Perturbation of the Electron Transport Mechanism by Proton Intercalation in Nanoporous TiO2 Films

This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties of nanoporous TiO ₂ films exposed to an ethanol electrolyte containing different percentages of water (0-10%). These measurements revealed that increasing the water content in the electrolyte led to increased proton intercalation into the TiO ₂ films, slower transport, and a dramatic change in the dependence of the thermal activation energy (E _a) of the electron diffusion coefficient on the photogenerated electron density in the films. Random walk simulations based on a microscopic model incorporating exponential conduction band tail (CBT) trap states combined with a proton-induced shallow trap level with a long residence time accounted for the observed effects of proton intercalation on E _a. Application of this model to the experimental results explains the conditions under which E _a dependence on the photoelectron density is consistent with multiple trapping in exponential CBT states and under which it appears at variance with this model.