Surface photovoltage spectroscopy of dye-sensitized solar cells with TiO2,Nb2O5, and SrTiO3 nanocrystalline photoanodes: Indication for electron injection from higher excited dye states

The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized s olar cells (DSSCs) with different mesoporous, wide-band gap electron conduc tor anode materials, viz., TiO2 (anatase), Nb2O5 (amorphous and crystalline ), and SrTiO3 using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the con duction band edge energies (E-CB) Of these oxides. This is manifested in a blue-shift for cells with Nb2O5 and SrTiO3 compared to those with TiO2. The ECB levels of Nb2O5 and SrTiO3 are known to be some 200-250 meV closer to the vacuum level than that of our anatase films, while there is no signific ant difference between the optical absorption spectra of the dye on the var ious films. We, therefore, suggest that the blue shift is due to electron i njection from excited-state dye levels above the LUMO into Nb2O5 and SrTiO3 . Such injection comes about because, in contrast to what is the case for a natase, the LUMO of the adsorbed dye in the solution is below the ECB Of th ese semiconductors, necessitating the involvement of higher vibrational and /or electronic levels of the dye, with the former being more likely than th e latter. While for Nb2O5 hot electron injection has been proposed earlier, on the basis of flash photolysis experiments, this is the first evidence f or such ballistic electron-transfer involving SrTiO3 a material very simila r to anatase but with a significantly smaller electron affinity. Additional features in the SPV spectra of SrTiO3 and amorphous Nb2O5 (but not in thos e of crystalline Nb2O5) can be understood in terms of hole injection from t he dye into the oxide via intraband gap surface states.