Ambipolar Diffusion of Photocarriers in Electrolyte-Filled, Nanoporous TiO2

Abstract

We report transient photocurrent measurements on solar cell structures based on dye-sensitized, porous TiO 2 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 TiO 2 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 cm 2 s -1 at low density to 10 -4 cm 2 s -1 for densities of 10 18 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 ( T 0 = 650 K) of electron trap levels in TiO 2 . We infer an electron recombination cross section less than 2 × 10 -27 cm 2; 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 TiO 2 matrix from electrolyte ions only nanometers distant.