Coherent Phonons, Localization and Slow Polaron Formation in Lead-free Gold Perovskite

TL;DR

This study investigates the coupled electronic and vibrational dynamics in lead-free Caesium gold bromide perovskite, revealing strong exciton-phonon interactions, slow polaron formation, and potential for controlling coherent phonons with light.

Contribution

It provides new insights into charge transport and polaron formation in lead-free perovskites using ultrafast spectroscopy and band structure calculations.

Findings

Strong coupling between charge transfer states and phonons.

Observation of slow polaron formation within 10-20 ps.

Estimated polaron binding energy of ~80 meV.

Abstract

Lead-free metal halide perovskites are emerging as less-toxic alternatives to their lead-based counterparts. However, their applicability in optoelectronic devices is limited, and the charge transport dynamics remain poorly understood. Understanding photo-induced charge and structural dynamics is critical for unlocking the potential of these novel systems. In this work, we employ ultrafast optical and Raman spectroscopy combined with band structure calculations to investigate the coupled electronic and vibrational dynamics in Caesium gold bromide, a promising lead-free perovskite. We find that the band-edge charge transfer states are strongly coupled to Au-Br stretching phonon modes, leading to frequency modulation of absorption by impulsively excited coherent phonons. Early-stage relaxation is characterized by dynamics of delocalized charge transfer excitation and slowly decaying coherent phonons. The electronic and vibrational relaxation reveals a slow formation of a localized polaronic state in the 10-20 ps timescale. Using a displaced harmonic oscillator model, the polaronic binding energy is estimated to be ~80 meV following lattice relaxation along the phonon modes. Strong exciton-phonon coupling and slow polaron formation via coupling to lattice modes make this material a promising testbed for the control of coherent phonons and localized polaronic states using light.