Anisotropy and exciton self-trapping in the 1D perovskite C₄N₂H₁₄PbBr₄ from first principles

Low-dimensional organic-inorganic metal halide hybrids have remarkable optical and electronic properties and better stability against heat and moisture. We study a 1D perovskite of formula C₄N₂H₁₄PbBr₄, consisting of PbBr chains separated by organic cations. Experiments showed a large Stokes shift (0.83 eV) with broadband emission [Nat Commun 8, 14051 (2017)] which hints at interesting photo-physics involving self-trapped excitons. We calculate the highly anisotropic optical, bandstructure, vibrational, and transport properties of this 1D perovskite, which could be used for polarized photodetectors and LEDs. The bands are highly dispersive along PbBr chains and nearly flat along other directions, leading to a factor of 100 in conductivity as calculated by Boltzmann transport. We find an indirect gap and a direct gap which is just slightly higher in energy. Our GW/Bethe-Salpeter equation calculations using BerkeleyGW show strong anisotropy in absorption, especially in the lowest exciton which has a binding energy of about 1 eV. We calculate excited-state forces based on these results and our vibrational calculations to find the coupling of excitons and phonons, from which we can predict exciton self-trapping and mechanisms of broadband emission.