(Invited) Use of the Doping Dependence of Collection Efficiencies for Dye Sensitized Photocurrents to Demonstrate Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces

The different physical limits of an Onsager-based model for charge collection at a semiconductor electrode are revealed through an experimental examination of the doping density dependence of photocurrents at single crystal ruitile TiO₂ electrodes sensitized with the N3 chromophore and a trimethine cyanine dye. As the doping density of the electrodes was varied from 10¹⁵ cm^-3 to 10²⁰ 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 an electrode bias in the depletion region. At highly doped electrodes tunneling currents played a significant role, especially in the recombination processes. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.