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Title: Optical and electrical excitation of conjugated polymers
Author: Harrison, Nicholas Torquil
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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The study of luminescent conjugated polymers, and their use in light-emitting diode (LED) structures, is now a well established field of research in both the academic and industrial sectors. Since their discovery, conjugated polymers with many different properties have been developed; and light-emitting diodes (LEDs) with different emission colours, improved efficiencies, and enhanced stability manufactured. However, there is still much that is unknown or poorly controlled with respect to both polymer devices, and the polymer materials themselves. The work in this thesis investigates both materials and device physics. In particular, there is still debate about the precise nature of the electronic excited state - the exciton - which is responsible for light emission in these materials. The exciton may be envisaged as an electron-hole pair which is coupled to the geometry of the polymer chain; when the electron and hole recombine a photon is produced in the process of luminescence. There is discussion about whether the exciton is confined to one polymer chain (an intrachain excitation), or spread over two or more chains (an interchain excitation). Furthermore, it is not clear whether the absorption of light (photoexcitation) in conjugated polymers forms mainly excitons, or if a non-emissive interchain state, which has been called a polaron-pair, is the predominant generated species. The work presented in this thesis is of relevance to these two issues. Site-selective fluorescence measurements are presented for poly(p-phenylenevinylene) (PPV) which is the prototypical luminescent conjugated polymer, and four derivatives of this material. The results suggest that an intrachain exciton is responsible for luminescence in three of the five polymers (including PPV), but that interchain states form the emitting species in two of the materials. Excitation spectra, which provide information about the efficiency of exciton generation, were taken on PPV and three derivative materials. The results suggest that, for polymers prepared in Cambridge, excitons are the primary product of photoexcitation. However, similar measurements on samples of photo-oxidised PPV suggest that material quality is crucial to the process of exciton formation. This conjecture is supported by numerical modelling of the oxidation process. The research in the final third of the thesis involved driving PPV-based polymer LEDs with short high-voltage pulses, with a view to understanding a new excitation regime for these devices. The results show that extremely high peak current-densities and brightness can be achieved in these devices using these driving conditions. These results are discussed with reference to the application of polymer LEDs in multiplexed displays, and as electrically-pumped polymer lasers. The measurements also illustrate how polymeric devices may be used as a tool for elucidating basic physics, as many of the results can be explained by a strongly field-enhanced hole-mobility in the active polymer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available