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. 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 and Pb L3 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 temperature-dependent XAS results rules out thermal effects as the primary source of distortion of the Pb–Br bonding motif during photoexcitation. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. This work provides a comprehensive description of the CsPbBr3 perovskites’ photophysics and reconciles the temperature-dependent modifications of the optical band gap, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.
Contact
Dr. Grigory Smolentsev
SuperXAS beamline
Operando spectroscopy
Swiss Light Source, Paul Scherrer Intitute
5232 Villigen-PSI, Switzerland
Telephone: +41 56 310 51 73
E-mail: grigory.smolentsev@psi.ch
Original Publications
O. Cannelli, J. Wiktor, N. Colonna, L. Leroy, M. Puppin, C. Bacellar, I. Sadykov, F. Krieg, G. Smolentsev, M. V. Kovalenko, A. Pasquarello, M. Chergui and G. F. Mancini "Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites" J. Phys. Chem. Lett (2022) https://doi.org/10.1021/acs.jpclett.2c00281
O. Cannelli, N. Colonna, M. Puppin, T. Rossi, D. Kinschel, L. Leroy, J. Löffler, J. Budarz, A.M. March, G. Doumy, ; A. Al Haddad, M.-F. Tu, Y. Kumagai, D. Walko, G. Smolentsev, F. Krieg, S. Boehme, M. Kovalenko, M. Chergui, G. Mancini "Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskites Nanocrystals" J. Am. Chem. Soc. 143 (2021) 9048
https://doi.org/10.1021/jacs.1c02403