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Title: Electroabsorption investigations of polymer light-emitting diodes with efficient electrodes
Author: Brown, T. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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This thesis is concerned with the application of electroabsorption (EA) spectroscopy to study conjugated polymer light-emitting diodes (PLEDs). Firstly, the design and development of an electroabsorption system that enables the accurate measurement of the built-in potential in PLEDs is described. The EA spectrometer is utilized to investigate PLEDs with advanced structures for effective injection of both types of carriers. A common fundamental structure for PLEDs is the Indium-Tin-Oxide(ITO)/Polymer/Ca-Al sandwich structure. In order to fabricate efficient devices, alterations to this basic structure have to be carried out. The insertion of a variety of thin interlayers, both organic or and inorganic, between the electrodes and the emitting polymer, has been shown to greatly enhance LED performance, although the physical mechanisms underlying this effect remain unclear. The stratagem used in this thesis is to investigate a series of devices differing in either the composition of the anode or that of the cathode. In particular, focus is on a doped conducting polymer anode, and on a variety of cathodic structures incorporating thin fluoride interlayers that improve device performance considerably. It is shown that EA determinations of the built-in potential in PLEDs permit quantitative measurement of the variations in the Schottky barrier heights with respect to a reference electrode. Since this can be done on a finished device, the EA results are directly comparable to the electroluminescent characteristics of the device itself. EA is used to provide conclusive evidence that these interlayers introduce a large reduction in the injection barriers. The results of this thesis can also be used as a test of the theoretical models for the energy level line-up and the injection barrier heights at interfaces between the emitting polymer and the four types of electrodes utilized: i) metals (Al and Ca), ii) ITO, iii) a doped conductive polymer (PEDOT:PSS) and iv) fluoride/metal bilayers. It is found that the Schottky-Mott model is a better approximation than those models where pinning or interface dipoles determine the alignment, except for those electrodes which react extensively with the underlying polymer, introducing a large concentration of deep gap states in the organic layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available