Strategies To Construct n-Type Si-Based Heterojunctions for Photoelectrochemical Water Oxidation

Silicon (Si) is a suitable absorber for constructing highly efficient solar cells and photoelectrodes, because of its narrow band gap (1.12 eV), Earth abundance, and excellent optoelectronic property. When it serves as photoanodes for solar water oxidation, Si corrodes continuously, because of thermodynamically favorable anodic oxidation of Si surface to SiOx and the subsequent dissolution in alkaline electrolytes. The instability becomes the main bottleneck that limits its further application, and thus stabilization of Si under operation conditions constitutes a key step in developing an integrated device for large-scale and practical PEC water splitting systems. Although a variety of designs are currently being proposed to create stable and efficient n-Si photoelectrodes, a comprehensive understanding of the operation and failure mechanism of these photoelectrodes is still lacking. Here, we outline the basic principles and recent advancements of n-Si photoanodes with different device configurations and discuss the merits and major challenges of these protection strategies. Emphasis is placed on the construction of a novel multifunctional protective layer that simultaneously optimizes the charge carrier transport and corrosion resistance of n-Si photoelectrodes. Finally, a perspective regarding the key challenges and future direction for the design of novel structures, the engineering of the interface, the enhancement of stability, and the utilization of advanced in situ techniques is also presented.