Ab Initio Theory of Exciton Polarons and Self-Trapped Excitons

Neutral excitons could interact with the hosting lattice and lead to self-localization, forming exciton polarons or self-trapped excitons. Exciton polarons have been suggested to actively participate in photocatalytic processes, give rise to broadband luminescence, and result in Stokes shift. Furthermore, they could serve as precursors to permanent defects and are thus crucial for quantum technologies. However, the broad interest in the physics of exciton polarons is asymmetric with the scarce ab initio characterizations of this excited-state species, primarily due to the necessity of using large supercells and the incurred heavy computational cost. In this talk, I will present a supercell-free theory of exciton polarons that is amenable to first-principles calculations. This theory allows us to identify the ubiquitous existence and significant impact of exciton polarons with disparate length-scales in a range of systems, including lithium-ion battery electrode materials, photocatalytic semiconductors, and optoelectronic halide perovskites. I will further demonstrate how this theory can be reduced to a simplified model that sheds light on the formation condition and mechanism of exciton polarons.