Direct Observation of Ultrafast Lattice Distortions during Exciton–Polaron Formation in Lead Halide Perovskite Nanocrystals

Abstract

The microscopic origin of slow hot-carrier cooling in lead halide perovskites remains debated and has direct implications for applications. Slow hot-carrier cooling of several picoseconds has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (>1018 cm–3). These effects cannot be unambiguously disentangled with optical experiments alone. However, they can be distinguished by direct observations of ultrafast lattice dynamics, as these effects are expected to create qualitatively distinct fingerprints. To this end, we employ femtosecond electron diffraction and directly measure the sub-picosecond lattice dynamics of weakly confined CsPbBr3 nanocrystals following above-gap photoexcitation. While we do not observe signatures of a hot-phonon bottleneck lasting several picoseconds, the data reveal a light-induced structural distortion appearing on a time scale varying between 380 and 1200 fs depending on the excitation fluence. We attribute these dynamics to the effect of exciton–polarons on the lattice and the slower dynamics at high fluences to slower sub-picosecond hot-carrier cooling, which slows down the establishment of the exciton–polaron population. Further analysis and simulations show that the distortion is consistent with motions of the [PbBr3]− octahedral ionic cage, and closest agreement with the data is obtained for Pb–Br bond lengthening. Our work demonstrates how direct studies of lattice dynamics on the sub-picosecond time scale can discriminate between competing scenarios proposed in the literature to explain the origin of slow hot-carrier cooling in lead halide perovskites.

Light-Induced Transient Lattice Dynamics and Metastable Phase Transition in CH3NH3PbI3 Nanocrystals

“…53 Mechanistically it is relevant to compare the observed behavior in MAPbI 3 NCs with that of CsPbBr 3 NCs, previously reported in the literature. 11,19,54 Analogous experiments on CsPbBr 3 NCs by Kirschner et al identified a photoinduced distortion attributed to an orthorhombic to cubic transition, distinct from a strictly thermal phase transition in that it bypassed the tetragonal phase and the reverse cubic to orthorhombic transition occurred at a much higher temperature than the orthorhombic thermal transition. 11 More recently, Cannelli et al found using transient Xray absorption spectroscopy (XAS) that a polaronic distortion is likely responsible for much of the structural rearrangement in CsPbBr 3 .…”

Biexciton-like Auger Blinking in Strongly Confined CsPbBr3 Perovskite Quantum Dots

“…To fully explain our observations, a noncharging model including multiexcitonic states needs to be invoked. Trapped excitons (TEs), in which the electron–hole pairs are localized and stabilized by the lattice polarons, have been proposed in CsPbBr 3 perovskites. ,− In our SCPQDs, given the softness of the surface lattices and the strong exciton–surface interaction promoted by the size confinement, we propose that exciton–surface lattice polarons can form, which leads to TE formation.…”

Nonlinear terahertz control of the lead halide perovskite lattice

“…Accordingly, the dynamic screening picture in LHPs is incomplete, and its microscopic mechanism continues to be debated (27,28). Furthermore, identifying and characterizing polaronic behavior is experimentally difficult (26,(28)(29)(30)(31)(32). Optical Kerr effect (OKE) in LHPs (33,34) did not succeed in unveiling a lattice response and can be explained by an instantaneous electronic polarization (due to hyperpolarizability) instead (35).…”