Shedding light on the stability and structure-property relationships of two-dimensional hybrid lead bromide perovskites

Two-dimensional (2D) hybrid lead iodide perovskites have gained prominence due to their remarkable structural tunability, optoelectronic features, and moisture stability, which have rendered them as attractive alternatives to 3D MAPbI3 for optoelectronic devices. 2D multilayer lead bromide perovskites remain an unfathomed phase space with the lack of systematic studies to establish the structure, photophysical properties and stability behavior of this family of 2D halide perovskites. Herein, we present new members of bilayer lead bromide perovskites (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 6-8) that belong to the Ruddlesden-Popper structure type, incorporating long chain alkyl-monoammonium cations (CmH2m+1NH3) of hexylammonium (m = 6), heptylammonium (m = 7), and octylammonium (m = 8). A universal solution synthetic methodology for bulk multilayer lead bromide perovskites is presented with all structures solved and refined using single crystal X-ray diffraction. The studied bilayer lead bromide perovskites demonstrate a decrease in the lattice rigidity and lattice match of the inorganic perovskite layer-organic layer, as the alkyl-monoammonium chain length increases. In comparison to their iodide analogues, the bilayer lead bromide compounds exhibit elongation of their stacking axis despite the smaller dimensions of the [PbBr6]4- lattice, while their internal lattice strain was calculated to be reduced, inferring a greater lattice match between the inorganic [PbBr6]4- perovskite layer and organic layer. The (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6-8) compounds exhibit narrow-band emission near 2.5 eV. Time-resolved photoluminescence (PL) displays longer carrier lifetimes on the nanosecond time scale comparing to their iodide analogues, where electronic structure calculations indicate that the increase of the alkyl chain length and, thus, lattice softness enhances nonradiative recombinations. A complete set of air, light, and heat stability tests on unencapsulated thin films of (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6-8) and MAPbBr3 show they are stable in ambient air for at least 5 months, exhibiting greater extrinsic stability than the 2D lead iodide congeners. Extraordinarily, 3D MAPbBr3 films prove to be more stable than films of 2D lead bromide perovskites, in contrast to MAPbI3 which is less stable than the 2D lead iodide perovskites.