PHOTOELECTRIC PROPERTIES OF COLLOIDAL SEMICONDUCTOR-FILMS

We report results on photocarrier transport in colloidal TiO2 films with the aim of elucidating the fundamental features of the transport kinetics. We find the transport process in these films to be dispersive. Typical tripping lifetimes are of the order of microseconds and mobility lifetime products several 10(-11) Cm2 V-1. The transport properties are markedly improved when deep trap states are saturated by excess carriers. We show that trapping lifetimes can be altered over several orders of magnitude. Recombination is studied in Schottky barrier type samples under double injection conditions. We find the recombination lifetime to decrease when the injection level is raised, indicative of bimolecular kinetics. Typical recombination times are in the range 10 mus to 1 ms. It is concluded from this study that specific operation conditions can result in transport properties greatly different from those near equilibrium. Recent success in using these films in photovoltaic applications may be due to the improved transport properties under high carrier injection.