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

Abstract

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 VRHE for over 100 min.

Key Strategies on Cu2O Photocathodes toward Practical Photoelectrochemical Water Splitting

“…However, TiO2 has been widely applied to the protection layer of Cu2O photocathodes, rather than the overlayer for the heterojunction effect, because it is an intrinsically stable oxide in the aqueous solution. Although the ZnO overlayer is also effective to improve the charge transport by the formation of heterojunction with Cu2O, the aluminum-doped zinc oxide (AZO) overlayer is more efficient for the heterojunction effect because it is more conductive compared to the ZnO overlayer [83,84]. Minami et al reported that the gallium oxide (Ga2O3)/Cu2O heterostructure improves the photovoltage of Cu2O-based solar cells due to the decreased defect levels at the interfaces [85].…”

Enhancing photoelectrocatalytic activity and stability of p-Cu2O photocathode through n-TiO2 coating for improved H2 evolution reaction

“…Therefore, there have been many studies on the renewable energy such as solar energy to replace fossil fuels [1]. Solar water splitting is a promising technology to convert solar energy into chemical energy stored within H 2 molecules regarded as the next generation of green energy carriers [2][3][4][5][6]. A photoelectrochemical cell (PEC) used for solar water splitting is constituted of a photoanode and a photocathode where O 2 and H 2 are generated, respectively.…”