Ambipolar diffusion of photocarriers in electrolyte-filled, nanoporous TiO₂

We report transient photocurrent measurements on solar cell structures based on dye-sensitized, porous TiO₂ films filled with a liquid electrolyte. The measurements are interpreted as ambipolar diffusion; under most measurement conditions, the ambipolar diffusion coefficient is dominated by electrons diffusing in the TiO₂ matrix. We report a strong dependence of the ambipolar diffusion coefficient upon the photoexcitation density, as has been proposed previously. The coefficients vary from 10^-8 cm² s^-1 at low density to 10^-4 cm² s^-1 for densities of 10¹⁸ cm^-3. At a specified photoexcitation density, ambipolar diffusion coefficients measured using weak laser pulses and optical bias are about 10 times larger than coefficients measured using large-intensity laser pulses. We describe trapping models for these effects based on an exponential distribution (T₀ = 650 K) of electron trap levels in TiO₂. We infer an electron recombination cross section less than 2 × 10^-27 cm²; this value is nearly 10 orders of magnitude smaller than typical values in compact semiconductors and indicates the extraordinarily effective separation of electrons in the TiO₂ matrix from electrolyte ions only nanometers distant.