Phase Transition Kinetics of MAPbI for Tetragonal-to-Orthorhombic Evolution.

Despite the commonly observed phase-instability-induced photovoltaic degradation of MAPbI, the phase transition kinetics at the atomic level remains elusive. Herein, by developing a stepwise NEB method, we clarify a nonsynergistic minimum-energy pathway for the tetragonal-to-orthorhombic phase transition. It is kinetically driven by the tilting of PbI that induces a spontaneous small rotation of adjoining MA and ends with stepwise ∼110° reorientations of two nonadjacent MA enabled by the cavity expansion. Compared to the common concerted mechanism, this process gives a low barrier of 0.08 eV/unit, demonstrating the easiness of the transition at extremely low temperatures and the importance of rotational entropies in regulating transition at elevated temperatures. With an explicit phase transition mechanism, we explore the structure-induced property response and reveal that introducing even low content of large-sized organic cations could help maintain the quasi-stable low-temperature performance of MAPbI solar cells.