Type-II Ga2SeTe/AlAs heterostructure: excellent optoelectronic properties for photocatalytic water splitting and solar cells

• Janus-Ga 2 SeTe/DLHS-AlAs heterojunction exhibits a moderate band gap of 1.73 eV with a type-II band arrangement. • Janus-Ga 2 SeTe/DLHS-AlAs heterojunction has potential application for photocatalytic water splitting. • High solar-to-hydrogen efficiency of 36.06 % is obtained in Janus-Ga 2 SeTe/DLHS-AlAs heterojunction under 2% biaxial strain. • The photoelectric conversion efficiency of the heterojunction is as high as 18.75%. Two-dimensional (2D) semiconductors have become a research hotspot in photovoltaic field because of their remarkable photoelectric properties. However, the rapid recombination of photogenerated charge carriers imposes a critical constraint on the optoelectronic efficiency. In this study, a type-II heterostructure consisting of Janus Ga 2 SeTe (Janus-Ga 2 SeTe) monolayer and AlAs monolayer with a double layer hexagonal structure (DLHS-AlAs) is systematically designed to enable efficient spatial separation of photogenerated carriers, ultimately enhancing optoelectronic performance as demonstrated by first-principles calculations. Our calculations indicate that Janus-Ga 2 SeTe/DLHS-AlAs heterostructure displays excellent stability and semiconducting character, showing a moderate bandgap and a distinct type-II band arrangement. It exhibits high visible-light absorption and water-splitting-compatible band alignment. Moreover, the system achieves notable solar-to-hydrogen conversion efficiency (26.64 %) and power conversion efficiency (18.75 %). Continuous bandgap control (0.67–1.75 eV) and reversible type-I/II switching are achieved via biaxial strain. Meanwhile, strain regulation (−2% to 2 %) optimizes light absorption while maintaining band positions suitable for water splitting, yielding a maximum STH efficiency of 36.06 % at 2 % strain. These results emphasize the material’s dual applicability in photocatalytic water splitting and solar cells.