  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 CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$), 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
[PbI$_6$]$_4$-octahedron along the [100]/[010] direction, leading to the
transformation from corner-sharing [PbI$_6$]$_4$-octahedron in bulk MAPbI$_3$
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 MAPbI$_3$ repels both electrons and holes while serves
as a fast ion-migration channel, causing a rapid decomposition into PbI$_2$
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.
