Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskites Nanocrystals

TL;DR

This study identifies and quantifies photoinduced polaronic lattice distortions in CsPbBr3 perovskite nanocrystals, revealing the role of electron-phonon interactions in their optoelectronic behavior.

Contribution

It combines advanced X-ray absorption spectroscopy and ab-initio simulations to precisely characterize structural changes during photoexcitation, a novel approach in this context.

Findings

Photoinduced lattice distortions are due to polaronic effects involving an 18 meV phonon mode.

Carrier recombination reversibly restores the original lattice structure.

Thermal effects are ruled out as the primary cause of observed distortions.

Abstract

The development of next generation perovskite-based optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron-phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of time-resolved and temperature-dependent studies at Br K-edge and Pb L3-edge X-ray absorption with refined ab-initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperature-dependent XAS results rules out thermal effects as the primary source of distortion of the Pb-Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr3 perovskites photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.