Strong Coupling of Self-Trapped Excitons to Acoustic Phonons in Bismuth Perovskite $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$

TL;DR

This study reveals that in bismuth-based perovskite Cs3Bi2I9, carriers form self-trapped excitons rapidly and induce long-lived strain fields via acoustic phonons, highlighting unique carrier-phonon interactions relevant for optoelectronic applications.

Contribution

It provides a detailed experimental and theoretical analysis of carrier-phonon coupling and exciton dynamics in Cs3Bi2I9, a bismuth-based perovskite, revealing strong self-trapping and acoustic phonon interactions.

Findings

Carriers form self-trapped excitons on ultrafast timescales.

Long-lived excitons induce coherent acoustic phonons and strain fields.

Limited initial coupling to optical phonons, with significant acoustic phonon effects at longer times.

Abstract

To assess the potential optoelectronic applications of metal-halide perovskites, it is critical to have a detailed understanding of the nature, strength, and dynamics of the interactions between carriers and the polar lattices. Here, we report the electronic and structural dynamics of bismuth-based perovskite $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ revealed by transient reflectivity and ultrafast electron diffraction. A cross-examination of these experimental results combined with theoretical analyses allows the identification of the major carrier-phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ form self-trapped excitons on an ultrafast time scale. However, they retain most of their energy and their coupling to Fr\"ohlich-type optical phonons is limited at early times. Instead, the long-lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub-ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye-Waller random atomic motions on longer times. The current results suggest the potential use of bismuth-based perovskites for applications other than photovoltaics to take advantage of carriers' stronger self-trapping and long lifetime.