Identifying a "Raoult's Law" Relationship To Modulate the Stoichiometry of Hybrid Perovskite Films by Amino-Deliquescence/Efflorescence in Mixed Amine Vapors

Hybrid perovskites (HPs) are a promising class of semiconductormaterials for optoelectronic applications, and the organic componentswithin them offer a singular opportunity to modulate physical properties.Control over the composition of films with more than one organic component-suchas the class of quasi-two-dimensional Ruddlesden-Popperhybrid perovskites (RPHPs) containing two organic ammoniums-isimportant for the rational design of targeted properties. Here, wereport the fundamental interactions between RPHP films and amine vaporsto understand the potential for vapor-based methods to modulate filmcomposition. In a vapor mixture of methylamine and n-butylamine above a critical pressure, we found that RPHP films spontaneouslyliquefy by amino-deliquescence, driven by a highly exothermic heatof solution. Upon decreasing the amine pressure, the films recrystallizedby amino-efflorescence but with a composition dramatically alteredcompared to the original film. Analysis of optical absorption andX-ray diffraction data demonstrates that the ratio of methylammoniumand n-butylammonium in the films depends approximatelylinearly on the corresponding amine ratios in the vapor, followinga relationship analogous to Raoult's law. The results demonstratethat an amino-deliquesced liquid state is a unique phase in whichamine concentrations can be controlled via exchange with the vapor,yielding RPHP films after amino-efflorescence with a stoichiometryand crystal phase that depends upon the vapor environment. The fundamentalunderstanding of RPHP vapor/solid interactions developed herein couldopen the door to more advanced vapor-based methods to engineer thestructure and properties of HP films.