Lead halide perovskites have emerged as highly promising materials for solar energy conversion, with single-junction power conversion efficiencies now exceeding 26%. Their soft crystal lattice combines high defect tolerance with mechanical flexibility, but also enables halide ions to migrate through the lattice, particularly under external stimuli such as light. This phenomenon, commonly referred to as halide segregation, is widely regarded as detrimental for photovoltaic performance, as it can reduce charge-carrier mobility and open-circuit voltage. At the same time, controlled halide redistribution has been proposed as a functional mechanism in applications such as light-emitting devices and optical memory. A detailed understanding of halide segregation is therefore essential, both to suppress and to exploit it where desired.

Halide segregation is most often investigated using photoluminescence (PL) spectroscopy. However, this approach is inherently challenging, as optical excitation itself can drive ion migration. Furthermore, most studies focus on mixed-halide perovskites with intermediate bromide–iodide ratios relevant for tandem and wide-bandgap solar cells. In this regime, small changes in halide composition typically result in only subtle PL shifts, complicating the sensitive and spatially resolved detection of early-stage segregation.

In this work, we introduce an approach to sensitively probe halide segregation while minimizing light-induced ion migration, using MAPbBr₃ thin films containing very low iodide concentrations (<1%). We find that iodide ions do not distribute homogeneously across the films, but instead form micron-scale clusters with locally elevated iodide content. By exciting the films with photon energies below the bandgap of pristine MAPbBr₃ but above that of the iodide-rich regions, we perform two-dimensional PL mapping that selectively probes these regions. By varying laser power and excitation wavelength, we can control growth and redissolution of the iodide-rich clusters. Because the bulk MAPbBr₃ remains optically inactive under these conditions, halide segregation is not induced in the surrounding material. This provides a clean and sensitive method to directly observe ion migration dynamics within iodide-rich clusters, offering new insight into the mechanisms governing halide segregation in mixed-halide perovskites.