No experimental procedure is known for measuring directly the position of the minority carrier quasi-Fermi level, φ, at the surface of a semiconductor electrode1, φ is an important parameter in studies of photoelectrolysis (p.e.) (ref. 2), of surface states, of catalysis, of the anomalous intensity dependence of the photovoltage of Schottky barriers3, and of the efficiency and stability against corrosion of low-cost photoelectrochemical solar cells. Hence a means for determining ψ should be of wide interest. I describe here a technique which results from an effort to measure changes in the potential versus standard calomel electrode (SCE), VAu, of a thin porous Type B gold film4 evaporated on the surface of n-GaP in a 〈Au/GaP/K2SO4/Pt〉 p.e. cell when the cell is illuminated with photons having energy hv≥EG, the 2.23 eV energy gap of GaP. Changes in VAu of ∼ + 1.0 V have been observed. It is shown below that VAu∼ψ, the minority hole quasi-Fermi level in the illuminated n-GaP anode. This series of measurements provides other valuable information. The valence band (VB) of the Au/GaP anode is shown to > 0.21 V below, or anodic to, the OH−/H2O redox potential, so that oxidation of OH− is possible. But, p.e. with liberation of H2 and O2 is shown to be highly improbable. The results provide evidence that the Type B gold film does not protect the GaP surface against electrochemical corrosion. The variation with time of VAu provides a sensitive indicator of the existence and rate of GaP corrosion. The technique for determining ψ should be applicable to semiconductors, both n- and p- type, other than GaP.
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PINSON, W. Quasi-Fermi level measurement in an illuminated GaP photoelectrolysis cell. Nature 269, 316–318 (1977). https://doi.org/10.1038/269316a0
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