On the Accessibility of Higher‐n 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)2FAn–1PbnI3n+1 and (PDMA)FAn–1PbnI3n+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.