Boosting the performance of WO₃/n-Si heterostructures for photoelectrochemical water splitting: from the role of Si to interface engineering

Metal oxide/Si heterostructures make up an exciting design route to high-performance electrodes for photoelectrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO₃/n-Si heterostructures are untangled. It shows that band bending near the WO₃/n-Si interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO₃/n-Si interface helps in establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Z-scheme design idea, which focuses on increasing the conductivity of an interface layer to facilitate charge transport, but ignores the potential profile around the interface. Based on the analysis, a high-work-function metal is predicted to provide the best interface layer in WO₃/n-Si heterojunctions. Indeed, the fabricated WO₃/Pt/n-Si photoelectrodes exhibit a 2 times higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and a 10 times enhancement at 1.6 V versus RHE compared to WO₃/n-Si. Here, it is essential that the native SiO₂ layer at the interface between Si and the metal is kept in order to prevent Fermi level pinning in the Schottky contact between the Si and the metal.