Ag⁺-driven valence band engineering and defect suppression enable a record 0.75 V open-circuit voltage in...

Solution-processed copper indium sulfide (CuInS₂) is an attractive absorber for thin-film photovoltaics due to its non-toxicity and favorable optoelectronic properties. However, low-temperature processed planar heterojunction (PHJ) devices based on CuInS₂ nanoparticles have long been constrained to power conversion efficiencies (η) below 5-6%, primarily due to poor crystallinity and high trap density in CuInS₂. Here, we introduce an Ag⁺-mediated crystallization (AMC) strategy that dramatically enhances CuInS₂ film quality. The incorporation of Ag⁺ cations into the precursor promotes the chalcopyrite phase formation, effectively passivates copper and sulfur vacancy defects, and downshifts the valence band of the resulting CuInS₂ film. This synergistic suppression of deep-level defects, combined with valence band engineering, modulates the valence band offset at the CuInS₂/CuSCN interface in all-inorganic PHJ devices, yielding an optimized band alignment. Consequently, the Ag⁺-mediated CuInS₂ solar cell achieves a notable efficiency of η = 7.31% with a high open-circuit voltage (V_oc) of 0.75 V, due to a significantly suppressed non-radiative recombination and a greatly enhanced charge collection efficiency. The V_oc represents the highest value among the low-temperature processed all-inorganic CuInS₂-based PHJ solar cells, while the efficiency approaches the current record for such devices. Our work establishes a powerful and versatile strategy for defect control and interfacial band engineering in solution-processed chalcogenide photovoltaics.