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 (V_Cu) 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 V_Cu, 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.