Electric potential distribution and short-range screening in nanoporous TiO2 electrodes

Impedance spectroscopy and electrochemical dye desorption experiments were employed to study the electrical characteristics of nanoporous TiO2 electrodes in the dark. The results show an unusual potential distribution across the TiO2 films, caused by the semiconducting nature of the TiO2 and the ion motion through the porous film that neutralizes applied electric fields over a short range. These results may have several implications for the functioning of dye-sensitized solar cells: (1) The potential of much of the TiO2 film will not necessarily correspond to the potential applied to the substrate electrode. (2) Charge carrier motion through the TiO2 is expected to occur primarily via diffusion rather than drift. (3) The transient electric field generated upon illumination will be quickly neutralized under steady state conditions, but may be an important factor in transient measurements. (4) Systems without excess supporting electrolyte, such as some solid state versions of the dye-sensitized cells, may not be able to efficiently neutralize the field generated by photoinduced charge separation, leading to enhanced charge recombination.