The role of a dark exciton reservoir in the luminescence efficiency of two-dimensional tin iodide perovskites

We report on the excitonic luminescence of polycrystalline films composed of two-dimensional (2D) tin iodide perovskites. By combining steady-state and time-resolved spectroscopies between room temperature and 5 K, we identify an optically inactive, dark excitonic state within the spectral fine structure at the exciton energy, ubiquitous in the optical spectra of 2D perovskites. Lying at about 10 meV below the bright exciton that is responsible for luminescence, the dark state mediates non-radiative decay of the photo-excited population at temperatures below 100 K. However, at about 100-120 K, we observe a thermally activated transfer of population from the dark state back to the bright state which results in an increase in luminescence efficiency. Based on quantitative analysis of the observed exciton dynamics, we argue that the dark state acts as a reservoir of photo-excitations at ambient temperature. By feeding the bright state at a rate that is slower than the radiative rate, the dark state mitigates the loss of photo-excited population to other non-radiative pathways. Our work provides insights into the dynamics of the inevitable dark states in 2D perovskites and their relevance in enhancing the emissivity of the technologically relevant lead-free material architectures.