Cu2O photocathodes with band-tail states assisted hole transport for standalone solar water splitting.

Photoelectrochemical water splitting provides a promising solution for harvesting and storing solar energy. As the best-performing oxide photocathode, the Cu2O photocathode holds the performance rivaling that of many photovoltaic semiconductor-based photocathodes through continuous research and development. However, the state-of-the-art Cu2O photocathode employs gold as the back contact which can lead to considerable electron-hole recombination. Here, we present a Cu2O photocathode with overall improved performance, enabled by using solution-processed CuSCN as hole transport material. Two types of CuSCN with different structures are synthesized and carefully compared. Furthermore, detailed characterizations reveal that hole transport between Cu2O and CuSCN is assisted by band-tail states. Owing to the multiple advantages of applying CuSCN as the hole transport layer, a standalone solar water splitting tandem cell is built, delivering a solar-to-hydrogen efficiency of 4.55%. Finally, approaches towards more efficient dual-absorber tandems are discussed.