Cuprous oxide photocathodes for solar water splitting

Solar water splitting is a promising technique for harvesting solar energy and converting abundant sunlight into storable hydrogen fuel. The cuprous oxide photocathode, one of the best-performing oxide photocathodes, possesses a theoretical photocurrent density of up to 14.7 mA cm−2 and a photovoltage as large as 1.6 V, making it possible to convert solar energy into hydrogen energy in a low-cost way. Herein, a comprehensive review of improving the solar water splitting performance of the cuprous oxide photocathode is presented with a focus on the crucial issues of increasing photocurrent density, photovoltage, and durability from the aspects of solving the incompatibility between the electron diffusion length and optical absorption distances, improving interfacial band alignment, revealing the impact of deficiencies, and introducing protective overlayers. We also outline the development of unassisted solar water splitting tandem devices with the cuprous oxide photocathode as a component, emphasizing the critical strategies to enhance the transmittance of the cuprous oxide photocathode, laying a solid foundation to further boost solar to hydrogen conversion efficiency. Finally, a perspective regarding the future directions for further optimizing the solar water splitting performance of the cuprous oxide photocathode and boosting solar to hydrogen conversion efficiency of the unbiased tandem device is also presented.