On the Accessibility of Higher‐ <i>n</i> Phases in Formamidinium‐Based Ruddlesden‐Popper and Dion–Jacobson Layered Hybrid Perovskites

Abstract 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.