Photovoltaics based
on metal halide perovskites have recently achieved a certificated
efficiency of 25.2%. One of the factors that limit further development
of these devices comes from the defective boundaries between crystalline
domains in perovskite solar cells (PSCs). Such boundaries represent
a significant loss channel causing nonradiative recombination, but
systematic optimization procedures have not been developed yet to
control their properties. Herein, we propose a facile but effective
defect healing method to passivate the defects along the grain boundaries
in PSCs by post-treatment of formamidinium iodide (FAI) solution in
isopropyl alcohol (IPA). We use a combination of methods including
space-charge-limited current, steady-state and time-resolved photoluminescence,
confocal laser scanning microscopy, and transient absorption spectroscopy
to show the reduction of density of defect states in perovskite films
processed with 1 mg/mL FAI solution. The resultant FAI healed PSCs
achieve an average power conversion efficiency of 19.26% (with a champion
efficiency of 20.62%), higher than that of 16.45% in the control cell.
FAI healed devices without encapsulation maintain nearly 95% of the
initial efficiency after 60-day storage under N2 environment
and nearly 78% of the initial efficiency after 30-day storage under
the ambient condition with varied humidity. Our results demonstrate
that FAI healing is an effective way to passivate the defect states
along grain boundaries for high-efficiency and stable PSCs.