Publication
State-of-the-art organic light-emitting diodes (OLEDs) used in commercial display technology are complex multilayer structures. In contrast, the light-emitting electrochemical cell (LEC) can be built from a single emissive layer sandwiched between two electrodes. Due to the presence of mobile ions - intermixed with the active material - the single layer can perform all the tasks that take place in an OLED, i.e., electrical charge injection, transport, exciton formation, and radiative recombination. Therefore, LECs have the potential to be fabricated by low-cost solution coating and printing processes. While the structure is simple, the simultaneous occurrence of electronic and mobile ionic charge introduces a complex device functionality. Basic operation principles have been discussed for years in the field and consensus has been reached on the electrochemical doping model. The focus of this thesis is to unravel the interplay of electronic and ionic charges in LECs and to highlight the boundaries and gaps of the firmly established electrochemical doping model. Used methods and conclusions are not restricted to the polymer-based LECs and can be applied to various systems with combined electronic and ionic conduction, as demonstrated for perovskite LEDs. First, effects of ion concentration and active layer thickness variation in the super yellow (SY) polymer LEC (PLEC) are investigated, which play a critical role on the performance. The dependence on the layer thickness reveals the often-overlooked microcavity/Purcell effect, which leads to a strong dependence of the optical outcoupling efficiency on the emitter position (EP) within the active layer. For thicker PLECs, a continuous change of the emission colour during operation provides a direct visual indication of a moving EP. Drift-diffusion simulation of the electrical measurements pinpoints the EP shift to ion dynamics and demonstrates that the only precondition for this event to occur is unequal cation and anion mobilities. Quantitative ion profiles reveal that the movement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. The unresolved origin of device degradation after the ionic steady state has been reached leads to the study of the SY polymer LED (PLED) with the idea to isolate the electronic from ionic effects. Response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours reveals a surprisingly slow response of electron traps. Subsequently, the characteristic response of the electron traps to electrical breaks is identified in the PLEC as well. Further investigation by electroabsorption measurement finds that hole trap formation limits the device lifetime, in the exact same manner as for PLEDs. The methodology to unravel dynamic processes due to mobile ions, electronic charges and trap development is applied to perovskite light-emitting diodes (PeLEDs). The application of the opt