Improving the photovoltage of CuO photocathodes with dual buffer layers.

Cuprous oxide (CuO) 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 CuO and the n-type buffer layer to improve the photovoltage of CuO photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, CuO photocathodes with a single buffer layer (GaO) and dual buffer layers (GaO/ZnGeO) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeO), the onset potential of the CuO 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 GaO and TiO is created when ZnGeO is introduced, which eliminates the potential barrier at the interface of GaO/TiO and improves the photovoltage of the CuO photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.