Tandem Cuprous Oxide/Silicon Microwire Hydrogen-Evolving Photocathode with Photovoltage Exceeding 1.3 V

Large research efforts have been devoted to optimizing the output of earth-abundant photoabsorbers in solar-to-fuel (S2F) devices. Here, we report a Cu₂O/Ga₂O₃ heterojunction/Si microwire photocathode with an underlying buried radial Si p-n junction, which achieves efficient light harvesting across the visible spectrum to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%, with a photocurrent onset above +1.35 V vs RHE, a photocurrent density of ∼10 mA/cm² at 0 V vs RHE, and an ideal regenerative efficiency of 5.51%. We show step-by-step the effects of every photocathode design element (i.e., Si p-n junction, Cu₂O layer thickness, microwire length, microwire pitch, etc.) on the overall efficiency of our final microwire Si/Cu₂O photocathode by comparing every addition to a baseline Cu₂O photocathode. Lastly, we show a stable operation exceeding 200 h at a bias potential of +1.0 V vs RHE, with an average current density of 4.5 mA/cm²