Cu: NiO as a hole-selective back contact to improve the photoelectrochemical performance of CuBi₂O₄ thin film photocathodes

P-type CuBi₂O₄ has recently been reported as a promising photocathode material for photoelectrochemical water reduction due to its optimal optical band gap and positive photocurrent onset potential. However, despite these favourable attributes, CuBi₂O₄ photocathodes have shown limitations in charge carrier transport within CuBi₂O₄ and across the interface with n-type fluorine doped tin oxide (FTO). To overcome the later limitation, a very thin and transparent p-type Cu doped NiO (Cu:NiO) back contact layer is inserted between the FTO substrate and CuBi₂O₄. The Cu:NiO layer is prepared by electron beam evaporation of Ni and Cu followed by post annealing in air. CuBi₂O₄ photocathodes with a 7 nm thick Cu:NiO back contact layer produce photocurrent densities up to 2.83 mA cm^-2 at 0.6 V versus RHE under back illumination with H₂O₂ as an electron scavenger, which is 25% higher than photocathodes without the back contact layer. This is also the highest reported photocurrent density for CuBi₂O₄ to date. The observed improvement in photocurrent density with the Cu:NiO back contact layer is attributed to hole selective transport across the CuBi₂O₄-Cu:NiO interface with a decrease in barrier height compared to the CuBi₂O₄-FTO interface. © 2019 The Royal Society of Chemistry.