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 SrTiO_3-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.