Activating Selenium for Atomic‐Level Defect Passivation and Band Engineering Enables High‐Efficiency Bulk Heterojunction CuInS 2 Solar Cells

ABSTRACT Copper indium sulfide (CuInS 2 ) stands out as a promising candidate for next‐generation photovoltaics due to its exceptional stability, high absorption coefficient, and eco‐friendly composition. However, realizing high‐quality CuInS 2 absorbers typically demands high‐temperature processing, which not only limits substrate compatibility but also invites interfacial degradation from elemental interdiffusion. Here, we overcome this challenge by introducing a low‐temperature selenium ion activation (LTSA) strategy that enables near‑surface atomic‑level defect passivation and band‑structure modulation in solution‐processed CuInS 2 absorbers. Beyond conventional passivation, LTSA orchestrates three synergistic effects: (i) selenium ion diffusion tailors a favorable band alignment; (ii) atomic substitution effectively passivates critical deep‐level defects (such as In Cu and V S ) through robust chemical bonding; and (iii) interfacial charge transport is markedly enhanced while carrier recombination is suppressed. These combined benefits yield CuInS 2 solar cells on transparent conducting oxide substrates with a record‐competitive power conversion efficiency of 11.43% and the highest fill factor of 66.88% reported to date. By establishing LTSA as a low‐temperature route for near‑surface atomic‐level defect passivation and structural reconstruction, this work unlocks new pathways toward high‐performance, cost‐effective inorganic photovoltaics.