Nanoscale Strain and Octahedral Tilting Removes Structural and Nonradiative Defects in 2D-Templated FAPbI₃

The halide perovskite formamidinium lead iodide (FAPbI₃) is a prime candidate for photovoltaics due to its excellent optoelectronic properties, but its application has been limited due to its structural instability. The large size of the FA cation results in metastability of the photoactive cubic phase and a facile degradation into thermodynamically stable hexagonal phases at room temperature. Recently, the incorporation of two-dimensional (2D) Ruddlesden-Popper halide perovskite seeds into a FAPbI₃ precursor solution was shown to template the growth of and stabilize cubic FAPbI₃. Here, we investigate the nanoscale structural and optoelectronic mechanisms behind the observed bulk stabilization using synchrotron-based X-ray microscopies. Nanoprobe X-ray diffraction reveals 2D-templated FAPbI₃ films exhibit an average compressive strain normal to the substrate of -3.3%, 2-fold larger than that of MACl-stabilized FAPbI₃. This compression creates locally templated regions composed of tetragonal-phase FAPbI₃ distributed nonuniformly throughout the film with fewer crystalline defects than purely cubic regions. Scanning X-ray excited optical luminescence (X-ray analog of photoluminescence) reveals that this local templating results in increased radiative recombination and red-shifted band edge and emission. Our results provide insight into the microscopic mechanism for the phase stabilization of FAPbI₃ using 2D perovskites as templates.