A study of the dynamic properties of photo-induced charge carriers at nanoporous TiO2/conductive substrate interfaces by the transient photovoltage technique

Two interfaces with opposite orientations of the built-in electric field, nanoporous TiO2 film/FTO electrode and nanoporous TiO2 film/semitransparent Pt substrate, were constructed. The separation and transport of photo-induced charge carriers in the two systems of nanoporous TiO2/conductive substrate were studied by the transient photovoltage technique. Various transient photovoltage responses were obtained when the laser beam was incident from the surface of the nanoporous TiO2 film (top illumination) or the TiO2/substrate interface (bottom illumination). A linear and a logarithmic dependence of the photovoltage amplitude on the excitation level were observed for top illumination and bottom illumination, respectively. The results indicate that diffusion is the major way for the separation of charge carriers in the nanoporous TiO2 film and that the excess carriers were separated by drift under the built-in electric field at the TiO2/substrate interfaces.