  Exposure to environmental factors is generally expected to cause degradation
in perovskite films and solar cells. Herein, we show that films with certain
defect profiles can display the opposite effect, healing upon exposure to
oxygen under illumination. We tune the iodine content of methylammonium lead
triiodide perovskite from understoichiometric to overstoichiometric and expose
them to oxygen and light prior to the addition of the top layers of the device,
thereby examining the defect dependence of their photooxidative response in the
absence of storage-related chemical processes. The contrast between the
photovoltaic properties of the cells with different defects is stark.
Understoichiometric samples indeed degrade, demonstrating performance at 33% of
their untreated counterparts, while stoichiometric samples maintain their
performance levels. Surprisingly, overstoichiometric samples, which show low
current density and strong reverse hysteresis when untreated, heal to maximum
performance levels (the same as untreated, stoichiometric samples) upon the
photooxidative treatment. A similar, albeit smaller-scale, effect is observed
for triple cation and methylammonium-free compositions, demonstrating the
general application of this treatment to state-of-the-art compositions. We
examine the reasons behind this response by a suite of characterization
techniques, finding that the performance changes coincide with microstructural
decay at the crystal surface, reorientation of the bulk crystal structure for
the understoichiometric cells, and a decrease in the iodine-to-lead ratio of
all films. These results indicate that defect engineering is a powerful tool to
manipulate the stability of perovskite solar cells.
