Ambipolar diffusion of photocarriers in electrolyte-filled, nanoporous TiO2

We report transient photocurrent measurements on solar cell structures base d on dye-sensitized, porous TiO2 films filled with a liquid electrolyte. Th e measurements are interpreted as ambipolar diffusion; under most measureme nt conditions, the ambipolar diffusion coefficient is dominated by electron s diffusing in the TiO2 matrix. We report a strong dependence of the ambipo lar diffusion coefficient upon the photoexcitation density, as has been pro posed previously. The coefficients vary from 10(-8) cm(2) s(-1) at low dens ity to 10(-4) cm(2) s(-1) for densities of 10(18) cm(-3) At a specified pho toexcitation density, ambipolar diffusion coefficients measured using weak laser pulses and optical bias are about 10 times larger than coefficients m easured using large-intensity laser pulses. We describe trapping models for these effects based on an exponential distribution (T-0 = 650 K) of electr on trap levels in TiO2. We infer an electron recombination cross section le ss than 2 x 10(-27) cm(2): this value is nearly 10 orders of magnitude smal ler than typical values in compact semiconductors and indicates the extraor dinarily effective separation of electrons in the TiO2 matrix from electrol yte ions only nanometers distant.