  The commercialization of perovskite solar cells is hindered by the poor
long-term stability of the metal halide perovskite (MHP) light absorbing layer.
Solution processing, the common fabrication method for MHPs, produces
polycrystalline films with a wide variety of defects, such as point defects,
surfaces, and grain boundaries. Although the optoelectronic effects of such
defects have been widely studied, the evaluation of their impact on the
long-term stability remains challenging. In particular, an understanding of the
dynamics of degradation reactions at the atomistic scale is lacking. In this
work, using reactive force field (ReaxFF) molecular dynamics simulations, we
investigate the effects of defects, in the forms of surfaces, surface defects
and grain boundaries, on the stability of the inorganic halide perovskite
CsPbI$_{3}$. Our simulations establish a stability trend for a variety of
surfaces, which correlates well with the occurrence of these surfaces in
experiments. We find that a perovskite surface degrades by progressively
changing the local geometry of PbI$_{\mathrm{x}}$ octahedra from corner- to
edge- to face-sharing. Importantly, we find that Pb dangling bonds and the lack
of steric hindrance of I species are two crucial factors that induce
degradation reactions. Finally, we show that the stability of these surfaces
can be modulated by adjusting their atomistic details, either by creating
additional point defects or merging them to form grain boundaries. While in
general additional defects, particularly when clustered, have a negative impact
on the material stability, some grain boundaries have a stabilizing effect,
primarily because of the additional steric hindrance.
