Exciton-polaron spectral structures in two dimensional hybrid lead-halide perovskites

TL;DR

This study models excitonic absorption in 2D hybrid lead-halide perovskites, revealing multiple excitonic states, vibronic progressions, and strong phonon coupling influenced by organic cation chemistry and temperature.

Contribution

It introduces a quantitative Wannier-based framework capturing excitonic lineshapes, multiple states, and vibronic effects, highlighting the hybrid and polaronic nature of excitons in these materials.

Findings

Four distinct excitonic states separated by ~35 meV

Significant variation of Huang-Rhys factors with temperature and cation

Evidence of strong coupling to localized vibrations and polaronic effects

Abstract

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. We present a phenomenological, yet quantitative framework to simulate excitonic absorption lineshapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by $35\pm5$\,meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the lineshape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.