Quasi-Fermi level measurement in an illuminated GaP photoelectrolysis cell

Abstract

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), V_Au, 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≥E_G, the 2.23 eV energy gap of GaP. Changes in V_Au of ∼ + 1.0 V have been observed. It is shown below that V_Au∼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 V_Au 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.