Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p‐Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting

Silicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO₂ could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO₂ would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (O_vac ) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of O_vac distribution, O_vac along GBs and O_vac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p-Si/TiO₂ /Pt heterojunction photocathode with controllable O_vac distribution in TiO₂ shows a +400 mV onset potential shift and yields an applied bias photon-to-current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.