Enhancing Direct Solar Water Splitting via ALD of Multifunctional TiO ₂ /Pt Nanoparticle Coatings With Engineered Interfaces to GaAs/GaInP Tandem Cells

Abstract Direct solar water splitting is a promising approach for sustainably producing hydrogen, but significant materials challenges must be overcome to achieve high efficiency and long‐term stability. This work demonstrates a tailored interface treatment combined with multifunctional surface coatings that significantly enhance the efficiency and lifetime of GaAs/GaInP tandem cells capable of unassisted solar water splitting. In particular, it is shown that exposure of the top AlInP window layer to a remote H 2 plasma effectively reduces the interfacial oxide, enhancing charge extraction and maximizing the available photovoltage. Subsequent atomic layer deposition (ALD) of a bilayer coating comprising a TiO 2 corrosion protection layer and Pt nanoparticles enhances the durability of the device, enables efficient electron extraction, and provides high catalytic activity. By tuning the Pt ALD process, a nanoparticulate morphology is achieved, ensuring high catalytic activity at low loading, thus minimizing parasitic light absorption and improving adhesion. The optimized dual‐junction photoelectrode achieves an initial maximum solar‐to‐hydrogen (STH) conversion efficiency of 17.1%, stabilizing at 16.2% for 170 min of continuous operation. Importantly, the tailored interfaces of the device result in a considerable photovoltage surplus, providing a route to systems offering higher STH efficiencies or for integration of Group III‐V semiconductor‐on‐Si tandems.