Thermodynamic and Kinetic Modulation of Methylammonium Lead Bromide Crystallization Revealed by In Situ Monitoring

Abstract

Hybrid organic–inorganic perovskite (HOIP) crystals are promising optoelectronic materials, but little is known about either the thermodynamic and kinetic controls on crystal growth or the underlying growth mechanism(s). Herein, we use fluid-cell atomic force microscopy (AFM) and solution nuclear magnetic resonance (NMR) spectroscopy to investigate the growth of the model HOIP crystal CH₃NH₃PbBr₃ (MAPbBr₃) and to determine how formic acid (HCOOH) modulates the thermodynamics and kinetics of growth. The results show that growth of MAPbBr₃ in dimethylformamide (DMF) proceeds through the classical pathway by the spreading of molecular crystal steps generated at screw dislocations on the {100} surface. Temperature-dependent step velocity measurements demonstrate that with increasing concentration, HCOOH decreases the solubility of MAPbBr₃. From the AFM data, we also determine the apparent kinetic coefficient (β) of step movement as a function of HCOOH concentration. ¹H NMR measurements indicate that HCOOH increases the lifetime of the methylammonium (MA⁺) ions and promotes the association of MAPbBr₃, thus tuning the solubility of the perovskite. We further propose that HCOOH alters the molecular tumbling motion and bulk diffusion of the MA⁺ ions, possibly via H-bonding. Our findings establish a direct correlation between the mesoscale crystal growth kinetics and the molecular-scale interactions between organic additives and constituent ions, providing unprecedented insights for developing predictive syntheses of HOIP crystals with defined size, crystal habit and shape, and defect distribution.