Local Order and Dynamics in Hybrid Metal Halide Perovskites

Hybrid metal halide perovskites (MHPs) are a rediscovered class of semiconductor with remarkable optoelectronic performance in applications such as solar absorbers, radiation detectors, spintronics, and light emitting diodes. Recent computational results have demonstrated that the optoelectronic properties of MHPs are not explained by classic semiconductor models, instead it is the large structural fluctuations, driven by strong anharmonicities, which govern the behavior [1,2]. Despite recent progress in understanding the fundamental physics underlying thermal and charge carrier transport in these materials, there still exist detrimental issues like lead toxicity and halide migration which hinder widespread deployment of MHP-based devices [2].<br/>In this talk I will present two studies which highlight the uniqueness of neutron scattering to investigate (i) the lattice dynamics contributions to thermal conductivity in lead-free MHPs, and (ii) the two-dimensional dynamic local order in the prototypical MHPs, CH₃NH₃PbI₃ and CH₃NH₃PbBr₃. In the first study, we use triple-axis neutron spectroscopy to measure the acoustic phonon lifetimes in Cs₂AgBiBr₆ and find that the acoustic mode contributions to the lattice thermal conductivity are larger than in other MHPs. In the second study, we utilize neutron and X-ray single crystal total scattering to reveal that the simple cubic structure of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ is in fact and assembly of two-dimensional tilted regions of PbX₆ octahedra which induce further correlations on the CH₃NH₃⁺ sublattice. I will connect this dynamic local structure to unresolved questions surrounding the optoelectronic properties and halide migration in these materials.<br/><br/>[1] M. Z. Mayers, L. Z. Tan, D. A. Egger, A. M. Rappe, and D. R. Reichman, <i>How Lattice and Charge Fluctuations Control Carrier Dynamics in Halide Perovskites</i>, Nano Lett. <b>18</b>, 8041 (2018).<br/>[2] D. A. Egger, A. Bera, D. Cahen, G. Hodes, T. Kirchartz, L. Kronik, R. Lovrincic, A. M. Rappe, D. R. Reichman, and O. Yaffe, <i>What Remains Unexplained about the Properties of Halide Perovskites?</i>, Advanced Materials <b>30</b>, 1800691 (2018).