Enhancing Crystallization Kinetics in Glass-Forming Halide Perovskites using Organic Cation Isomers

The recent exploration of glass-forming hybrid metal halide perovskites (MHPs) has opened new possibilities to extend their applications beyond the well-established realm of optoelectronic research, which primarily focuses on their crystalline variants. In this regard, it is imperative to diversify the range of glass-forming MHP compositions and manipulate their crystallization kinetics through synthetic structural engineering. Herein, we conducted a comparative study involving two MHPs that possess subtly different structural characteristics, utilizing isomer organic cations while maintaining the same chemical composition. Our investigation sheds light on how these alterations in the position of functional groups profoundly influence the kinetics of both glass formation and cold crystallization. One of the MHPs, (S)-(−)-1-(1-naphthyl)ethylammonium lead bromide (S(1-1)NPB), exhibits a lower melting point (<i>T_m</i>) of 175 °C and readily transforms into a glassy state at a critical cooling rate (CCR) of 20 °C/min. In contrast, (S)-(−)-1-(2-naphthyl)ethylammonium lead bromide (S(1-2)NPB) displays a higher <i>T_m</i> of 193°C, and requires a CCR of 150,000°C/min, necessitating the use of ultrafast calorimetry for glass formation. The examination of the underlying crystallization kinetics, performed using iterative calorimetry and the Kissinger modeling technique, further indicates a small activation energy barrier (<i>E_A</i>) for crystallization in S(1-2)NPB.<br/>We also investigate the interplay between structural and thermodynamic features that engenders distinct glass formation and crystallization behavior using a comprehensive analysis of the organic-inorganic hydrogen bonding interactions in the crystalline state and a careful examination of the enthalpy and entropy balance across the melting transition. These analyses highlight the strengthened hydrogen bonding in S(1-2)NPB and the reduced entropy of melting as the key factors contributing to the higher <i>T_m</i> and CCR, as well as a lower <i>E_A</i>, observed in comparison to S(1-1)NPB. The results presented in this study establish a framework for evaluating the impact of structural elements on the kinetics of glass formation and crystallization, serving as a new material design strategy to diversify the family of glass-forming MHPs and extending their potential applications in areas demanding faster switching speeds such as memory, computing, metamaterials, and reconfigurable photonic devices.