Defect Density-Dependent Electron Injection from Excited-State Ru(II) Tris-Diimine Complexes into Defect-Controlled Oxide Semiconductors

Dye-sensitized solar cells and photocatalysts that consist of a light-absorbing dye and a wide gap oxide semiconductor substrate have been studied extensively as a means of solar energy conversion. Although defects existing at an oxide surface have a significant impact on the electron injection efficiency from the excited state dye-molecule into the oxide, the effects of defects on the electron injection process have not been fully understood in any dye-sensitized system. In this study, we present a systematic evaluation of electron injection into defects using emissive Ru­(II) complexes adsorbed on oxide substrates (HCa2Nb3O10 nanosheets and nonstoichiometric SrTiO3−δ), which had different defect densities. Using these oxides, electron injection from adsorbed Ru­(II) complexes was observed by time-resolved emission spectroscopy. It was shown that electron injection from the excited state Ru­(II) complex into an oxide was influenced by the defect density of the oxide as well as by the excited state oxidation potential (E ox*) of the Ru­(II) complex. Electron injection was clearly accelerated with increasing defect density of the oxide, and was inhibited with increasing electron density of the oxide because of a trap-filling effect. Even though the E ox* of the Ru­(II) complex was more positive than the conduction band edge potential of the oxide, electron injection into defects could be identified when a defective oxide was employed. The electron injection event is discussed in detail, on the basis of the defect density and the energy levels of oxides as well as the E ox* values of the Ru­(II) complexes. Overall, the results suggest that it is possible to estimate the potential of surface defect states in oxide by changing E ox* of an emissive complex dye.