Nanoscale Heterogeneity in 2D Templating of FAPbI₃ Controls Structural and Optoelectronic Behavior in Durable Perovskite Solar Cells

Despite the outstanding optoelectronic properties of formamidinium lead iodide (FAPbI₃), the large size of the FA cation destabilizes the photoactive cubic phase in favor of a photoinactive hexagonal phase. In recent work¹, we demonstrated that 2D Ruddlesden-Popper perovskites with lattice parameters well-matched to cubic FAPbI₃ can template the growth of cubic FAPbI₃ with its inherent bandgap of 1.48eV for efficient and highly durable perovskite solar cells. Here, we investigate the origins of this templating and its impacts on structural and optoelectronic properties at the nanoscale. Using synchrotron nanoprobe x-ray diffraction (nano-XRD) and x-ray excited optical luminescence (XEOL), we demonstrate that the templating occurs locally and that significant heterogeneity in crystallographic and optoelectronic structure remains at the nanoscale in these durable films.<br/> We explore templating effects in 3 dimensions by performing nano-diffraction measurements of 2D-stabilized samples both in-plane (transmission geometry) and out-of-plane (reflection geometry). We find substantial heterogeneity in the local structure of 2D-stabilized FAPbI₃, with locally tetragonal (<i>I4/mcm</i>) and cubic (<i>Pm3m</i>) structures throughout the film, suggesting that templating occurs heterogeneously throughout the film. We find some preferential orientation of the templating effect in the out-of-plane direction. Templating also appears to be seeded around 2D clusters roughly 100-300nm in size. The local microstrain is significantly decreased in the tetragonal, templated regions relative to purely cubic regions, suggesting that the locally-templated growth may reduce structural defects. XEOL mapping reveals locally redshifted regions with bright luminescence intensity in 2D-stabilized FAPbI₃, indicating that the templating improves optoelectronic quality by reducing nonradiative defect centers. These results highlight the three-dimensional distribution of templating that occurs in FAPbI₃ crystallized using 2D perovskite templates and offer insights as to pathways for homogenizing templating to further reduce nonradiative defects and degradation.<br/> <br/>1. Sidhik, S. <i>et al.</i> Two-dimensional perovskite templates for durable, efficient formamidinium perovskite solar cells. <i>Science</i> <b>384</b>, 1227–1235 (2024).