Infrared optical absorption of Fröhlich polarons in metal halide perovskites

The formation of Fröhlich polarons in metal halide perovskites, arising from the charge carrier–longitudinal optical (LO) phonon coupling, has been proposed to explain their exceptional properties, but the effective identification of polarons in these materials is still a challenging task. Herein, we theoretically present the infrared optical absorption of Fröhlich polarons based on the Huang–Rhys model. We find that multiphonon overtones appear as the energy of the incident photons matches the multiple LO phonons, wherein the average phonon number of a polaron can be directly evaluated by the order of the strongest overtone. These multiphonon structures sensitively depend on the scale of electronic distribution in the ground state and the dimensionality of the perovskite materials, revealing the effective modulation of competing processes between polaron formation and carrier cooling. Moreover, the order of the strongest overtone shifts to higher ones with temperature, providing a potential proof that the carrier mobility is affected by LO phonon scattering. The present model not only suggests a direct way to verify Fröhlich polarons but also enriches our understanding of the properties of polarons in metal halide perovskites.