Fluorescence and phosphorescence in polymeric light-emitting diodes

• Main Author: Chan, Khai Leok
• Published: University of Cambridge 2006
• Subjects: 541.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597421

This dissertation describes work done to improve the stability and efficiency of conjugated-polymer organic light-emitting diodes (OLEDs) through the design of novel polymers. The work begins with an investigation of the failure mechanism for the most widely used blue emitter, polyfluorene 1.11. Using fluorenone-containing oligofluorenes as model compounds, it was established that oxidation at the fluorine bridgehead followed by fluorenone excimer formation was responsible for the emergence of low energy bands in the emission spectrum of polyfluorene 1.11 following degradation. The problem of oxidation was solved by designing polymers in which the vulnerable bridgehead carbon was substituted by heteroatoms. Thus, polydibenzosilole 3.66 and polydibenzogermole 3.68 were synthesised and showed significant stability improvement over their carbon analogue. The silicon and germanium atoms were also found to modulate the electron affinity of the π-conjugated system such as to improve electron injection into the polymers. (Fig. 16198B) Finally, the use of phosphorescent systems was investigated to improve the efficiency of OLEDs. The same dibenzosilole motif as in polymer 3.66 was used to synthesise the 3,6-linked polydibenzosilole 4.16. The polymer 4.16 was found to possess the necessary criteria for hosting the green phosphorescent guest 4.01, and complete energy transfer from the polymer to the guest at a low turn on voltage was observed in a simple electroluminescent device using a guest-host blend.