Glass Transition, Liquid Dynamics, and Thermal Degradation in 2D Hybrid Halide Perovskites

2D hybrid organic-inorganic perovskites (2D HOIPs) are of interest for optoelectronic and phase-change applications. Using ultra-fast (flash) differential scanning calorimetry (FDSC), this study shows the 2D HOIPs (S-Cl-MBA)₂PbI₄ and (R-Cl-MBA)₂PbBr₄ (Cl-MBA referring to 4-chloro-α-methylbenzylamine) form a glass on cooling. Both show evidence of a liquid-to-glass transition during quenching from the liquid state; on reheating, a glass-to-liquid transition is followed by crystallization and melting. Using continuous heating in FDSC, the temperature dependence of the liquid viscosity of (S-Cl-MBA)₂PbI₄ is characterized. The kinetic fragility of the liquid is similar to that of bulk metallic glass-formers and significantly lower than that of organic and phase-change chalcogenide liquids. On cooling the liquid, glass formation is first impeded by thermal degradation, then crystallization. The stages of thermal degradation can be related to known mechanisms. This study highlights the reduced glass-transition temperature and the liquid fragility as key parameters in guiding the optimization of 2D HOIP compositions for targeted applications.