Polymer Light-Emitting Electrochemical Cell Blends Based on Selection of Lithium Salts, LiX [X = Trifluoromethanesulfonate, Hexafluorophosphate, and Bis(trifluoromethylsulfonyl)imide] with Low Turn-On Voltage

Light-emitting electrochemical cell (LEEC) performance is drastically affected by the selection of suitable electrolyte. With the limited n-type doping in MEH-PPV near the cathodic interface, we hypothesized that anions in the electrolyte are critical to attain a higher conductivity p-doped area. Poly(ethylene oxide)-lithium salts electrolyte systems have been investigated in the LEEC devices to determine the influence of anions on device turn-on voltage and operation. The TFSI^- anions showed higher resonance states, low turn-on voltages near the optical bandgap of the emissive conjugated polymer, high ionic conductivity in solid state (1.05 × 10^-4 S cm^-1), and larger electrochemical stability window compared to conventional CF₃SO₃^- anion. Device maximal brightness (∼100 cd/m²) can be achieved. Modulated differential scanning calorimetry (mDSC) studies of the polymer blend films correlate the thermal stability and doping effectiveness during operation and delineate the onset of degradation that entails burnout of the devices.