Wafer-Scale Si-Based Metal−Insulator−Semiconductor Photoanodes for Water Oxidation Fabricated Using Thin Film Reactions and Multiple-layer Electrodeposited Catalysts

Solar-driven photoelectrochemical (PEC) water splitting offers a promising and environmentally friendly route for the conversion of renewable solar energy to hydrogen gas. A crystalline Si absorber is especially attractive due to its moderate bandgap, high charge mobility, long carrier diffusion length, costeffectiveness, and scalability in manufacturing. To improve the stability of Si-based PEC cells in operation, metal−insulator− semiconductor (MIS) structures have been widely employed. In this work, we employ simple and highly scalable processes to fabricate high-performance, extremely stable Si-based MIS photoanodes, and demonstrate their application to the fabrication of wafer-scale photoanodes. Localized conduction paths formed via an Al/SiO2 thin-film reaction enable low-resistance charge extraction even through thick insulating layers, yielding photoanodes with excellent stability. To improve the efficiency, we demonstrate a twostep Ni/NiFe electrodeposition process to create efficient oxygen evolution reaction catalysts. The Ni/NiFe catalyst allows for a high Schottky barrier between Si and Ni, lowering the photoanode onset potential, while the NiFe surface layer improves the catalytic performance. An unassisted solar-driven water splitting system incorporating a wafer-scale photoanode and monocrystalline Si solar cells is demonstrated and yields a solar-to-hydrogen conversion efficiency of 6.9% under simulated AM 1.5G sunlight illumination.