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 semiconductor materials for optoelectronic applications, and the organic components within them offer a singular opportunity to modulate physical properties. Control over the composition of films with more than one organic component─such as the class of quasi-two-dimensional Ruddlesden–Popper hybrid perovskites (RPHPs) containing two organic ammoniums─is important for the rational design of targeted properties. Here, we report the fundamental interactions between RPHP films and amine vapors to understand the potential for vapor-based methods to modulate film composition. In a vapor mixture of methylamine and n-butylamine above a critical pressure, we found that RPHP films spontaneously liquefy by amino-deliquescence, driven by a highly exothermic heat of solution. Upon decreasing the amine pressure, the films recrystallized by amino-efflorescence but with a composition dramatically altered compared to the original film. Analysis of optical absorption and X-ray diffraction data demonstrates that the ratio of methylammonium and n-butylammonium in the films depends approximately linearly on the corresponding amine ratios in the vapor, following a relationship analogous to Raoult's law. The results demonstrate that an amino-deliquesced liquid state is a unique phase in which amine concentrations can be controlled via exchange with the vapor, yielding RPHP films after amino-efflorescence with a stoichiometry and crystal phase that depends upon the vapor environment. The fundamental understanding of RPHP vapor/solid interactions developed herein could open the door to more advanced vapor-based methods to engineer the structure and properties of HP films.