Investigating the Operating Mechanisms of Polymer Light Emitting Diodes

Abstract

This work uses a broad range of optoelectronic characterisation techniques to understand – at a fundamental level – the operating mechanisms of PLEDs. The electromodulation (EM) technique particularly provides a straightforward means of determining the electric field strength inside operational devices, and is used here to investigate the improved device performance due to the insertion of an interlayer between 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. Together with poly(3,4-ethylenedioxythiophene-styrenesulfonate) (PEDOT:PSS), VPP-PEDOT is a viable alternative anode to indium tin oxide (ITO), capable of yielding superior efficiencies in otherwise identical PLEDs. Finally, a simulation code is developed for organic semiconductor devices to systematically study the charge and electric field distributions in model devices. This code, based on drift-diffusion model, can be used to study light-emitting electrochemical cells (LECs). The simulation results indicate that there are high electric fields at both electrodes due to ionic charge distribution, which in turn facilitates electronic charge injection and thus leads to high recombination rates and luminous efficiencies.