Electron transport in particulate ZnO electrodes:: A simple approach

The charge transport in nanoparticulate ZnO layers held at different potentials was studied by analyzing the anodic transient photocurrents. The electrodes used in the electrochemical cell consisted of ZnO films coated onto conductive ITO substrates. In these experiments, three contributions to the photocurrent could be distinguished which were ascribed to a fast initial charge transport, a slow transport via deep trap states, and a transport via conduction band states or shallow traps. For the latter process, a simple phenomenological model is derived which enables a representation of the transients using only three parameters. These parameters account for the transport through the particulate ZnO layer, for the transport through the substrate, and for the amount of charge initially being present. The decay of the photocurrent signal is limited by transport through the ITO and the electrical connections, whereas the rise reflects mainly the transport through the ZnO itself. If the experimental results are interpreted in terms of a diffusion according to the theory of a random walk, the effective diffusion constant for the electron transport through the ZnO film held at positive potentials is. calculated to be 1.7 x 10(-4) cm(2)/s which is in good agreement with diffusion constants reported for particulate electrodes.