Layer number dependent exciton dissociation and carrier recombination in 2D Ruddlesden-Popper halide perovskites

It has been a consensus that the exciton binding energies in two-dimensional (2D) Ruddlesden–Popper perovskites is closely correlated with the number of layers. However, the detailed recombination dynamics of excitons and free carriers in these 2D perovskites are still highly controversial. Using transient spectroscopic techniques, carrier dynamics of 2D Ruddlesden–Popper perovskites are investigated at both room and low temperatures. We confirmed that 2D perovskites of (BA)2PbI4, with the number of [PbX6]4− layers n = 1, exhibit clearly exciton characteristic properties. Meanwhile, (BA)2(MA)Pb2I7 and (BA)2(MA)2Pb3I10, n = 2 and 3, exhibit free carrier behaviour, which is inconsistent with their binding energies of more than 100 meV. Such anomalous exciton and free carrier behaviours are attributed to the effective exciton in-plane transport and dissociation by the edge state. This investigation provides novel insights into exciton and charge carrier dynamics of 2D perovskites as well as the influence of edge states.