Bright and dark excitons in two-dimensional perovskite semiconductors

Lead halide perovskites are known for their great potential in high-performance light-harvesting devices. We investigated the exciton recombination properties of 2D perovskites. We resolved two bright (optically allowed) exciton doublets and a dark (optically forbidden) exciton. Particularly, under the inherently strong electron-hole exchange interaction, each bright exciton doublet is split into two orthogonally orienting dipoles with large energy splitting of 2 meV, which is the largest experimental values in two-dimensional semiconductors. Furthermore, we observed an efficient transfer of oscillator strengths from the bright excitons to a dark exciton, which originates from strong spin-mixing between bright and dark excitons induced by external magnetic fields, and the optical emission from the dark exciton is brightened. Our results reveal that the physics on exciton recombination in 2D perovskites is rich, while the optical emission properties can be manipulated by external fields