Polymer light-emitting diodes : influence of morphology and structure

The work presented in this thesis focuses on the influence of morphology and device structure on the performance of polyfluorene light-emitting diodes (LEDs). The influence of blend morphology has been investigated by considering how certain processing parameters give rise to different blend morphologies and the impact these morphologies have on device characteristics and efficiency. An understanding of how different blend morphologies arise has allowed the development of techniques to control blend morphology. In this way, vertically segregated polymer blend devices have been fabricated that increase device efficiency by up to 100%. The influence of multiple semi-conducting polymer layer structures on the efficiency of polymer LEDs has also been investigated and the results show that bilayer structures increase device efficiency relative to comparable blend devices by 20%. These bilayer structures have been adapted to incorporate one-dimensional periodic structures, which result in a further 20% increase in efficiency relative to unpatterned bilayer devices. The effect of these periodic structures on the electroluminescent radiation pattern has also been investigated and it has been found that they produce distinct non-Lambertian behaviour perpendicular to the pattern direction. Finally, one- and two-dimensional self-organised periodic structures have been induced into polymer blend LEDs by using surface energy modifications to control the blend morphology. It has been found that the 1- and 2-D patterned devices are 40% and 100% more efficient than comparable blend devices. As in the cased of the 1-D patterned bilayer devices, the radiation patterns of these periodically structured devices have been investigated and exhibit non-Lambertian behaviour perpendicular to the patterned directions. Both the 1-D patterned bilayer and 1- and 2-D self-organised blend device structures have been modelled to allow the observed optical behaviour to be accounted for. The results show that the increases in efficiency and the non-Lambertian radiation patterns are due to the periodic structures coupling light trapped within the organic-anode layers out of the devices in the forward direction without altering the device characteristics.EThOS - Electronic Theses Online ServiceGBUnited Kingdo