Theoretical insights into the role of lattice fluctuations on the excited behavior of lead halide perovskites

Lead halide perovskites have been extensively studied as a class of materials with unique optoelectronic properties. A fundamental aspect that governs optical and electronic behaviors within these materials is the intricate coupling between charges and their surrounding lattice. Unravelling the role of charge-lattice interactions on the optoelectronic properties in lead halide perovskites is necessary to understand their photophysics. Unlike traditional semiconductors where a harmonic approximation often suffices to capture lattice fluctuations, lead halide perovskites have a significant anharmonicity attributed from the rocking and tilting motions of inorganic framework. Thus, while there is broad consensus on the importance of the structural deformations and polar fluctuations on the behavior of charge carriers and quasiparticles, the strongly anharmonic nature of these fluctuations and their strong interactions render theoretical descriptions of lead halides perovskites challenging. In this Account, we review our recent efforts to understand how the soft, polar lattice of this class of materials alter their quasiparticle binding energies and fine structure, charge mobilities, and lifetimes of phonons and excess charges. Throughout, these are aimed at characterizing the interplay between lattice fluctuations and optoelectronic properties of lead halide perovskites and are reviewed in the context of the effective models we have built, and the novel theoretical methods we have developed to understand bulk crystalline materials, as well as nanostructures, and lower dimensionality lattices. By integrating theoretical advances with experimental observations, the perspective we detail in this account provides a comprehensive picture that serves as both design principles for optoelectronic materials and a set of theoretical tools to study them when charge-lattice interactions are important.