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 δ-FAPbI₃ to active α-FAPbI₃. 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 δ-to-α transition in epitaxial FAPbI₃ thin films. We adopt polarized light microscopy that offers enhanced contrast to distinguish isotropic α-FAPbI₃ from the anisotropic δ-FAPbI₃. Facilitated by in situ heating, it allows us to identify heterogeneous nucleation of α-FAPbI₃ 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 δ-to-α 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 δ-to-α structure transformation. Besides filling the knowledge gap on the phase transition behavior in FAPbI₃, our work also demonstrates a multiscale microscopy approach to interrogate the phase transition mechanism in hybrid perovskites.