Enhanced Carrier Transport and Bandgap Reduction in Sulfur-Modified BiVO₄ Photoanodes

Recent progress on bismuth vanadate (BiVO₄) has shown it to be among the highest performing metal oxide photoanode materials. However, further improvement, especially in the form of thin film photoelectrodes, is hampered by its poor charge carrier transport and its relatively wide bandgap. Here, sulfur incorporation is used to address these limitations. A maximum bandgap decrease of ∼0.3 eV is obtained, which increases the theoretical maximum solar-to-hydrogen efficiency from 9 to 12%. Hard X-ray photoelectron spectroscopy measurements as well as density functional theory calculations show that the main reason for the bandgap decrease is an upward shift of the valence band maximum. Time-resolved microwave conductivity measurements reveal a ∼3 times higher charge carrier mobility compared to unmodified BiVO₄, resulting in a ∼70% increase in the carrier diffusion length. This work demonstrates that sulfur incorporation can be a promising and practical method to improve the performance of wide-bandgap metal oxide photoelectrodes.