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.