Transport of a charge carrier packet in nanoparticulate ZnO electrodes

Abstract

The apparent changes in the transient photocurrents of nanoparticulate ZnO layers have been investigated by varying the extrinsic conditions of the measurement such as the composition of the electrolyte solution, the intensity of the incident light and the temperature. The effect of the light intensity on the transient photocurrents is interpreted in terms of a local increase of the electron density in the charge carrier packet. From the analysis of measurements performed at different temperatures, the mechanism of an electron exchange between adjacent particles is identified as a tunnelling process. A simple model is derived from which information about the height and the width of the potential barrier between the particles is gained. The energetic position of the conduction band edge of the ZnO is found to follow the Nernst equation with respect to the pH of the electrolyte solution. In comparison with the steady-state measurements, the transient photocurrents obtained for different hole scavenger concentrations in the electrolyte solution are explained by kinetic arguments. The changes of the charge transport by varying the concentration of the supporting electrolyte are interpreted in terms of the tunnelling approach by a screening of the potential barrier by the cations in the electrolyte solution.