  We use scanning probe microscopy to study ion migration in the formamidinium
(FA)-containing halide perovskite semiconductor
$Cs_{0.22}FA_{0.78}Pb(I_{0.85}Br_{0.15})_3$ in the presence and absence of
chemical surface passivation. We measure the evolving contact potential
difference (CPD) using scanning Kelvin probe microscopy (SKPM) following
voltage poling. We find that ion migration leads to a ~100 mV shift in the CPD
of control films after poling with 3V for only a few seconds. Moreover, we find
that ion migration is heterogeneous, with domain interfaces leading to a larger
shift in the CPD. Application of (3-aminopropyl)trimethoxysilane (APTMS) as a
surface passivator further leads to 5-fold reduction in the CPD shift from ~100
meV to ~20 meV. We use hyperspectral microscopy to show that APTMS-treated
perovskite films undergo less photoinduced halide migration than control films.
We interpret these results as due to a reduction in halide vacancy
concentration due to passivation with APTMS.
