The Relationship Between Local Structural Order and Optoelectronic Performance of Lead Halide Perovskites

Empirical A-site cation substitution has advanced the stability and efficiency of hybrid organic-inorganic lead halide perovskites solar cells and the functionality of X-ray detectors. Yet, the fundamental mechanisms underpinning these improvements in this novel class of semiconductors remain elusive. Our study unveils the link between microscopic structural dynamics and macroscopic optoelectronic properties in these materials by utilising X-ray diffuse scattering, inelastic neutron spectroscopy and optical microscopy complemented by molecular dynamics simulations. In this talk I will present the evidence of the presence of dynamic, lower-symmetry local nanodomains embedded within the higher-symmetry average structures in various perovskite compositions. The local structure is tunable via the A-site cation selection: methylammonium induces anisotropic, planar nanodomains of out-of-phase octahedral tilts, while formamidinium favours isotropic, spherical nanodomains with in-phase tilting, even when crystallography reveals cubic symmetry on average. As a result, these dynamic nanodomains lead the electronic structure disorder by locally widening the band gap. This widening is more pronounced in the planar nanodomains of methylammonium systems compared to the spherical nanodomains in formamidinium-based systems. Thus we propose this low electronic disorder is a key factor in the superior optoelectronic performance of formamidinium-based devices. By demonstrating that the selection of the A-site cation dictates the local structure and, in turn, macroscopic properties, we establish a foundation for local order engineering in lead halide perovskite materials as a means of controlling their optoelectronic performance and stability.