Odd‐Even Alkyl Chain Effect of Aromatic Monoammoniums on the Interlayer Architecture and Stability of Ruddlesden–Popper 2D Perovskites

Ruddlesden–Popper (RP) 2D perovskites with organic bilayers attract great interest for optoelectronics owing to their high stability and tunable functionality. However, the relationship between the interlayer architecture, molecular configuration of aromatic cations, and resulting stability remains poorly understood. Herein, we systematically investigate phenyl alkylammonium‐based RP 2D perovskites with varying alkyl chain lengths (─(CH 2 ) m NH 3 + with m = 1–4) of organic cations. We find that odd‐carbon cations (m = 1, 3) preferentially form type‐I 2D perovskites featuring interdigitated organic cations, which induces substantial inorganic distortion and lower octahedral symmetry to the C 1 point group. Even‐carbon cations (m = 2, 4) favor type‐II 2D perovskites with well‐defined interlayer gaps, wherein the alkylammonium groups rotate to optimize registry with the inorganic sublattice, minimizing distortion and yielding higher‐symmetry D 2 h octahedra. Furthermore, we demonstrate that intrinsic structural stability of RP 2D perovskites is primarily governed by π–π packing within the organic layer, whereas air stability of the thin films is determined by surface hydrophobicity. The type‐II films exhibit higher hydrophobicity, conferring enhanced stability under ambient conditions compared to type‐I films. These findings establish odd‐even alkyl chain parity as a design principle for tailoring structurally diverse yet stable RP 2D perovskites.