Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces as Revealed by the Doping Density Dependence of the Collection Efficiency of Dye Sensitized Photocurrents

The doping density dependence of photocurrents has been experimentally measured at single crystal rutile TiO2 electrodes sensitized with the N3 chromophore and a trimethine dye. As the doping density of the electrodes was varied from 10(15) to 10(20) cm(-3), three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at electrode bias in the depletion region. At highly doped electrodes, tunneling currents played a significant role especially in the recombination processes. These different forms of the current-voltage curves could be fit to an Onsager-based model for charge collection at a semiconductor electrode. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.