Atomic Structure and Electrical/Ionic Activity of Antiphase Boundary in CH <sub>3</sub>NH <sub>3</sub>PbI <sub>3</sub>

Defects in organic-inorganic hybrid perovskites (OIHPs) greatly influence their optoelectronic properties. Identification and better understanding of defects existing in OIHPs is an essential step towards fabricating high-performance perovskite solar cells. However, direct visualizing the defects is still a challenge for OIHPs due to their sensitivity during electron microscopy characterizations. Here, by using low dose scanning transmission electron microscopy techniques, we observe the common existence of antiphase boundary (APB) in CH3NH3PbI3 (MAPbI3), resolve its atomic structure, and correlate it to the electrical/ionic activities and structural instabilities. Such an APB is caused by the half-unit-cell shift of [PbI6]4- octahedron along the [100]/[010] direction, leading to the transformation from corner-sharing [PbI6]4- octahedron in bulk MAPbI3 into edge-sharing ones at the APB. Based on the identified atomic-scale configuration, we further carry out density functional theory calculations and reveal that the APB in MAPbI3 repels both electrons and holes while serves as a fast ion-migration channel, causing a rapid decomposition into PbI2 that is detrimental to optoelectronic performance. These findings provide valuable insights into the relationships between structures and optoelectronic properties of OIHPs and suggest that controlling the APB is essential for their stability.