Origin of discrete donor-acceptor pair transitions in 2D Ruddlesden-Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light-matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by mu-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths ( similar or equal to 120 mu eV) and long decay times (5-8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden-Popper perovskites, (PEA)(2)(MA)(n-1)Pb(n)I3(n+1), particularly in phases with n >= 2, has been identified as a mechanism of donor-acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of similar or equal to 10 nm. The analysis of the spectral and electronic characteristics of these single donor-acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.