Composition-driven anion vacancy control in I–III–VI CuIn(S1−xSex)2 quantum dots for efficient photoelectrochemical hydrogen production

Heavy-metal-free I–III–VI quantum dots (QDs) are promising candidates for solar energy conversion owing to their strong light absorption, broadly tunable bandgap spanning the full solar spectrum, and environmentally benign composition. However, precise control over their composition and defect states remains challenging due to their complex multinary synthesis chemistry, which critically governs their optoelectronic properties. Here, we demonstrate composition-driven control of anion vacancies in p -type CuIn(S 1− x Se x ) 2 QDs and elucidate their impact on photoelectrochemical (PEC) hydrogen production. Systematic variation of the chalcogenide S/Se ratio enables modulation of anion vacancy concentrations without altering QD size or Cu/In stoichiometry. Notably, CuIn(S 0.5 Se 0.5 ) 2 QDs exhibit the lowest density of anion vacancies, attributed to enhanced anion coordination and reduced lattice distortion. These structural improvements result in increased hole concentration and extended carrier lifetimes. As a result, when integrated into TiO 2 -based photoanodes, these QDs deliver a high photocurrent density of 15.1 mA cm −2 at 0.6 V RHE for PEC hydrogen generation, exceeding the performance of most previously reported heavy-metal-free QD-based systems. This work provides fundamental mechanistic insights into the role of anion vacancies in I–III–VI QDs and offers a promising strategy for advancing their PEC performance. • Precise control of anion vacancies in heavy-metal-free I–III–VI QDs is achieved through chalcogenide alloying. • Systematic S/Se ratio tuning modulates vacancy levels without altering QD size or cation ratio. • CuIn(S 0.5 Se 0.5 ) 2 QDs show minimized vacancy density, increased carrier concentrations, and longer carrier lifetimes. • Optimized QD photoanodes deliver 15.1 mA cm −2 for photoelectrochemical hydrogen production, outperforming most heavy-metal-free QD-based systems.