  It is unmistakably paradoxical that the weakest point of the photoactive
organic-inorganic hybrid perovskite is its instability against light. Why and
how perovskites break down under light irradiation and what happens at the
atomistic level during the degradation still remains unanswered. In this paper,
we revealed the fundamental origin and mechanism for irreversible degradation
of hybrid perovskite materials from our new experimental results and ab initio
molecular dynamics (AIMD) simulations. We found that the photo-generated
charges trapped along the grain boundaries of the perovskite crystal result in
oxygen-induced irreversible degradation in air even in the absence of moisture.
The present result, together with our previous experimental finding on the same
critical role of trapped charges in the perovskite degradation under moisture,
suggests that the trapped charges are the main culprit in both the oxygen- and
moisture-induced degradation of perovskite materials. More detailed roles of
oxygen and water molecules were investigated by tracking the atomic motions of
the oxygen- or water-covered CH3NH3PbI3(MAPbI3) perovskite crystal surface with
trapped charges via AIMD simulation. In the first few picoseconds of our
simulation, trapped charges start disrupting the crystal structure, leading to
a close-range interaction between oxygen or water molecules and the
compositional ions of MAPbI3. We found that there are different degradation
pathways depending on both the polarity of the trapped charge and the kind of
gas molecule. Especially, the deprotonation of organic cations was
theoretically predicted for the first time in the presence of trapped anionic
charges and water molecules. We confirmed that a more structurally stable,
multi-component perovskite material(MA0.6FA0.4PbI2.9Br0.1) exhibited a much
longer lifespan than MAPbI3 under light irradiation even in 100% oxygen
ambience.
