Identifying Key Properties of Electrolytes for Light-Emitting Electrochemical Cells

The electrolyte is a key component in light-emitting electrochemical cells (LECs), as it facilitates in situ electrochemical doping and associated attractive device features. LiCF 3 SO 3 dissolved in hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) constitutes an electrolyte with which we have attained high stability and efficiency for polymer LECs, but the turn-on time of such devices is unfortunately slow. By replacing hydroxyl with methoxy as the TMPE end-group, we produced LECs with a desired combination of high efficiency, good stability, and fast turn-on time. Specifically, we showed that the turn-on time to high luminance (300 cd/m 2 ) at a current density of 7.7 mA/cm 2 is lowered from 1740 to 16 s, that the efficiency is improved by ∼20%, and that the other device properties are either maintained or improved. In a parallel modeling and experimental effort, we demonstrated that the faster kinetics following the shift in the TMPE end-group is attributed to a marked decrease in the level of both inter- and intramolecular interactions of the electrolyte, as manifested in a lowered electrolyte viscosity, faster ion transport, and more facile ion release during doping.