Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation

Lead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent-free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)(2)(MA)(n-1)PbnI3n+1 series) at the quite high temperature T-m (T-melting) of 171 degrees C. Here, we report on the solvent-free preparation (n=2, 4), thermal behaviour and melt-processed thin films of a series of HP (GABA)(2)(MA)(n-1)PbnI3n+1 (n=1, 2, 4; GABA(+): 4-ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent T-m of 126 degrees C and 136 degrees C, respectively, and, for n=1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)(2)PbI4 or (GABA)(2)(MA)Pb2I7 with MAPbI(3) in 1/1 (T=130 degrees C) or 1/2 (T=135 degrees C) ratio leads to pure phases of the n=2 HP and (GABA)(2)(MA)(3)Pb4I11 (n=4, T-m=185 degrees C), respectively. Melt-processed thin films of n=1, n=2 and n=4 HP have also been prepared.