Enhancing Direct Solar Water Splitting via ALD of Multifunctional TiO₂/Pt Nanoparticle Coatings With Engineered Interfaces to GaAs/GaInP Tandem Cells

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 H₂ 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 TiO₂ 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.