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.
© 2012 American Chemical Society