CHARGE-TRANSFER AND RECOMBINATION KINETICS AT PHOTOELECTRODES - A QUANTITATIVE-EVALUATION OF IMPEDANCE MEASUREMENTS

Impedance measurements were performed with illuminated photoelectrodes (bare and Pt-coated p-InP) at frequencies between 0.1 Hz and 1 MHz. Two different five-element equivalent circuits are discussed, both resulting in a satisfying fit of the impedance data. Mott-Schottky data and the photocurrent-voltage behaviour cannot be explained by a Maxwell-type circuit (involving surface states). A Voigt-type circuit, however, does describe adequately the recombination behaviour of the photo-electrode, the electrochemical charge transfer reaction (hydrogen evolution), and anomalies in Mott-Schottky evaluations. From the recombination and charge transfer resistances, the Schottky diode ideality factor, the electrochemical exchange current density, and the cathodic charge transfer coefficient are derived. Measured Helmholtz capacity values are at 3-30-mu-F/cm2 with bare p-InP during current flow. The results are in agreement with the model of a Schottky barrier (photovoltage build-up) with subsequent charge transfer across the metal (catalyst)/electrolyte interface.