Electron-Phonon Coupling in Weakly Quantum-Confined Perovskite Nanocrystals

Lead-halide perovskite nanocrystals (NCs) have attracted increasing research interest because of their exceptional optoelectronic properties, in which electron-phonon coupling (EPC) plays a crucial role by dictating how charge carriers interact with lattice vibrations. The underlying EPC mechanism is dominated by the longitudinal optical (LO) phonon scattering mediated by the Fröhlich interaction. Although EPC has been extensively studied in bulk systems, a clear understanding of how crystal-structure factors influence EPC in NCs remains lacking. In particular, a comprehensive picture of EPC experienced by both hot carriers and band-edge carriers is still elusive. Herein, we systematically investigate these effects in lead-halide perovskite NCs with varying sizes and representative A-site cations (A = MA⁺, FA⁺, or Cs⁺) by combining analyses of photon scattering during hot-carrier relaxation and steady-state photoluminescence (PL), complemented by theoretical calculations. At room temperature, the EPC for energetic carriers is found to be weaker in MAPbBr₃ and CsPbBr₃ NCs than in FAPbBr₃ NCs. Further analysis reveals that the EPC is governed by the combined effects of dynamic screening and the vibronic motions of the A-site cations. Furthermore, EPC becomes progressively stronger as NC size decreases with a notably pronounced enhancement when the NC size falls below the exciton Bohr diameter, likely due to the modified excitonic properties arising from the stronger quantum confinement. Lastly, temperature-dependent LO-phonon broadening of the PL further confirms the roles of cation dynamics and NC size and, importantly, indicates that the hot carriers and band-edge carriers experience similar phonon scattering environments.