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

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