Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid–Base Theory

The correct identification
of all gases released during hybrid perovskite degradation is of great
significance to develop strategies to extend the lifespan of any device
based on this semiconductor. CH3X (X = Br/I) is a released
degradation gas/low boiling point liquid arising from methylammonium
(MA+) based perovskites, which has been largely overlooked
in the literature focusing on stability of perovskite solar cells.
Herein, we present an unambiguous identification of CH3I release using microwave (rotational) spectroscopy. An experimental
back-reaction test demonstrates that the well-known CH3NH2/HX degradation route may not be the ultimate degradation
pathway of MAPbX3 in thermodynamic closed systems. Meanwhile,
the CH3X/NH3 route cannot back-react selectively
to MAX formation as occurred for the former back-reaction. Metadynamics
calculations uncover the X halide effect on energy barriers for both
degradation reactions showing a better stability of Br based perovskite
ascribed to two aspects: (i) lower Brönsted–Lowry acidity
of HBr compared to HI and (ii) higher nucleophilic character of CH3NH2 compared to NH3. The latter property
makes CH3NH2 molecules stay preferentially attached
on the electrophilic perovskite surface (Pb2+) during the
dynamic simulation instead of being detached as observed for the NH3 molecule.