On the Accessibility of Higher-<i>n</i> Phases in Formamidinium-based Ruddlesden-popper and Dion-jacobson Layered Hybrid Perovskites

Layered (2D) hybrid perovskites offer a promising alternative for stabilizing halide perovskite materials, with a growing interest in formamidinium (FA(+)) lead iodide derivatives for photovoltaics due to their exceptional optoelectronic properties. While their potential increases with the number of inorganic layers (n), the experimental evidence suggests that obtaining n > 2 phases is challenging for FA-based layered perovskites. To address this challenge and identify the conditions governing the formation of higher-n phases, representative FA-based layered hybrid perovskite materials containing aromatic spacer cations, namely benzylammonium (BNA) and 1,4-phenylenedimethanammonium (PDMA)-are investigated as model systems for the corresponding Ruddlesden-Popper and Dion-Jacobson phases based on (BNA)(2)FA(n-1)Pb(n)I(3n+1) and (PDMA)FA(n-1)Pb(n)I(3n+1) formulations (n = 1-3), respectively. Moreover, the effect of Cs+ cations on the formation of n > 1 phases is explored through a combination of X-ray scattering measurements, solid-state NMR spectroscopy, optoelectronic characterization, and density functional theory calculations. Despite improved photovoltaic performances, the formation of higher (n > 2) phases is excluded, even in the presence of Cs+, due to the favorable formation of other low-dimensional phases revealed by the theoretical investigation. The results contribute to a comprehensive understanding of these materials of broad interest to their application in optoelectronics.