The nature of dynamic local order in CH3NH3PbI3 and CH3NH3PbBr3

NJ Weadock and TC Hemamala and JA Vigil and A Gold-Parker and IC Smith and B Ahammed and MJ Krogstad and F Ye and D Voneshen and PM Gehring and AM Rappe and HG Steinr├╝ck and E Ertekin and HI Karunadasa and D Reznik and MF Toney, JOULE, 7 (2023).

DOI: 10.1016/j.joule.2023.03.017

Hybrid lead-halide perovskites (LHPs) are semiconductors with novel properties that are distinctively governed by structural fluctu-ations. Diffraction experiments sensitive to long-range order reveal a cubic structure in the device-relevant, high-temperature phase. Local probes find additional short-range order with lower symmetry that may govern structure-function relationships. However, our un-derstanding is impeded by unresolved dimensionality, participating atoms, and dynamics of short-range order. Here, we determine the true structure of two hybrid LHPs, CH3NH3PbI3 and CH3NH3PbBr3, using a combination of single-crystal diffuse scattering, neutron in-elastic spectroscopy, and molecular dynamics simulations. The remarkable collective dynamics, not observed in previous studies, consist of a network of local, two-dimensional, circular regions of dynamically tilting lead-halide octahedra (lower symmetry) that induce longer-range intermolecular CH3NH3+ correlations. The dy-namic local structure may introduce transient ferroelectric or anti-ferroelectric domains that increase charge carrier lifetimes and strongly affect halide migration, a poorly understood degradation mechanism.

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