Single-electron migration in nanostructured TiO2

Electron migration in nanostructured anatase TiO2 films is investigated with intensity-modulated photocurrent spectroscopy. Transport of electrons can be described with a macroscopic diffusion model. By fitting the data to this model it is possible to determine the absorption coefficient, the diffusion coefficient, and the lifetime of the electrons. Upon illumination, electrons accumulate in the nanostructured film. A fraction of these electrons is stored in deep surface states. Another fraction resides in the conduction band and is free to move. It is found that the average concentration of the excess conduction band electrons equals about one electron per nanoparticle, irrespective of the type of electrode, the film thickness, or the irradiation intensity. Equilibration of the quasi-Fermi level in combination with infinite fast extraction of electrons at the substrate explains this saturation effect.