Transient and spectral photovoltage (PV) is investigated in nanocrystalline
porous TiO2, which belongs to a class of materials with a very low electri
cal 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 TiO2. 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 TiO2 nanoparticles and generation of defect states. The ban
d gap of porous TiO2 (rutile, anatase) and the preferential formation of el
ectron traps below the band gap are analyzed.