Exciton Fine Structure in 2D Perovskites: The Out-of-Plane Excitonic State

2D Ruddlesden-Popper metal-halide perovskites feature particularly strong excitonic effects, making them a fascinating playground for studying exciton physics. A complete understanding of the properties of this quasi-particle is crucial to fully exploit the tremendous potential of 2D perovskites (2DP) in light emission applications. Despite intense investigations, some of the exciton properties remain elusive to date, for example, the energy-ordering of the exciton states within the so-called fine structure manifold. Using optical spectroscopy, it demonstrates that in the archetypical 2DP (PEA)(2)PbI4, in contradiction to theoretical predictions, the energy of the bright out-of-plane exciton state is higher than that of two in-plane states. Having elucidated the order of exciton fine structure, it determines the g-factor of the dark exciton transition, together with the values of the electron and hole g-factors in the direction parallel to the c-axis of the crystal. In this way, it provides for the first time, a complete picture of the exciton fine structure in (PEA)(2)PbI4 2DP.