Exciton Properties and Broadband Emission in Two-Dimensional Ruddlesden-Popper Perovskites

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites are considered as promising optoelectronic materials due to their intriguing broadband emission properties. However, the origin of the broadband emission in RP perovskites remains a controversial issue. Herein, we have systematically analyzed the exciton behavior and luminescence properties of 2D (PEA)₂PbI₄. Our calculations demonstrate that 2D (PEA)₂PbI₄ exhibits significant quantum confinement effects, which provide a prerequisite for the formation of self-trapped excitons (STEs). However, the intrinsic STEs are absent in pristine lattices. In contrast, the iodine vacancy (V_I1) is energetically favorable and exhibits a shallow charge transition level above the valence band maximum at ∼2.17 eV, which further induces large lattice deformation around the defect to accommodate STEs. Further excited-state electronic structure calculations strongly suggest that the V_I1-associated extrinsic STEs are responsible for the broadband emission in 2D (PEA)₂PbI₄. This mechanistic understanding of exciton properties provides fundamental design principles for high-efficiency light-emitting diodes.