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

The halide perovskite formamidinium lead iodide (FAPbI) 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 FAPbI precursor solution was shown to template the growth of and stabilize cubic FAPbI. 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 FAPbI films exhibit an average compressive strain normal to the substrate of -3.3%, 2-fold larger than that of MACl-stabilized FAPbI. This compression creates locally templated regions composed of tetragonal-phase FAPbI 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 FAPbI using 2D perovskites as templates.