How Electrode Material Affects the Performance of Polymer Light-Emitting Electrochemical Cells

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Hohertz, Donna
Light Emitting Electrochemical Cells
Polymer light emitting electrochemical cells (LECs) are solid-state devices containing an active layer blend of luminescent polymer, ion transport material and salt sandwiched between two electrodes. They operate on the principal of in situ electrochemical doping. Doping entails the injection of electronic charge from the electrodes, causing the reduction/oxidization of the luminescent polymer, and accompanied by charge compensation through the redistribution of salt counter-ions. Due to the high conductivity of the doped polymer, a fully turned on LEC has a dramatically reduced contact and bulk resistance. This gives the LEC certain intrinsic advantages such as balanced charge injection, low operating voltage and high quantum efficiencies, even when stable metal or symmetric electrodes are used. These properties have led to the popular assumption that the electrode work function is not a critical device parameter for LEC operation. In this thesis, I describe my original research to determine how the electrode composition influences LEC performance. A series of sandwich and planar configuration LECs with various electrodes on identical MEH-PPV (poly[5-(2-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene]):PEO (poly ethylene oxide):LiTr (Lithium trifluoromethanesulfonate) based films are constructed. I demonstrate that the doping profile, doping propagation speed, emission zone shape, emission zone location, electro-luminescence (EL) turn-on, and EL efficiency are all strongly affected by the choice of electrode materials. LECs with asymmetrical electrodes optimized for both electron and hole injection result in the best overall performance. Using an optimized electrode configuration, I am able to realize extremely large crown ether based planar LECs. MEH-PPV: dicyclohexano-18-crown-6 (DCH18Cr6): LiTr and 108GE:DCH18Cr6:LiTr devices with various symmetric and asymmetric electrode configurations were constructed, where 108GE is the fluorene copolymer poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-(2-methoxy-5-(ethylhexyloxy)-1,4-phenylene)]. I demonstrate ii i and image the first ever crown ether-based planar LECs with millimeter inter-electrode spacing. Due to minimal phase separation, crown ether-based LECs display highly uniform doping propagation and very smooth emission zones. Junction relaxation, de-doping and reverse bias operation experiments are also presented, and results compared to behavior in PEO based LECs. Additionally, I demonstrate that crown ether-based LECs do not exhibit frozen junction behavior at room temperature.
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