Time-dependent p–i–n structure and emission zone in sandwich-type light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) are one of the simplest electroluminescent devices and consist of a single emissive organic/salt layer sandwiched between two electrodes. The unique attribute of an operated LEC is the development of a p-doped/intrinsic/n-doped (p–i–n) structure in the active layer in which the doped regions allow for facile transport of electronic charges to the intrinsic region, where charge recombination and light emission occur. However, due to the complex and simultaneous motion of ionic and electronic charges, the examination of the p–i–n structure and the zone where the light is emitted (EZ) is challenging during operation. By analyzing incident photon-to-current conversion efficiency and angular emission measurements with optical simulations, and correlating the results with capacitance measurements, we are able to obtain a clear picture of the p–i–n situation and the EZ within the active layer of a sandwich-type LEC during operation. It is found that the p-doped zone grows to the center of the active layer while the EZ stays closer to the metal electrode and is unchanged over time. Furthermore, optical simulations reveal that the determined EZ limits the external quantum efficiency of the LEC by outcoupling efficiencies of less than 10%