MoOX is commonly considered to be a high work-function semiconductor. From x-ray photoelectron spectroscopy and photoelectrochemical analysis, it is shown that MoOX 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 MoOX/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 MoOX applications beyond the conventional solar cells, including electrochemical energy storage and chemical conversion systems.@en

Recent advances in photoelectrochemical redox flow cells, such as solar redox flow batteries, have received much attention as an alternative integrated technology for simultaneous conversion and storage of solar energy. Theoretically, it has been reported that even single- photon devices can demonstrate unbiased photo-charging with high solar-to-chemical con- version efficiency; however, the poor redox kinetics of photoelectrodes reported thus far severely limit the photo-charging performance. Here, we report a band alignment design and propose surface coverage control to reduce the charge extraction barrier and create a facile carrier pathway from both n- and p-type photoelectrodes to the electrolyte with the respective redox reaction. Based on these observations, we develop a single-photon photo- charging device with a solar-to-chemical conversion efficiency over 9.4% for a redox flow cell system. Along with these findings, we provide design principles for simultaneous optimisa- tion, which may lead to enhanced conversion efficiency in the further development of solar- rechargeable redox flow cells.@en