Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid–Base Theory
2019
Juarez-Perez, Emilio J. | Ono, Luis K. | Uriarte, Iciar | Cocinero, Emilio J. | Qi, Yabing
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.
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