Improving the photovoltage of Cu₂O photocathodes with dual buffer layers

Cuprous oxide (Cu₂O) is a promising oxide material for photoelectrochemical water splitting (PEC), and increasing its photovoltage is the key to creating efficient overall PEC water-splitting devices. Previous reports are mostly focused on optimizing the energy band alignment between Cu₂O and the n-type buffer layer to improve the photovoltage of Cu₂O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, Cu₂O photocathodes with a single buffer layer (Ga₂O₃) and dual buffer layers (Ga₂O₃/ZnGeO_x) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeO_x), the onset potential of the Cu₂O photocathode increases by 0.16 V. Operando electrochemical impedance spectroscopy measurements and analysis of the energy-level diagrams of each layer show that an energy level gradient between Ga₂O₃ and TiO₂ is created when ZnGeO_x is introduced, which eliminates the potential barrier at the interface of Ga₂O₃/TiO₂ and improves the photovoltage of the Cu₂O photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.