Glass Formation in Metal Halide Perovskite using Ultrafast Calorimetry

While the existence of a glassy state has been established in numerous materials families, including chalcogenide semiconductors, metal halide perovskites (MHPs), currently one of the most exciting and extensively studied classes of semiconductors, have primarily been known to exist only in crystalline states. However, our recent discovery of reversible crystal-glass switching in MHPs challenged this notion by employing structural engineering techniques in an exemplary [S-(−)-1-(1-naphthyl)ethylammonium]₂PbBr₄ (SNPB) perovskite, achieving slow melt ordering kinetics that resulted in a stable melt-quenched glassy state.^[1] Nevertheless, it is challenging to vitrify and conduct kinetic studies on the broader MHP family, particularly members with flexible and non-bulky aliphatic organic cations, thereby limiting the glass forming compositional space. In this study, we expand the range of MHP glass formation across a broader range of organic (fused ring to branched aliphatic) and halide (bromide to iodide) compositions by employing an unconventional technique called <i>ultrafast calorimetry</i> that enables rapid cooling and heating of samples (4-5 order higher than achievable through conventional calorimetry). For the exemplary case of the 1-MeHa₂PbI₄ (1-MeHa = 1-methyl-hexylammonium) perovskite, a low melting MHP with <i>T_m</i> ~170 °C, we demonstrate the ability to obtain a glass by melt-quenching at a rate of 6000°C/s, assisted with a partial mass loss of ~15%. The obtained glass shows a glass transition temperature of ~16 °C and faster crystallization kinetics compared to SNPB.^[2] Furthermore, through iterative calorimetric and viscosity measurements, and a combination of kinetic, thermodynamic, and rheological modeling techniques, we construct an Angell plot and determine important kinetic parameters for glass formation and crystallization, such as the activation energy of glass crystallization (<i>E_A</i> = 124 to 50 kJ/mol for 45 to 97 °C), fragility index (<i>m</i> = 72), and crystal growth rates (<i>U_max</i> = 0.21 m/s), providing a deeper understanding of the system's behavior. Ultrafast calorimetry can thus have immense potential to expand the compositional range of glass-forming MHPs, offering a greater range of glass-forming abilities. The results and techniques employed in this study also contribute to establishing a framework for selecting suitable MHP candidates for applications beyond conventional photovoltaics, emitters, and sensors, opening possibilities in areas such as cost-effective memory, computing, metamaterials, and reconfigurable photonic devices.<br/><br/>[1] A. Singh, M. K. Jana, D. B. Mitzi, Adv. Mater. 2021, 33, 2005868.<br/>[2] A. Singh, D. B. Mitzi, ACS Mater. Lett. 2022, 4, 1840.