Anisotropic δ-to-α Phase Transition in Formamidinium Lead Iodide Thin Films

Thermal annealing on hybrid perovskites is essential to prepare high-quality solar cells with extraordinary efficiency, whose benefits include transformation of inactive phases such as delta-FAPbI(3) to active alpha-FAPbI(3). The detailed mechanism for such critical phase transition, however, has not yet been adequately studied. Here, we present multiscale microscopic observation to unravel the anisotropic delta-to-alpha transition in epitaxial FAPbI(3) thin films. We adopt polarized light microscopy that offers enhanced contrast to distinguish isotropic alpha-FAPbI(3) from the anisotropic delta-FAPbI(3). Facilitated by in situ heating, it allows us to identify heterogeneous nucleation of alpha-FAPbI(3) and the subsequent diffusional phase transition preferentially occurring along < 0001 >, which is underpinned by the smaller activation energy along the face-sharing direction of PbI6 octahedra. We further reveal the morphology and orientation relationship at the delta-to-alpha transition front using four-dimensional scanning transmission electron microscopy (4D-STEM), evincing the surface energy dominated orientation rather than the interfacial energy. The presence of high-density planar defects is also discovered at the transition front, which can be considered as an intermediate state facilitating delta-to-alpha structure transformation. Besides filling the knowledge gap on the phase transition behavior in FAPbI(3), our work also demonstrates a multiscale microscopy approach to interrogate the phase transition mechanism in hybrid perovskites.