Theoretical Insights into the Role of Lattice Fluctuations on the Excited Behavior of Lead Halide Perovskites

Abstract

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 in 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 to the rocking and tilting motions of the 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 halide perovskites challenging. In this Account, we review our recent efforts to understand how the soft, polar lattice of this class of materials alters their excited state properties. We highlight the influence of the lattice on static properties by examining the quasiparticle binding energies and fine structure. With perovskite nanocrystals, we discuss how incorporating lattice distortion is essential for accurately defining the exciton fine structure. By considering lattices across various dimensionalities, we are able to illustrate that the energetics of excitons and their complexes are significantly modulated by polaron formation. Beyond energetics, we also delve into how the lattice impacts the dynamic properties of quasi-particles. The mobilities of charge carriers are studied with various charge-lattice coupling models, and the recombination rate calculation demonstrates the molecular origin on the peculiar feature in the lifetime of charge carriers in these materials. In addition, we address how lattice vibrations themselves relax upon excitation from charge-lattice coupling. Throughout, these examples 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. These insights may further guide the development of next-generation optoelectronic devices with improved efficiency and stability while also inspiring new research directions to explore emerging quantum phenomena in these materials.