The solar energy conversion efficiency of semiconductor-liquid junctions is determined by their photovoltage and by their photocurrent under illumination. Photoelectrochemistry (PEC) gives information on the current, but not on the photovoltage (the quasi Fermi level splitting), which for slow redox couples is obscured by kinetic overpotentials and surface states. Here we show that surface photovoltage spectroscopy (SPS) provides direct measurement of the quasi fermi level splitting potential (qFLS) in BiVO₄-electrolyte contacts. Measurements were done with a vibrating Kelvin probe on FTO deposited BiVO₄ films in contact with aqueous electrolytes containing triiodide/iodide, oxygen/ hydrogen peroxide, or sulfate/sulfite redox couples and for iron-oxyhydroxide modified BiVO₄ films. The triiodide/iodide couple, for example, yields qFLS=0.78 V under 49 mW/cm^-2 (400 nm LED) illumination, comparable to the photovoltage from PEC (0.79 V). Furthermore, the reversibility of the redox reactions determines the reversibility of the SPS transients. For example, the triiodide/iodide couple gives nearly complete SPS reversibility on the 20 sec timescale, while the sulfate/sulfite couple gives an irreversible photovoltage signal. Lastly, by combining the SPS data with the majority carrier potential from open circuit potential measurements, the minority carrier electrochemical potentials, E_fp can be obtained for all systems. For instance, E_fp = 1.0 V vs RHE for the BiVO₄ - triiodide/iodide junction and E_fp = 0.26 V vs RHE for the sulfate/sulfite junction. These shows that BiVO₄ can photoelectrochemically oxidize both redox couples, without applied bias. These results shed new light on the photoelectrochemistry of BiVO₄, an important photoanode material for solar hydrogen production from water.

Hervin Errol Mendoza

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