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

Silicon is a promising photocathode 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 (Ovac) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac 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 Ovac 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.