Organic light-emitting electrochemical cells (LECs) are very simple electroluminescent devices compared to OLEDs and consist of a single emissive organic/salt layer sandwiched between two electrodes. Due to the presence of mobile ions, the single layer can perform all the tasks that take place in an electroluminescence device, i.e. facile electrical charge injection, transport, exciton generation and radiative recombination. Firstly, electronic double layer formation at the interface to the electrodes – as it is observed in perovskite solar cells – leads to screening of the electric field and enables electronic charge injection into the active layer. Secondly, electrochemical doping results in a p-doped/intrinsic/n-doped (p-i-n) structure in the active layer. The doped regions enable transport of electronic charges to the intrinsic region where charge recombination and light emission occurs. The LEC device concept sounds simple, but the working principle behind it is truly complicated; despite twenty years of research, resolving the dynamics of the ions and injected electrical charges as well as identifying the zone where light is emitted in sandwich devices present considerable scientific challenges. In particular, a remaining puzzle is the observation that the intrinsic zone position and width are dynamic and change over an operation time of many hours.

We tackled these questions by investigating effects of ion concentration and active layer thickness, which play a critical role on the LEC performance. Expanding on a pioneering materials system comprising the Super Yellow (SY) polymer and the electrolyte trimethylolpropane ethoxylate (TMPE) / Li⁺CF₃SO₃^-, we report that a slightly lowered salt concentration and layer thickness result in a substantial efficiency increase, and that this increase is confined to a narrow concentration and thickness range. For a film thickness of 70 nm, a blend ratio SY : TMPE :  Li⁺CF₃SO₃^- = 1 : 0.075 : 0.0225 and a current of 7.7 mA cm^-2 the current efficacy was 11.6 cd A^-1, on a par with SY light-emitting diodes. This optimization hints at an unexploited performance potential in many other LEC materials systems. The optimized salt content can be explained by increased exciton quenching at higher concentrations and hindered carrier injection and conduction at lower concentrations, while the optical dependence on the layer thickness is due to weak microcavity effects. We present a comprehensive optical modeling study that includes the doping-induced changes of the refractive indices and self-absorption losses due the emission-absorption overlap of intrinsic and doped SY. The analysis indicates that the position of the emitter in the active layer has a critical influence on the performance, and the situation in our device can be explained either by a thickness-independent emitter position (EP) close to the anode or a thickness-dependent EP, shifted to the cathode for increased thicknesses [1]. To further unravel the complex (temporal) device dynamics, it is necessary to know where the EP is situated. By fitting electroluminescence spectra measurement at different angles with the optical model, the position and shape of the emission zone can be determined over time [2]. This is possible because the shape of the spectra depend on the position of the emission zone due to microcavity effects. Further insight was gained by looking at the ion profile at different points in time during operation, which we achieved by quantitative Tof-SIMS in-situ depth profiling at liquid nitrogen temperature [3].