Exciton binding energy of CH3NH3PbX3 under low magnetic field: implications of strong exciton-phonon coupling

Lead halide perovskites are a fascinating class of materials for a wide range of applications because of their unique optoelectronic features. In spite of the recent discussions on the strong electron-phonon coupling of halide perovskites, its effects on the excitonic properties are not yet completely understood. Here, we investigate the exciton binding energies of CH3NH3PbX3 [X = I, Br, and Cl] under weak magnetic fields below 7 T where the cyclotron energy is sufficiently small compared with LO phonon energy and exciton Rydberg energy. Highly sensitive magneto-reflectance spectroscopy was employed to detect the high-order exciton Rydberg states and Landau levels. The larger reduced masses and smaller exciton binding energies under the lower magnetic field suggest the strong exciton-phonon coupling. We discuss the field-dependent excitonic properties in terms of phonon screening effect and derive the polaronic and excitonic material parameters that can account for the magneto-optical properties of lead halide perovskites.