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 Pb²⁺ 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.