Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals

Abstract

The development of next-generation perovskitebased 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 CsPbBr 3 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 timeresolved and temperature-dependent studies at Br K and Pb L 3 X-ray absorption edges 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 temperaturedependent 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 CsPbBr 3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

Direct and real-time observation of hole transport dynamics in anatase TiO2 using X-ray free-electron laser

“…The excited electrons have two lifetime components, one of 0.3 ns and one of 6 ns 20 . However, direct evidence for hole dynamics has never been observed in TMOs, although indirect evidence has been reported for hole polarons in ZnO 23 , 24 and in perovskites 25 , 26 .…”

Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites

“…Moreover, these results highlight the difference between thermal and light-induced structural responses in CsPbBr 3 perovskites, the former being intrinsically random in nature and the latter selectively driven by electron–phonon coupling. 26 The deeper understanding of the perovskite responses upon different stimuli will open new opportunities for manipulating and stabilizing the lattice structure in realistic applications.…”

Direct measurement of radiative decay rates in metal halide perovskites

“…There exists a growing body of experimental evidence in literature indicating that the perovskite lattice relaxes around electrons and holes to form large charged polarons. [53][54][55] Lattice reorganization in the excited state to form charged polarons shifts the chemical equilibrium between excitons and charged carriers towards these latter. In other words, the presence of polarons reduces the concentration of light-emitting excitons, being polarons heavier and closer in energy to excitons with respect to lattice-unrelaxed charges.…”