  Despite soaring performance of organic-inorganic hybrid perovskite materials
in recent years, the mechanism of their decomposition at actual operation
condition has been elusive. Herein, we elucidated the decrystallization process
of CH3NH3PbI3 perovskite crystals via localized charges and identified
polarity-dependent degradation pathway by carrying out time-evolution
measurements for absorption spectra of perovskite films with underlying
different charge transport layers and ab initio molecular dynamics
calculations. It was found that the carrier polarity (hole-rich or
electron-rich) inside the perovskite films played a critical role in the
degradation rate, and polarity-dependent degradation pathway strongly depended
on the combination of surrounding gaseous molecules. The hole-rich perovskite
films degraded more rapidly in the existence of H2O than the electron-rich one,
while the degradation trend became opposite in only-oxygen ambient. Strikingly,
the hole-rich one was extremely weak to atmospheric air containing both H2O and
O2, whereas the MAPbI3 film with excessive electrons rather stabilized in air.
Ab initio molecular dynamics (AIMD) simulation was also done to find the
detailed degradation pathway of MAPbI3 under atmosphere for different polarity
of localized charge, which are in good agreement with experimental results.
Furthermore, X-ray assisted spectroscopic measurements confirmed the production
of Pb(OH)I as predicted from the simulation result.
