Glass-Crystal Transformation Kinetics in a Hybrid Metal Halide Perovskite

Metal halide perovskites (MHPs) are the widely celebrated family of crystalline semiconductors that has led to significant advancement in the fields of photovoltaics, emitters, and sensors. Though most of the research is condensed towards studying the crystalline MHPs, the study of MHPs that can access a glassy/amorphous state could help broaden the fundamental understanding of the structure-property relationship as well as their application space.<br/><br/>Our recent discovery of glass formation and reversible crystal-glass switching in an exemplary 2D lead bromide perovskite [1], with distinct optoelectronic properties in either state, has spurred interest into their potential applicability in areas where phase change characteristics could be exploited. To recognize the suitability of glass forming MHPs into such application space, it is imperative to study the kinetics of glass crystallization. Such a study would offer the potential to understand the underlying nucleation and crystal growth mechanisms, as well as the activation energy requirement to drive such a change, providing important information for prospective phase-change applications.<br/><br/>In this work, we demonstrate the underlying kinetic effects of glass crystallization in a representative MHP family member, [S-(−)- 1-(1-naphthyl)ethylammonium]₂PbBr₄. By making use of iterative calorimetry and numerical modelling techniques, such as the Ligero [2] and Kissinger [3] kinetic models, we extract the activation energy of glass crystallization in the studied MHP. The activation energy (~350 kJ/mol) is found to be higher than those determined for most fast-crystallizing chalcogenides but smaller than for most of the stable oxide glasses. Furthermore, the extracted Avrami parameter of ~2 suggests a heterogeneous surface-mediated nucleation mechanism with two-dimensional crystal growth, as also corroborated by in-situ and ex-situ microscopy. Our present work serves as the first study to model the glass crystallization kinetics of MHPs and thus, with the advent of more glass forming MHPs, these results are expected to serve as a starting point for comparing crystallization kinetics parameters as a function of MHP composition and organic cation choice. The results should therefore also help in establishing a framework for selecting suitable candidates for use in new prospective applications for MHPs, such as memory, computing, phase change energy storage, glass-composites, metamaterials, and reconfigurable photonic devices.<br/><br/>[1] Singh, A. et. al. <i>Adv. Mater.</i> <b>2021,</b> <i>33</i> (3), 2005868.<br/>[2] Ligero, R. et. al. <i>J. Mater. Sci. </i><b>1991,</b> <i>26</i> (1), 211-215.<br/>[3] Kissinger, H. E. et. al. <i>J. Res. Natl. Bur. Stand. (U. S.)</i> <b>1956,</b> <i>57</i> (4), 217-221.