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 photo-voltage 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/cm(2) and 2.34 mA/cm(2), 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. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.