Ultrafast Terahertz Tracking of Carrier Diffusion Across Thermally Induced Phase Transitions in Hybrid Perovskite MAPbBr ₃

The outstanding performance of organic–inorganic perovskite solar cells benefits from the exceptional electron–phonon interaction properties of both electrons and holes featured by the soft lattice in the material. In this study, by combining time‐resolved photoluminescence with optical‐pump terahertz probe spectroscopy, we systematically unveil the electron–phonon coupling characteristics of MAPbBr 3 single crystals across the complete sequence of structural phase transitions, as well as their impact on carrier transport. The results reveal pronounced differences in carrier–lattice interactions among distinct phases. We observe that the characteristic peak frequency (around 1.0 THz) of excited‐state‐phonon coupling undergoes a blue shift with increasing temperature after crossing the cubic‐to‐tetragonal phase transition, indicating a transformation from thermally activated small polarons in the orthorhombic phase to large polarons with moderate mobility (around 300 ) in the tetragonal and cubic phases. The Raman experiments further demonstrate that the cations in the tetragonal phase provide strong dielectric screening, suppressing bimolecular recombination and thereby maximizing the diffusion length in this phase. At low temperatures, however, the polaron formation time in the orthorhombic phase is severely prolonged, leading to reduced transport efficiency. Fundamentally, these transport properties are governed by phase transitions dictated by lattice symmetry, in which the soft‐mode‐driven cubic‐to‐tetragonal transition and the mode coupling from tetragonal to orthorhombic play key roles. Our work elucidates the unique role of electron‐phonon coupling in perovskite carrier transport, offering new strategies for performance optimization of halide perovskites through phase engineering.