Exciton Self-Trapping in Hybrid Lead Halides: Role of Halogen

Low-dimensional hybrid lead halides have recently been reported as efficient white light emitters. However, unlike lead halide 3D perovskites, most of the reported low-dimensional materials with broad-band emission crystallize in different structure types according to their halogen composition (i.e., Cl, Br, and I) for a selected organic molecule. Because of the absence of isostructural halide series, the role of chemistry in the self-trapping of the excitons at the origin of the broad-band emission remains unclear. Among the most efficient white phosphors, hybrid lead bromide (TDMP)PbBr₄ (TDMP = trans-2,5-dimethylpiperazinium) built of post-perovskite type chains exhibits a record photoluminescence quantum yield for hybrid lead halides. In this article, the two new isostructural (TDMP)PbX₄ chloride and iodide analogues could be synthesized and structurally characterized. A comparison of the optical properties of the lead halide series reveals a strong dependence of the nature of the halogen (Cl, Br, or I) on the trapping/detrapping of the excitons and the resulting emission intensities, wavelengths, and band broadness.