A copper nickel mixed oxide hole selective layer for Au-free transparent cuprous oxide photocathodes

State-of-the-art cuprous oxide (Cu₂O) photocathodes for photoelectrochemical (PEC) water splitting have a long tradition of using gold (Au)-coated F-doped SnO₂ (FTO) substrates for the improvement of Cu₂O electrodeposition and overall PEC performance. Au is one of the best contact materials for Cu₂O photocathodes due to its large work function enabling proper alignment with the valence band level of Cu₂O. Due to its relatively large band gap (2.0 eV), Cu₂O is preferentially used as the top-cell absorber in tandem with a photoanode or a photovoltaic (PV) cell for overall solar-driven water splitting. However, the Au contact poses a major issue due to its poor transparency. Moreover, Au is a precious metal, which increases the cost and can hinder the scalability of PEC devices. In this work, we propose an effective replacement of the Au layer with a transparent and cost-efficient copper-nickel mixed oxide (CuO/NiO) thin film, which is prepared by a facile sequential sputtering deposition combined with an annealing process in air. We successfully demonstrate that a thin layer of the CuO/NiO film shows better transparency as well as well-aligned energy levels for efficient hole collection leading to an improved PEC performance compared to the performance of a Au-contact based equivalent device in a pH 5 electrolyte biased at 0 V versus the reversible hydrogen electrode. This new transparent and efficient CuO/NiO layer paves the way for the development of efficient, yet inexpensive PEC-PV or photocathode-photoanode stacked tandem devices for a hydrogen fuel based economy.