Structural secrets of hybrid perovskites The optoelectronic and photovoltaic applications of polycrystalline hybrid metal halide perovskite films are notable because grain boundaries in most materials cause scattering of charge carriers that decreases performance. Electron microscopy studies of these materials have been hindered by their rapid structural degradation under intense electron beams. Rothmann et al. now present an atomic crystallographic structure of formamidinium lead triiodide (FAPbI 3 ) polycrystalline thin films obtained by low-electron-dose scanning transmission electron microscopy with advanced image processing. The crystal structure sustains substoichiometry in the A-site cation, has a nearly perfect crystallographic alignment between PbI 2 impurity phases and the FAPbI 3 perovskite, and has atomically clean grain boundaries between polycrystalline domains. These features help to explain the films' surprising regenerative ability, their benign grain boundaries where strain and dislocations appear mostly absent, and why excess lead-iodide precursor can be counterintuitively beneficial. Science , this issue p. eabb5940