Intragrain impurities can negatively affect the efficiency and stability of perovskite solar cells (PSCs), but they are difficult to detect using conventional characterization methods. Using in situ scanning transmission electron microscopy (STEM), the authors reveal that intragrain impurity nano-clusters, which can originate from solution synthesis or post-synthesis storage, can revert to perovskites upon irradiation stimuli, leading to the unexpected improvement of crystalline grains. Computational modeling is used to atomically resolve the crystallographic transformation modes for the annihilation of these impurities and to investigate their impact on optoelectronic properties. This fundamental finding is translated into device advancements. By employing a scanning laser stimulus, the authors successfully heal intragrain impurity nano-clusters in formamidinium-cesium perovskite solar cells, enhancing both the efficiency and stability of the devices through improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, guided by atomic-scale in situ microscopic imaging, presents a new prototype for advancing solar cell technology.Intragrain impurities can negatively affect the efficiency and stability of perovskite solar cells (PSCs), but they are difficult to detect using conventional characterization methods. Using in situ scanning transmission electron microscopy (STEM), the authors reveal that intragrain impurity nano-clusters, which can originate from solution synthesis or post-synthesis storage, can revert to perovskites upon irradiation stimuli, leading to the unexpected improvement of crystalline grains. Computational modeling is used to atomically resolve the crystallographic transformation modes for the annihilation of these impurities and to investigate their impact on optoelectronic properties. This fundamental finding is translated into device advancements. By employing a scanning laser stimulus, the authors successfully heal intragrain impurity nano-clusters in formamidinium-cesium perovskite solar cells, enhancing both the efficiency and stability of the devices through improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, guided by atomic-scale in situ microscopic imaging, presents a new prototype for advancing solar cell technology.