Dynamic Phase Transitions in Mixed Halide Perovskites

Halide perovskites have emerged as promising materials for photovoltaic applications primarily due to their high efficiency. Among these, MAPbI₃ and FAPbI₃ are particularly favorable. However, these materials suffer from instabilities, whether inherent or caused by environmental conditions. Mixed halide perovskites, specifically MA_xFA_1-xPbI₃, have gained prominence as a potential solution, showing signs of greater stability. In this context, gaining a thorough understanding of the phase diagrams of these materials is crucial. This thesis investigates the various phases in mixed halide perovskites using state-of-the-art molecular dynamics simulations. The study focuses on mapping out the phase transitions of MA_xFA_1-xPbI₃ and identifying the crystal structures they exhibit at different temperatures and composition coordinate x.

The simulations reveal a map of the phase transitions in the mixed halide perovskite, capturing some key features from experimental studies and providing deep microscopic insights. Phonon mode projection unveil the crystal structure of the different phases. This analysis also indicates a morphotropic phase boundary around the composition MA_0.27FA_0.73PbI₃. This morphotropic boundary is explained by the coexistence of phases, as the system forms a layered structure with different crystal symmetries. This is suggested to be a higher order phase transition path between two crystal structures, providing a smooth transition between phases.