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published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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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. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-09-23 16:15:06.569

This thesis is concerned with the preparation of organic light-emitting diodes (LEDs) by using different thin film technologies: the Langmuir-Blodgett (LB) technique; spin-coating and thermal evaporation. The π-conjugated polymer, poly(2-methoxy-5-(5'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), was used as the emissive layer and was deposited onto patterned indium-tin oxide (ITO) glass using the LB technique or spin-coating. Y-type LB films of MEH-PPV were deposited at a surface pressure of 17 mN m(^-1) with a transfer ratio of 0.95 ± 0.03. Many efforts were made to improve the LB film device performance parameters, such as external quantum efficiency and operating lifetime, by inserting an electron transporting or insulating layer between the emissive layer and top cathode. Annealing the LB films was found to result in an improved operating lifetime. LEDs based on spun films possessed higher external quantum efficiencies than devices made from LB films. The more ordered LB films had a higher probability of intra- and intermolecular interactions and formed more excimer states within the structure. This led to a lower quantum efficiency compared to devices incorporating spun films. The operating lifetime of the LEDs was highly dependent on the morphology of the film surface. A smoother film surface is required for a longer device operating lifetime. A new electron transporting material, 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]pyridine (PDPyDP), was deposited on top of the MEH-PPV spun film. Despite a high external quantum efficiency of 0.7 %, this device suffered from dark regions in the electroluminescence output resulting in degradation of the device. The dark area formation was attributed to delamination of the aluminium electrode from the PDPyDP layer, which was lessened by: (a) annealing the degraded devices; (b) evaporating a thicker aluminium layer at a high rate and (c) inserting a buffer layer (Alq(_3)) between the PDPyDP and the Al top electrode.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references. Also available in electronic version.

Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Samuel Graham; Committee Member: J. Rhett Mayor; Committee Member: Yogendra Joshi. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.