Beyond Homogeneous Models: Intrinsic Halide Gradients Remodel Phase Segregation Pathways in Wide-Bandgap Perovskites

Wide-bandgap (WBG) mixed-halide perovskites are highly promising for tandem photovoltaic applications, yet their operational stability remains limited by ion-migration-driven phase segregation. Existing mechanistic understandings on the ion migration dynamics largely rely on idealized homogeneous halide distribution models, overlooking the influence of intrinsic halide concentration gradients. Here, we elucidate the dynamics of halide-gradient-driven ion migration in WBG perovskites, uncovering a pronounced kinetic inversion in the migration energy barriers between I − and Br − that depends on local halide stoichiometry. Driven by the coupled effects of concentration gradients and spatially heterogeneous migration barriers, halide redistribution fundamentally deviates from classical homogenization, culminating in a distinctive “sandwich-like” segregated architecture with I-rich regions at both interfaces and Br-rich domains in the bulk, as directly validated by cross-sectional photoluminescence spectral mapping. Our findings establish a new mechanistic framework for gradient-driven phase segregation and provide actionable insights for designing intrinsically stable WBG perovskites for high-performance tandem photovoltaics.