Electrochemical Reduction Properties of Extended Space Charge InGaP and GaP Epitaxial Layers

Abstract

Two lattice-matched epitaxial III-V phosphide films of thicknesses between 400 and 500 nm are grown by metal-organic chemical vapor deposition: InGaP on GaAs and GaP on Si. These structures are designed as photocathodes for solar-driven chemical reduction processes such as the hydrogen evolution reaction (HER) and CO2 reduction into higher-order hydrocarbons. By using p+ substrates and undoped epitaxial layers, an extended space-charge active region is achieved in the electrode with a design analogous to a p-i-n solar cell. When in contact with the methyl viologen MV+ / + + redox couple, the InGaP/GaAs and GaP/Si cathodes generate a photovoltage of 388 mV and 274 mV, respectively, under 1 sun illumination. Incident photon-to-current efficiency (IPCE) measurements confirm that the undoped active layers are exclusively performing light absorption and minority carrier diffusion-based charge transfer of high-energy photons. This shows that performance can be significantly boosted with lower-doped substrates. The InGaP/GaAs and GaP/Si electrodes are shown to drive the HER at saturation photocurrent densities of 9.05 mA/cm2 and 2.34 mA/cm2, respectively, under 1 sun illumination without a co-catalyst and under a large reduction bias. Thicker films did not show a corresponding increased performance, and can be explained through understanding of crystalline defects and the electrostatics of the junctions.