Charge Transfer Kinetics and Thermodynamics Control the Energy Conversion Efficiency of a Gallium Phosphide Solar Hydrogen Photocathode

P-type gallium phosphide (GaP) photocathodes for hydrogen evolution from water have a theoretical energy conversion efficiency of 12% based on the 2.4 eV optical band gap of the material. The performance of actual GaP photocathodes is much lower, for reasons not entirely clear. Here we use vibrating Kelvin probe surface photovoltage (VKP-SPV), open circuit potential (OCP) measurements, and photoelectrochemical (PEC) experiments to evaluate the kinetic and thermodynamic factors that control energy conversion with GaP photocathodes for the hydrogen evolution reaction (HER). We find that the open circuit photovoltage of the bare GaP-H