Homojunction Back Contacts Enable Efficient Cu 2 O Photocathodes

ABSTRACT Enhancing carrier separation and extraction is crucial to achieve efficient metal oxide photoelectrodes for photoelectrochemical (PEC) water splitting. Here, we report a highly efficient Cu 2 O photocathode based on p + /p back‐contact homojunction. The homojunction significantly enhances hole‐selective extraction while suppressing electron backflow at the back‐contact interface, leading to marked improvements in PEC performance. The optimized photocathode delivers a hydrogen evolution reaction (HER) onset potential of 1.1 V and a saturated photocurrent density of ∼8.0 mA cm −2 . Furthermore, the p + /p back homojunction design enables the fabrication of a semi‐transparent Cu 2 O photocathode, which, when integrated with a perovskite solar cell in a tandem configuration, achieves a solar‐to‐hydrogen (STH) efficiency of 7.31%. This work demonstrates a viable strategy for advancing metal oxide photoelectrodes via homojunction engineering.