Solution Aging Promotes the Formation of Hexagonal Polytypes in Mixed-Cation/-Halide Perovskites

Mixed-cation/-halide perovskite compositions are becoming increasingly popular, allowing the convenient tuning of photophysical and optoelectronic properties of photoactive films. However, the presence of multiple components in the solution and the demonstrated reactivity between some of them make the ink formulation quite complex and difficult to connect to the film properties and to a fundamental understanding, leading to the fabrication of reproducible and efficient optoelectronic devices. Herein, we report for the first time on the straightforward relationship between the ubiquitous presence of the N-methylformamidinium (MFA) adduct into MA/FA aged solutions and the formation of hexagonal polytypes (6H and 4H) during FA0.85MA0.15Pb(I0.85Br0.15)3 perovskite crystallization. The aging process in solution is evaluated through nuclear magnetic resonance (NMR), supported by optical, morphological, and crystalline structure characterizations. Density functional theory calculations unravel how the conformation of the MFA deeply alters the crystallization pathways, preferentially stabilizing the hexagonal yellow phases, known to be detrimental to the perovskite film performances in optoelectronic devices.