Mechanism of charge recombination in dye-sensitized nanocrystalline semiconductors: Random flight model

The trap-filling effect on the kinetics of charge recombination in dye-sensitized TiO2 nanoparticles is discussed on the basis of the random flight model. The model assumes that electrons move between trap sites on the surface of a nanoparticle by diffusion in the conduction band. After an electron is thermally detrapped into the conduction band, it can subsequently be captured by any empty trap or recombine with a cation within the same nanoparticle. This is an alternative to the nearest-neighbor random walk. A reasonable agreement with recent experimental data is observed. It is concluded, in accord with the previous studies, that the reaction is governed by the energy redistribution of trapped electrons rather than by spatial diffusion.