Understanding Dynamic Titled Domains in Halide Perovskites with Big-Box Refinements of X-Ray Total Scattering Data

While solar cell and light-emitting devices based on halide perovskite materials have become significantly more efficient over the past decade,^[1] much of this progress has been the result of empirical fabrication optimisation, and the community’s structural materials understanding lags behind in comparison. For example, off the back of recent single crystal diffraction^[2,3,4] and computational^[5,6] studies, the field is only just beginning to appreciate that there are transient nano-domains where the lead halide octahedra are mutually tilted embedded in a higher symmetry average perovskite structure. Such dynamic domains are important for the properties of halide perovskites,^[2] and functional materials more generally, where structural disorder is often property determining.^[7]<br/><br/>Here I present our current work characterising these tilted domains in mixed-component halide perovskites using X-ray total scattering in the form of powder pair distribution function (PDF) analysis. By designing an advanced big-box refinement protocol that simultaneously considers the Bragg scattering and PDF data, where atomic positions in a supercell are unconstrained by crystal symmetry but obey local chemical rules, we produce atomic configurations which show a snapshot in time of the atomic halide perovskite crystal structure. Analysis of these supercells reveals pancake-shaped nanodomains where octahedra are locally tilted to lower symmetry in addition to Pb displacement from octahedral centres. Crucially, we perform these analyses on samples that have mixed compositions (like the highest-performance formulations) to understand the effects of ion substitution at different crystallographic sites on the size, shape, and density of the tilted nanodomains. Importantly, our big-box refinement approach uncovers evidence for these local domains using powder PDF measurements which are i) easier and higher throughput than single crystal diffraction studies, and ii) amenable for use on the most technologically relevant samples using either powders from scraped films or native thin films^[8] as they would be found in devices. Our results inform device makers on how to best tune their perovskite formulations to realise more efficient and stable optoelectronics, and our approach can be applied to a variety of other materials systems where structural disorder is a key factor for material performance.<br/><br/>[1] Best Research-Cell Efficiency Chart. https://www.nrel.gov/pv/cell-efficiency.html<br/>[2] M. Dubajic <i>et al.</i>, arXiv:2404.14598<br/>[3] N. J. Weadock <i>et al.</i>, <i>Joule</i> <b>7</b>, 1051–1066 (2023)<br/>[4] T. Lanigan-Atkins <i>et al.</i>, <i>Nat. Mater.</i> <b>20</b>, 977–983 (2021)<br/>[5] X. Liang <i>et al.</i>, <i>J. Phys. Chem. C</i> <b>127</b>, 19141–19151 (2023)<br/>[6] W. J. Baldwin <i>et al.</i>, <i>Small</i> <b>20</b>, 2303565 (2024)<br/>[7] N. Roth & A. L. Goodwin, <i>Nat. Commun.</i> <b>14</b>, 4328 (2023)<br/>[8] Kirsten M. Ø. Jensen <i>et al.</i>, <i>IUCrJ</i> <b>2</b> 481–489 (2015)