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.
Highlights
Published: 22 March 2021Among the various technologies for the sustainable production of hydrogen, photoelectrochemical (PEC) water splitting using photoactive materials is a promising and low-cost method with almost zero carbon emissions [1,2]
There are three key factors to be considered: (1) Ensuring the generation of electron-hole pairs (EHPs) with no degradation of the optical absorption efficiency of p-type Cu2 O. This requires the use of transparent overlayers with wider bandgaps than those in Cu2 O. (2) An adequate band alignment favored by the charge separation and charge transport behaviors in the heterojunction
The details of the used overlayers are listed in Because the Atomic layer deposition (ALD) coating is typically homogeneous and there is no phase change at the used temperature, no apparent changes in the morphology of the microcrystals were detected in the samples before and after coating
Summary
Among the various technologies for the sustainable production of hydrogen, photoelectrochemical (PEC) water splitting using photoactive materials is a promising and low-cost method with almost zero carbon emissions [1,2]. It is necessary to emphasize the importance of photocathodes that can generate direct hydrogen products and to conduct extensive research in this field. Among various p-type photocathodes, copper-based oxide materials have received the most attention. Cu2 O is one of the most promising p-type semiconductors for reducing water to hydrogen owing to the suitable conduction band position and its good visible absorption [11,12]. Cu2 O can generate a photocurrent of 14.7 mA cm−2 under AM (air mass) 1.5G illumination (100 mW cm−2 ), which corresponds to a maximum solar-to-hydrogen (STH) conversion efficiency of 18% [13]
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