Investigating the Operating Mechanisms of Polymer Light Emitting Diodes

• Main Author: Jin, Rui
• Other Authors: Bradley, Donal ; de Mello, John
• Published: Imperial College London 2009
• Subjects: 621.381045
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.506025

This work uses a broad range of optoelectronic characterisation techniques tounderstand ? at a fundamental level ? the operating mechanisms of PLEDs. Theelectromodulation (EM) technique particularly provides a straightforward means ofdetermining the electric field strength inside operational devices, and is used here toinvestigate the improved device performance due to the insertion of an interlayerbetween the anode and the emissive layer. The effects of different interlayer materials(hole-transporting polymeric materials and one crosslinkable material) are studied in red,green and blue PLEDs. Interlayer devices yield better efficiencies and longer lifetimes,which can be attributed to charge accumulation near the anode/interlayer and (or)interlayer/emissive layer interfaces indicated by EM measurements. A promising alternative anode material ? vapour phase polymerised poly(3,4-ethylenedioxy thiophene)] (VPP-PEDOT) is another major focus of this thesis. Togetherwith poly(3,4-ethylenedioxythiophene-styrenesulfonate) (PEDOT:PSS), VPP-PEDOT is aviable alternative anode to indium tin oxide (ITO), capable of yielding superior efficienciesin otherwise identical PLEDs. Finally, a simulation code is developed for organic semiconductor devices tosystematically study the charge and electric field distributions in model devices. Thiscode, based on drift-diffusion model, can be used to study light-emitting electrochemicalcells (LECs). The simulation results indicate that there are high electric fields at bothelectrodes due to ionic charge distribution, which in turn facilitates electronic chargeinjection and thus leads to high recombination rates and luminous efficiencies.