed electron transfer rates for different semiconductors was qualitatively explained by the idea of the density of electron acceptor states; that is, the larger the density of states near the energy level of the excited molecules was, the faster the electron transfer took place.