Ag+-driven valence band engineering and defect suppression enable a record 0.75 V open-circuit voltage in low-temperature, all-inorganic CuInS2 solar cells.

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 (Voc) of 0.75 V, due to a significantly suppressed non-radiative recombination and a greatly enhanced charge collection efficiency. The Voc 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.