Photovoltage in nanocrystalline porousTiO2

Abstract

Transient and spectral photovoltage (PV) is investigated in nanocrystalline porous ${\mathrm{TiO}}_2,$ which belongs to a class of materials with a very low electrical conductivity, i.e., with a huge Maxwell relaxation time. The PV in such materials is caused by diffusion of excess charge carriers with different diffusion coefficients. Usually, the diffusion coefficient for electrons is larger than that for holes in porous ${\mathrm{TiO}}_2.$ The PV transients are strongly retarded in time with respect to the exciting light pulse. The retardation of the photovoltage transients becomes stronger with decreasing size of the interconnected ${\mathrm{TiO}}_2$ nanoparticles and generation of defect states. The band gap of porous ${\mathrm{TiO}}_2$ (rutile, anatase) and the preferential formation of electron traps below the band gap are analyzed.