Effects of\nA‑Site Cation Structure on the Stability\nof 2D Tin Halide Perovskites

Two-dimensional halide perovskites (2D-HPs) are of significant\ninterest for their applications in optoelectronic devices. Part of\nthis increased interest in 2D-HPs stems from their increased stability\nrelative to their three-dimensional (3D) counterparts. Here, the origin\nof higher stability in 2D-HPs is mainly attributed to the bulky ammonium\ncation layers, which can act as a blocking layer against moisture\nand oxygen ingression and ion diffusion. While 2D-HPs have demonstrated\nincreased stability, it is not clear how the structure of the ammonium\nions impacts material stability. Herein, we investigate how the structure\nof ammonium cations, including three <i>n</i>-alkyl ammoniums,\nphenethylammonium (PEA) and five PEA derivatives, anilinium (An),\nbenzylammonium (BzA), and cyclohexylmethylammonium (CHMA), affects\nthe crystal structure and air, water, and oxygen stability of 2D tin\nhalide perovskites (2D-SnHPs). We find that stability is influenced\nby several factors, including the molecular packing and intermolecular\ninteractions in the organic layer, steric effects around the ammonium\ngroup, the orientation distribution of the 2D sheets, and the hydrophobicity\nof the perovskite film surface. With superior hydrophobicity, strong\ninteractions between organic molecules, and a high extent of parallel\noriented inorganic sheets, the 2-(4-trifluoromethyl-phenyl)-ethylammonium\n(4-TFMPEA) ion forms the most stable 2D-SnHP among the 12 ammonium\ncations investigated.