Electron Diffusion Length in Mesoporous Nanocrystalline TiO2 Photoelectrodes during Water Oxidation

A long electron diffusion length (L) relative to film thickness is required for efficient collection of charge in photoelectrodes. L was measured in a nanocrystalline, mesoporous TiO2 electrode during photochemical water splitting by two independent methods: (1) analyzing the ratio of incident photon conversion efficiency (IPCE) measured under back and front side illumination, and (2) analyzing transient photovoltage rise and decay measurements. These gave values of L that agreed (L = 8.5-12.5 mu m between -0.15 and 0.1 V vs Ag/AgCl 3 M KCl at pH 2). The transient measurements were consistent with trap limited transport and recombination of electrons as previously observed in dye-sensitized solar cells. L similar to 10 mu m is sufficient to collect separated electrons with minor electrolyte recombination losses in thin electrodes. However, charge collection, may limit performance in doped mesoporous electrodes with weak visible light absorption where thicker films are required.