Homogeneous Cu2O p-n junction photocathodes for solar water splitting

Cu2O is considered as one of the most promising p-type semiconductors for photocathodes in solar water splitting due to its abundance, nontoxicity and appropriate band gap of 2.0 eV with favorable energy band positions. It has been long desired to find a proper n type semiconductor for p-Cu2O to form a p-n junction to generate a more positive onset potential. However, most previous research adopted heterogeneous p-n junctions due to the difficulty in obtaining n-type Cu2O, and the utilization of p-n Cu2O homojuncion was limited to solid-state photovoltaic devices. This paper describes a facile electrodeposition process for n-type Cu2O to construct the homogenous p-n junction, which is further protected by TiO2 for water splitting, demonstrating the possibility of applying Cu2O p-n homojunction in an aqueous condition for the first time. Compared with the heterogeneous Schottky junction counterpart, this p-n homojunction significantly increases the built-in potential for efficient charge separation, which leads to an anodic onset potential shift from 0.3 V to 0.7 V versus the reversible hydrogen electrode (RHE), reaching a photocurrent of 4.30 mA/cm2 at 0 V vs. RHE under AM1.5G. The performance of this electrodeposited p-n Cu2O photocathode without using the expensive Au back contact is comparable to the most efficient Cu2O photocathodes reported previously.