Observation of Structural Phase Transitions and PbI₂Formation during the Degradation of Triple-Cation Double-Halide Perovskites

The severe instability and rapid degradation of hybrid organic-inorganic metal-halide perovskite materials under adverse environmental conditions are one of the major obstacles facing perovskite solar cell (PSC) technology in the path toward large-scale utilization. In this work, the first investigation of the degradation dynamics of the state-of-the-art, triple-cation double-halide perovskite material Cs0.05MA0.10FA0.85Pb(I0.85Br0.15)3 aided by in situ transmission electron microscope (TEM) imaging is reported. The real-time progression of the degradation is visualized directly inside a TEM at the micron and atomic scales. The degradation originates predominantly at perovskite-substrate heterointerfaces or at grain boundaries, and it propagates through the surrounding grain boundaries and into grain cores through twin domains The degradation dynamics involves a phase transition from the black perovskite α-phase to the yellow nonperovskite δ-phase and formation of PbI2 nanoparticles, which later transform into metallic Pb particles. The energy barrier landscape and phase transition pathway are obtained using density functional theory calculations. The results indicate that phase transformations occurring during the degradation follow the reverse sequence of those that occur during perovskite crystallization.