Carrier transport in nanoporous TiO2 films

Recent results on electron transport in nanoporous TiO2 films with gas-filled, insulating pores are evaluated. Measurements on Pt/TiO2 Schottky barrier structures indicate a barrier height of 1.7 eV, compatible with an electron affinity of 3.9 eV for the TiO2 films. Below similar to 300 K, tunneling transport through the barrier occurs, resulting in barrier lowering effects. Carrier drift mobilities, recombination lifetimes and their dependence on injection level in TiO2 are reported. It is found that the mobility-lifetime product is independent of injection level, while drift mobility and recombination lifetime change strongly with injection. All experimental findings are discussed in terms of two different transport models, one based on trap filling, the other on the screening of potential fluctuations. The trap filling model appears as the more plausible model. Comparison with recent experiments on nanoporous films in contact with electrolytes indicate that the transport and recombination mechanism is qualitatively similar for the two cases.