Optimal n-Type Al-Doped ZnO Overlayers for Charge Transport Enhancement in p-Type Cu₂O Photocathodes

An effective strategy for improving the charge transport efficiency of p-type Cu₂O photocathodes is the use of counter n-type semiconductors with a proper band alignment, preferably using Al-doped ZnO (AZO). Atomic layer deposition (ALD)-prepared AZO films show an increase in the built-in potential at the Cu₂O/AZO interface as well as an excellent conformal coating with a thin thickness on irregular Cu₂O. Considering the thin thickness of the AZO overlayers, it is expected that the composition of the Al and the layer stacking sequence in the ALD process will significantly influence the charge transport behavior and the photoelectrochemical (PEC) performance. We designed various stacking orders of AZO overlayers where the stacking layers consisted of Al₂O₃ (or Al) and ZnO using the atomically controlled ALD process. Al doping in ZnO results in a wide bandgap and does not degrade the absorption efficiency of Cu₂O. The best PEC performance was obtained for the sample with an AZO overlayer containing conductive Al layers in the bottom and top regions. The Cu₂O/AZO/TiO₂/Pt photoelectrode with this overlayer exhibits an open circuit potential of 0.63 V and maintains a high cathodic photocurrent value of approximately -3.2 mA cm^-2 at 0 V_RHE for over 100 min.