Cu doped NiO (Cu:NiO) back contact layers are inserted between FTO substrates and CuBi₂O₄ thin films to improve the performance of CuBi₂O₄ photocathodes.

In this paper, we assess a tandem photoelectrochemical cell consisting of a W:BiVO4 photoanode top absorber and a CuBi2O4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO4 photoanode oxidizes water and produces oxygen at potentials ≥ 0.7 V vs. RHE when CoPi is added as a co-catalyst. However, the CuBi2O4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuOx is added as a co-catalyst because the photocurrent primarily goes towards photo-corrosion of CuBi2O4 rather than proton reduction. Protecting the CuBi2O4 photocathode with a CdS/TiO2 heterojunction and then adding RuOx as a co-catalyst prevents photo-corrosion and allows for photoelectrochemical production of hydrogen at potentials ≤ 0.3 V vs. RHE. A tandem photoelectrochemical cell composed of a W:BiVO4/Co-Pi photoanode and a CuBi2O4/CdS/TiO2/RuOx photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥ 0.4 V. Since the valence band of BiVO4 and conduction band of CuBi2O4 are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes i.e. the relatively low quasi-Fermi level splitting within BiVO4 and CuBi2O4. This work is the first experimental demonstration of hydrogen production from a BiVO4-CuBi2O4 based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi2O4 photocathodes.

A new method for enhancing the charge separation and photo‐electrochemical stability of CuBi₂O₄ photoelectrodes by sequentially depositing Bi₂O₃ and CuO layers on fluorine‐doped tin oxide substrates with pulsed laser deposition (PLD), followed by rapid thermal processing (RTP), resulting in phase‐pure, highly crystalline films after 10 min at 650 °C, is reported. Conventional furnace annealing of similar films for 72 h at 500 °C do not result in phase‐pure CuBi₂O₄. The combined PLD and RTP approach allow excellent control of the Bi:Cu stoichiometry and results in photoelectrodes with superior electronic properties compared to photoelectrodes fabricated through spray pyrolysis. The low photocurrents of the CuBi₂O₄ photocathodes fabricated through PLD/RTP in this study are primarily attributed to their low specific surface area, lack of CuO impurities, and limited, slow charge transport in the undoped films. Bare (without protection layers) CuBi₂O₄ photoelectrodes made with PLD/RTP shows a photocurrent decrease of only 26% after 5 h, which represents the highest stability reported to date for this material. The PLD/RTP fabrication approach offers new possibilities of fabricating complex metal oxides photoelectrodes with a high degree of crystallinity and good electronic properties at higher temperatures than the thermal stability of glass‐based transparent conductive substrates would allow.

CdS, BiVO ₄ , and Ga ₂ O ₃ buffer layers were tested between CuBi ₂ O ₄ and TiO ₂ in heterojunction photoelectrodes. Photoelectrochemical analysis and modulated surface photovoltage spectroscopy revealed that interface hole traps impacted device performance.

Compact spray-pyrolysed Cu₅V₂O₁₀ photocathodes with 1.9 eV bandgap show positive photocurrent onset voltage and promising photoresponse.