The nature of dynamic local order in CH3NH3PbI3 and CH3NH3PbBr3

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.

Automated summary

This study determines the true structure of two hybrid lead-halide perovskites using single-crystal diffuse scattering, neutron inelastic spectroscopy, and molecular dynamics simulations. The research reveals a remarkable collective dynamics that induces longer-range intermolecular correlations and may increase charge carrier lifetimes and affect halide migration.