Lead halide perovskites have emerged as highly promising materials for solar energy conversion, with single-junction power conversion efficiencies exceeding 26%. Their soft crystal lattice combines high defect tolerance with mechanical flexibility, but also enables halide ion migration, particularly under illumination. This phenomenon, known as halide segregation, is generally considered detrimental due to its impact on charge-carrier mobility and open-circuit voltage. However, controlled halide redistribution may also offer opportunities for engineering local band structure. Such control over the local band gap is particularly relevant at device interfaces, where band alignment and interfacial recombination velocities govern charge extraction in perovskite solar cells. Harnessing these light-induced changes could enable dynamic tuning of interfacial band offsets, potentially reducing the need for static interlayers.

Here, we show that halide segregation at low iodide concentrations gives rise to dynamic emission peaks in MAPbBr₃ thin films doped with ~1% iodide. Under continuous illumination, photoluminescence (PL) spectra exhibit a stable emission peak around 540 nm, accompanied by multiple dynamic peaks spanning the range between pure bromide and pure iodide emission. These transient features emerge and disappear on sub-second timescales and can shift by more than 100 nm within seconds. They are observed across different cation compositions and doping levels. This behavior contrasts with halide segregation at higher iodide fractions, which is typically characterized by a gradual redshift toward a stable emission peak.

Temperature-dependent PL measurements (300 K to 3 K) reveal a slowing of these dynamics at lower temperatures, with dynamic behavior still visible down to 192 K. This temperature dependence supports a mechanism based on light-induced ion migration, leading to the fast formation and dispersion of iodide-rich regions with lower band gap. If this dynamic halide redistribution could be spatially controlled, these fluctuations could be localized at perovskite device interfaces. This would enable in-situ tuning of band offsets and recombination velocities under operating conditions, rather than relying on static interfacial layers.

These results demonstrate a previously underexplored regime of halide segregation in which low dopant concentrations lead to dynamically evolving emission peaks. This provides a pathway toward tuning interfacial band offsets and recombination velocities, which could enable improved charge extraction and reduced interfacial losses in perovskite solar cells.