Crystallization Control Through Intermediates in Br‐Enriched Mixed‐Halide Wide‐Bandgap Perovskites

Br‐enriched wide‐bandgap halide perovskites are essential for tandem and indoor photovoltaic applications due to their tunable bandgaps and high open‐circuit voltages. However, increasing bromide content intensifies compositional instability and halide segregation, leading to voltage losses and long‐term operational degradation. Emerging evidence indicates that such instability is frequently preconditioned during film formation through crystallization intermediates, rather than merely manifesting as light‐induced halide segregation during operation. This review presents an intermediate‐centric framework for understanding crystallization in Br‐rich mixed‐halide perovskites. Crystallization intermediates are defined as emergent states encompassing both coordination‐defined species and structural intermediates (e.g., polytypes), governed by the competitive binding of halides, solvents, and additives to Pb ²⁺ centers, thereby regulating nucleation kinetics, halide redistribution, and lattice consolidation. Strong Pb–Br interactions restrict the range of kinetically accessible intermediate configurations, promoting halide‐segregated growth and kinetically frozen compositional gradients that subsequently appear as defect‐rich regions and ion‐migration pathways. Recent advances demonstrate that solvent and additive engineering reshape intermediate accessibility and lifetime, enabling stabilization, synchronization, reconstruction, or even elimination of intermediate states. An intermediate‐centered perspective on crystallization provides a chemically grounded basis for improving halide homogeneity, defect control, and stability in Br‐rich wide‐bandgap perovskite photovoltaics.