Buried Interface Ionic Engineering Enables Defect Passivation and Efficient Cu₂AgBiI₆ Solar Cells.

Lead-free perovskite-inspired materials, particularly Cu₂AgBiI₆ (CABI), hold promise for environmentally friendly photovoltaics. However, their performance is hampered by unidentified microscopic loss mechanisms. Here, combining depth-resolved chemical profiling with first-principles calculations, we observe a reproducible depth dependence in the Cu related signal during film formation, with a comparatively higher Cu/(Ag+Bi) ratio near the top region than in the inner bulk, indicating Cu mobility and redistribution under typical processing conditions. Density functional theory (DFT) further suggests that Cu vacancies (VCu) are among the most thermodynamically accessible defects and can act as deep recombination centers. To mitigate these losses, we introduce a thin KCl layer at the buried SnO₂/CABI interface. DFT indicates that K can bind favorably with VCu, consistent with an ionic passivation effect. With this buried interface treatment, planar devices reach a champion power conversion efficiency of 2.11%. This work provides mechanistic insights into defect mediated recombination in lead-free CABI and demonstrates a practical buried interface ionic passivation strategy that may be applicable to other multicomponent ionic semiconductors.