Enhancing Charge Transport in CuBi2O4 Films: The Role of a Protective TiO2 ALD Coating Probed by Impedance Spectroscopy

Abstract The common solution for protecting p‐type semiconductors against photocorrosion in a photo‐electrochemical (PEC) cell is by applying a TiO 2 over‐layer via atomic layer deposition (ALD). However, for the case of CuBi 2 O 4 (CBO), this approach leads to a significant decline in electrode performance, despite the small conduction band offset between CBO and TiO 2 . Here, electrochemical impedance spectroscopy (EIS) under light illumination is used to study how to enhance charge transport in CuBi 2 O 4 /TiO 2 photocathodes. A 15 nm TiO 2 overlayer enables a small charge transfer resistance to the electrolyte while preserving the performance and stability of the CBO film. When increasing the TiO 2 thickness from 15 to 20 nm, the photogenerated currents decrease by 74%. The EIS data are fit with an equivalent circuit model that enabled to extract the charge transfer resistances, capacitances, and time constants that influence the PEC performance of the electrode as a function of the TiO 2 layer thickness, together with the flat‐band potentials and doping densities of both the CBO and TiO 2 layers under light illumination. The decline in performance is attributed to accumulation and recombination of photogenerated carriers at the CBO‐TiO 2 interface, due to a band mismatch between the two semiconductors.