Electron-phonon interactions in halide perovskites

Strong electron-phonon interactions are frequently considered the origin of the unique electrical and optical properties of lead halide perovskites. Electron-phonon interactions induce the formation of a polaron, which is a charge carrier dressed with a phonon cloud. The details of polaron formation are crucial for carrier transport since polaron formation leads to a larger effective mass of a carrier. Several mechanisms have been proposed regarding the physics of polaron formation in halide perovskites, but the details are still under active debate. While the Fröhlich interaction plays an essential role in ionic crystals, we also need to consider the strong phonon anharmonicity of halide perovskites that may lead to the formation of an unconventional polaron. In this review article, we discuss the uniqueness of perovskite semiconductors from the viewpoint of electron-phonon interactions. We review the experimental results and the proposed models concerning the effective carrier mass and carrier mobility. Finally, we briefly explain two physical phenomena related to strong electron-phonon interactions: strong anti-Stokes photoluminescence and slow hot-carrier cooling. This review focuses on the electron-phonon interactions in lead halide perovskite semiconductors, which dominate their optical and electronic properties by forming polarons. We describe the uniqueness of perovskite semiconductors in terms of long-range and short-range electron-phonon interactions. The formation of polarons profoundly affects the carrier effective mass, mobility, and various optoelectronic properties. These topics are explained with recent experimental data.