(Invited) Surface Photovoltage Spectroscopy on BiVO4, Gallium Phosphide, and CuGa3Se5 Photoelectrodes in Contact with Aqueous Electrolytes

Abstract

The quasi-Fermi level splitting energy (photovoltage) is important for the solar to fuel conversion efficiency of photocatalysts and photoelectrodes. While electrochemical measurements can provide the electrochemical potential of the majority carriers at the semiconductor back-contact, the photovoltage, and the potential of the minority carriers at the front semiconductor/electrolyte are more difficult to determine experimentally, due to the lack of a direct electrical connection. Here we show that surface photovoltage spectroscopy (SPS) on photoelectrode films in contact with aqueous electrolytes can provide photovoltage information. For the measurements the semiconductor films (BiVO4, CuGa3Se5) or wafers (n-GaP or p-GaP) are immersed in aqueous electrolytes and placed underneath a vibrating Kelvin probe to monitor the light induced contact potential difference change. We find that the photovoltage (quasi-Fermi level splitting) of all photoelectrodes is increased by the aqueous electrolyte, and strongly depends on the electrochemical potential of the redox couple (TEOA, Na2SO3, KI, K4[Fe(CN)6] in the electrolyte, the back contact, and the light intensity. Electrochemical potentials of the minority carriers at the solid-liquid interface can be estimated by combining the photovoltage data with open circuit potential measurements that yield the quasi-Fermi levels of the majority carriers. This study provides new insight on the factors that affect photochemical charge separation at solid-liquid interfaces. Figure 1