  Perovskite solar cells have shown fast deterioration during actual operation
even with encapsulation, but its mechanism has been elusive. We found the
fundamental mechanism for irreversible degradation of perovskite materials in
which trapped charges regardless of the polarity play a decisive role. A novel
experimental setup utilizing different polarity ions revealed that the moisture
induced irreversible dissociation of perovskite materials is triggered by
charges trapped along grain boundaries. Our finding clearly explained the
intriguing observations why light soaking induces irreversible degradation
while in the dark, moisture only causes reversible hydration, and why
degradation begins from different side of interface for different charge
extraction layers. The deprotonation of organic cations by trapped charge
induced local electric field is attributed to the initiation of irreversible
decomposition.
