Transient Photoinduced Pb2+Disproportionation for Exciton Self-Trapping and Broadband Emission in Low-Dimensional Lead Halide Perovskites

Low-dimensional lead halide perovskites with broadband emission hold great promise for single-component white-light-emitting (WLE) devices. The origin of their broadband emission has been commonly attributed to self-trapped excitons (STEs) composed of localized electronic polarization with a distorted lattice. Unfortunately, the exact electronic and structural nature of the STE species in these WLE materials remains elusive, hindering the rational design of high-efficiency WLE materials. In this study, by combining ultrafast transient absorption spectroscopy and ab initiocalculations, we uncover surprisingly similar STE features in two prototypical low dimensional WLE perovskite single crystals: 1D (DMEDA)PbBr4and 2D (EDBE)PbBr4, despite of their different dimensionalities. Photoexcited excitons rapidly localize to intrinsic STEs within ∼250 fs, contributing to the white light emission. Crucially, STEs in both systems exhibit characteristic absorption features akin to those of Pb+and Pb3+. Further atomic level theoretical simulations confirm photoexcited electrons and holes are localized on the Pb2+site to form Pb+- and Pb3+-like species, resembling transient photoinduced Pb2+disproportionation. This study provides conclusive evidence on the key excited state species for exciton self-trapping and broadband emission in low dimensional lead halide WLE perovskites and paves the way for the rational design of high-efficiency WLE materials.