Tailored energy level alignment at MoO_X/GaP interface for solar-driven redox flow battery application

MoO_X is commonly considered to be a high work-function semiconductor. From x-ray photoelectron spectroscopy and photo-electrochemical analysis, it is shown that MoO_X can be considered as an effective hole transfer layer for the GaP-based device. Specifically, in the absence of carbon contamination using an ion beam cleaning step, the oxygen vacancy derived defect band located inside the bandgap becomes the main charge transfer mechanism. We demonstrate, for the first time, a device with a MoO_X/GaP junction that functions as an unbiased photo-charging cell for the redox flow battery system with AQS/AQSH₂|| I^-/I₃^- redox couples. This work has important implications toward enabling MoO_X applications beyond the conventional solar cells, including electrochemical energy storage and chemical conversion systems.