Phonon-Mediated Attractive Interactions between Excitons in Lead-Halide-Perovskites

TL;DR

This study investigates how electron-phonon interactions in lead-halide perovskites influence their optical properties, revealing strong coupling effects, especially in FAPbBr3, and demonstrating phonon-mediated attractive interactions between excitons.

Contribution

It provides the first direct measurement of lattice reorganization due to photoexcitation in lead-halide perovskites and uncovers superlinear scaling of exciton interactions mediated by phonons.

Findings

Strong electron-phonon coupling in FAPbBr3

Weaker coupling observed in CsPbBr3

Phonon-mediated attractive interactions scale quadratically with exciton number

Abstract

Understanding the origin of electron-phonon coupling in lead-halide perovskites (LHP) is key to interpreting and leveraging their optical and electronic properties. Here we perform femtosecond-resolved, optical-pump, electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in LHP nanocrystals. Photoexcitation is found to drive a reduction in lead-halide octahedra tilts and distortions in the lattice, a result of deformation potential coupling to low energy optical phonons. Our results indicate particularly strong coupling in FAPbBr3, and far weaker coupling in CsPbBr3, highlighting differences in the dominant machanisms governing electron-phonon coupling in LHPs. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multiexcitonic state constructively interfere, giving rise to a coupling strength which scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in LHPs.