Halide Segregation Induces Carrier Funneling and Polaron Mode Modulation in Mixed-Halide Perovskites

Mixed-halide perovskites enable bandgap tunability but suffer from light-induced halide segregation that limits device stability. While phase segregation is known to redistribute carriers, its influence on polaron dynamics and electron-phonon coupling remains unclear. Here, we combine optical pump-terahertz probe spectroscopy with microscale transient reflection microscopy to resolve carrier transport and polaron evolution with simultaneous temporal and spatial resolution. We show that slight compositional fluctuations trigger subpicosecond carrier funneling from Br-rich to I-rich domains, forming micrometer-scale, high-density carrier reservoirs within ∼20 ps. Despite pronounced spatial redistribution, ensemble-averaged mobility and photoconductivity remain largely unchanged. Frequency-resolved terahertz spectroscopy reveals that the locally elevated carrier density enhances polaron-polaron overlap, leading to strengthened anharmonic electron-phonon coupling. This is manifested as a blue shift (∼0.8 THz) of the polaron resonance, increased oscillator strength, and shortened phonon lifetime. These results demonstrate that early stage halide segregation primarily renormalizes polaron modes through carrier spatial accumulation rather than degrading intrinsic transport, providing mechanistic insight into the microscopic origins of performance loss in mixed-halide perovskite optoelectronics.