Demonstrating Metal Halide Perovskite Reversible Glass Transition via In Situ X-Ray Scattering

2D metal halide perovskites (2DPKs) are a rapidly expanding research interest. Implemented 2DPK technology has thus far been limited to the distinct properties of the crystalline phase in device heterostructures, but the recent realization of reversible crystal-glass switching capabilities provides unique optoelectronic structure-property relationships with significant potential for applications in advanced computing, phase-change memory, sensing, catalysis, and communications [1,2]. Using in-situ X-ray scattering pair distribution function measurements, we establish the design criteria for effective thermal tuning. Structural modification of the organic cation can modulate the characteristic transition temperatures over a 100 degree Celsius range, i.e., glass transition, crystallization, melting, and degradation temperatures.<br/><br/>We demonstrated the first reversible switching between glassy and crystalline states under moderate thermal conditions and ambient pressure with [S-(-)-1-(1-naphthyl)ethylammonium] lead bromide, “S-NPB” by modifying the alkylammonium-derived cation and selecting chirality [3]. We reported the lowest ever melting temperature for any lead-bromide MHP of 173.1 degrees Celsius, which is uniquely lower than the 205 degrees Celsius degradation temperature. This atypical thermal stability permits melt-processing of S-NPB, offering advantageous control over crystallographic orientation and removes solvent requirement. [5] The ordering kinetics are slowed by the increased configurational energy barrier for structural relaxation. We have accessed glass formation on melt-quenching at cooling rates below 20 degrees Celsius per minute and quantified the critical cooling rate and crystallization nucleation kinetics via DSC and in-situ X-ray scattering. By exploiting these techniques, we have made significant progress resolving these critical research gaps to realize technological implementation of this switching phenomena. Our development of an in-situ thermally controlled transition chamber has confirmed the stability of S-NPB in the amorphous phase and allowed us to discover the fully reversible crystallization behavior. This understanding of fundamental reorganization kinetics, amorphous phase stability, structural coordination, thermal cycling, and energy efficiency paves the way for implementation of these systems.