Probing Femtosecond Lattice Displacement upon Photo-carrier generation in Lead Halide Perovskite

TL;DR

This study uses impulsive vibrational spectroscopy to investigate ultrafast lattice dynamics and electron-phonon interactions in lead halide perovskites, revealing a specific phonon mode involved in carrier-induced lattice distortion.

Contribution

It provides direct evidence of a coherent phonon mode driving lattice distortion during photoexcitation in lead halide perovskites, highlighting polaronic effects.

Findings

Identification of a 106 cm-1 coherent phonon mode

Lattice distortion occurs within 80 fs after photoexcitation

Polaronic effects are involved in carrier-lattice interactions

Abstract

Electronic properties and lattice vibrations are supposed to be strongly correlated in metal-halide perovskites, due to the "soft" fluctuating nature of their crystal lattice. Thus, unveiling electron-phonon coupling dynamics upon ultra-fast photoexcitation is necessary for understanding the optoelectronic behaviour of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal ground and excited state vibrational modes of methylammonium lead-bromide perovskite. We observe a coherent phonon at 106 cm-1 (13 meV), pertaining to the inorganic octahedral, which is peculiar of the electronic excited state and generated via displacive excitation mechanism. This indicates the formation of a new geometry, reached after a quarter of the phonon period T/4=80 fs, and fully equilibrated within the phonon lifetime of about 1 ps. Our observations unambiguously prove that this mode drives the crystalline distortion occurring upon carrier generation, implying the presence of polaronic effects.