  The instability of organometal halide perovskites when in contact with water
is a serious challenge to their feasibility as solar cell materials. Although
studies of moisture exposure have been conducted, an atomistic understanding of
the degradation mechanism is required. Toward this goal, we study the
interaction of water with the (001) surfaces of CH$_3$NH$_3$PbI$_3$ under both
low and high water concentrations using density functional theory. We find that
water adsorption is heavily influenced by the orientation of the methylammonium
cations close to the surface. It is demonstrated that, depending on
methylammonium orientation, the water molecule can infiltrate into the hollow
site of the surface and get trapped. Controlling dipole orientation via poling
or interfacial engineering could thus enhance its moisture stability. We do not
see a direct reaction between the water and methylammonium molecules.
Furthermore, calculations with an implicit solvation model indicate that a
higher water concentration may facilitate degradation.
