Counter‐Halide‐Directed Ion‐Pair Passivation Enables High‐Performance Wide‐Bandgap Perovskites for Perovskite/Cu(in,Ga)Se 2 Tandem Solar Cells

Wide‐bandgap (WBG) perovskites (PVSK) are key top‐cell absorbers for tandem photovoltaics but remain limited by severe open‐circuit voltage ( V OC ) losses and photoinduced halide segregation. Since iodide‐ and bromide‐related defects coexist at the surface and bulk of Br/I mixed‐halide WBG PVSK, the counter‐halide identity in ammonium passivators may critically influence defect passivation effectiveness. Moreover, while deprotonation of ammonium passivator has emerged as an important factor governing interfacial stability, its discussion has focused mainly on organic cation structure, leaving the counter‐halide's role in regulating deprotonation‐driven side reactions largely unexplored. Here, we establish a counter‐halide‐directed ion‐pair passivation strategy using 4‐fluorobenzylammonium halides (FBz‐Cl, FBz‐Br, and FBz‐I). Density functional theory calculations combined with optoelectronic and interfacial characterizations reveal that the counter‐halide simultaneously governs defect‐binding affinity, nonradiative recombination, energy‐level alignment, and deprotonation‐driven degradation. Among the passivators, FBz‐Br achieves the optimal balance of strong defect passivation, favorable band alignment with C 60 , and enhanced stability. Consequently, FBz‐Br‐treated PVSK exhibits suppressed nonradiative recombination and reduced V OC deficit, delivering a power conversion efficiency (PCE) of 21.89% with improved resistance to moisture, thermal, and light‐induced degradation. Integrated with Cu(In,Ga)Se 2 (CIGS) bottom cells, the optimized WBG PVSK enables PVSK/CIGS tandem solar cells with PCEs of 29.51% in four‐terminal and 25.65% in two‐terminal configurations.