Olugbemiga, Caroline Teniola (2003) Heterocyclic ladder polymers and oligomers for electronic devices. (PhD thesis), Kingston University, .
Abstract
In this project it was planned to synthesise and investigate the physical, chemical and electrical properties of the "polyquinoxaline" family of materials. The compounds synthesised and studied included 5,14-dihydro-5, 7,12, 14-tetraazapentacene(L5H[sub]2), dibutyldihydro-5, 7,12, 14-tetraazapentacene (L5Bu[sub]2H[sub]2), 7, 16-dihydro-5, 7,9,14,16,18-hexaazaheptacene(L7H[sub]2), 7,20-dihydro-5, 7, 9, 11, 16, 18, 20, 22-octaazanonacene (L9H[sub]2), dibutyldihydro-5, 7, 9, 11, 16, 18, 20, 22- octaazanonacene (L9Bu[sub]2H[sub]2), “Polyquinoxaline"(poly[ 1, 6-dihydropyrazino(2, 3-g)-quinoxaline-2, 3,8-triyl-7(2H)-ylidene-7,8-dimethylidene) and the dibutyl-substituted (poly[ 1, 6-dihydropyrazino(2,3- g)-quinoxaline-2,3,8-triyl-7(2H)-ylidene-7,8-dimethylidene). Some alkyl-substituted oligomers were also made, so as to increase their solubility in organic solvents. Both liquid phase and solid phase synthesis have been used in preparing these compounds, the latter being done in an attempt to produce the compounds in their purest form. In the case of L7H[sub]2, this method gave the first successful synthesis of the compound. A number of techniques, such as FTIR, solid-state NMR and Visible / Ultraviolet spectroscopy have been used in characterising both the synthesised compounds and the first in the "quinoxaline" series of compounds phenazine (L3), which is commercially available. For solutions of the oligomers in concentrated sulphuric acid, the HOMO-LUMO absorption bands in the UV-visible decreased monotonically with the number of rings. The electrochromic properties of vacuum evaporated thin films of dihydrotetraazapentacene (L5H[sub]2) and dihydrohexaazaheptacene (L7H[sub]2) have been investigated using cyclic voltammetry. These investigations suggest that L7H[sub]2 thin films exhibit similar intermediate redox states to L5H[sub]2 films. Two-point electrical conductivity measurements have been carried out on pressed powder pellets of doped and undoped oligomers, substituted oligomers and ladder polymers. The electrical conductivities showed marked increases of five to six orders of magnitude as a result of doping the materials with hydrogen chloride vapour. The final major part of the project was the first fabrication of metal-semiconductor Schottky diode devices based on vacuum-evaporated thin films of dihydrotetraazapentacene (L5H[sub]2). Schottky diode devices on L5H[sub]2 films were successfully made using a range of metal contacts (Sn, Cu. AI, Pb, Au and Ag). Electrical measurements on various metal-doped L5H[sub]2 films gave I-V characteristics indicative of a Schottky barrier formed at a metal/n-type semiconductor interface. These electrical device measurements were mathematically modelled to determine the magnitude of the Schottky barriers at the various metal-semiconductor interfaces. This information and the electrochromic redox measurements on L5H[sub]2 thin films have helped to characterise the charge-transfer processes in these oligomer thin films.
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