Competition between Hot-Electron Cooling and Large Polaron Screening in CsPbBr₃ Perovskite Single Crystals

Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to form large polarons on subpicosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr₃ perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of -0.64 ± 0.06 eV/ps and -0.82 ± 0.08 eV/ps at 300 and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of unscreened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large polaron formation from electron-LO phonon interaction, with temperature-dependent polaron formation time constants of τ_p = 0.7 ± 0.1 and 2.1 ± 0.2 ps at 300 and 80 K, respectively. The increase in polaron formation rate with temperature is correlated with the broadening in phonon resonances, suggesting that phonon disorder and dephasing facilitate large-polaron formation. The large polaron formation rate is not competitive with the cooling rate of unscreened hot electrons in CsPbBr₃, in contrast to hybrid CH₃NH₃PbBr₃ (or CH₃NH₃PbI₃) where the two rates are similar. This contrast explains the observation of long-lived hot carriers in the latter but not the former.