Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden-Popper perovskites

Two-dimensional metal halide perovskites of Ruddlesden-Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier-phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons. The Ruddlesden-Popper type of halide perovskites are gaining interest but the origin of their broad band emission remains elusive. Here Kahmann et al. suggest that in-gap states, instead of the commonly believed self-trapped excitons, are responsible for the broad band emission in crystalline samples.