Doping-Induced Self-Absorption in Light-Emitting Electrochemical Cells

We report on the quantitative effects of doping-induced self-absorption in light-emitting electrochemical cells (LECs) as a function of active material (AM) thickness and doping concentration. For state-of-the-art polymer LECs with optimized doping concentration and comprising Super Yellow as the electroluminescent (EL) polymer and poly(ethylene oxide)-KCF3SO3 as the electrolyte, we find that the self-absorption loss at the EL peak wavelength is similar to 10% for a 100 nm thin AM and >70% for a 1 mu m thick AM. This implies that the utilization of micrometer-thick AMs fit for fault-tolerant large-scale fabrication can be concomitant with a notable penalty in device performance, and that spatial variations in AM thickness will be manifested in a corresponding spatial light-intensity variation. Moreover, we find that inclusion of a poly(ethylene oxide)-KCF3SO3 electrolyte can inhibit the out-coupling of light and suggest that the culprit is light scattering from dispersed crystalline-electrolyte domains. Finally, we demonstrate evidence for that the selected initial salt concentration in an LEC device dictates the maximum doping concentration that can be attained at steady-state operation.