Broadband emission originating from the stereochemical expression of 6s2 lone pairs in two-dimensional lead bromide perovskites

Abstract

The stereochemical expression of the 6s² lone pair on the lead atom has a significant impact on the crystal structures and physical properties of lead halide perovskites. Two-dimensional (2D) lead bromide perovskites often exhibit a broadband emission, yet the structural origin of the broadband emission has been under debate. Here, we report the synthesis and characterization of a 2D lead bromide hybrid (4-chlorophenylammonium)₂PbBr₄ that consists of a combination of the octahedral unit PbBr₆ and the rarely observed capped octahedral unit PbBr₇ through corner-sharing and edge-sharing linkages. The seven-coordination geometry indicates a strong stereo-active lone pair on the Pb²⁺ cation. By comparing this structure with two representative 2D perovskites, (benzylammonium)₂PbBr₄ and (4-aminotetrahydropyran)₂PbBr₄, we establish how the lone pair expression affects the local coordination geometry of the Pb²⁺ cation and the resulting optical and electronic properties. As the Pb–Br bond length increases, the lone pair expression leads to off-centering displacement of Pb²⁺ within the octahedra and then the formation of seven-coordination capped octahedra. Density functional theory calculations indicate that the off-centering distorted octahedra and capped octahedra are due to the asymmetric distribution of the Pb electrons that have both s and p orbital characteristics. Spectroscopic studies show the photoluminescence spectra evolving from narrowband emission to broadband emission with increasing LPE, as well as softer and more anharmonic lattice vibrations that facilitate exciton self-trapping. Our results demonstrate that lone pairs could be a powerful design rule for developing light emitting materials.