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Title: Carborane containing conjugated polymers for use in organic electronics
Author: Marshall, Jonathan
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Organic semiconductors have emerged as potential replacements for inorganic materials based on several promising advantages, such as their low-cost, high-throughput fabrication and tolerance for flexible substrates. While these attributes ensure that the use of organic semiconductors is appealing, further improvements in electronic performance and stability are still required. This work focuses on the design, synthesis and analysis of a novel range of organic semiconductors incorporating carborane units. Carboranes possess unusual properties which, in principle, make them highly attractive species to incorporate into optoelectronically active materials, although they have been largely untested as components of organic electronic devices. An uncommon three-dimensional delocalised bonding motif infers upon carboranes extreme thermal and chemical stability and a strong electron-withdrawing nature, traits that, if correctly exploited, could potentially lead to organic semiconductors with improved stabilities and optimised electronic properties. This thesis reports a number of approaches to the incorporation of carborane into semiconducting polymers. In the first, ortho-carborane is directly fused to the aromatic backbone via both carbon atoms. The novel difunctionalisation of benzocarborane, and its subsequent incorporation into two previously unreported polymer systems, is reported. Copolymerisation with electron-rich and electron-poor comonomers established that the incorporation of benzocarborane did not prevent electronic delocalisation along the polymer backbone, and that it acted as a mildly electron withdrawing comonomer. P-type behaviour in organic field effect transistors was observed. Based on these results, a second-generation monomer unit was synthesised, incorporating two fused thienyl rings and facilitating observable improvements in molecular ordering and charge-transporting behaviour. The second approach focuses on the attachment of carborane to the conjugated polymer backbone via the functionalisation of one of its carbon atoms, either directly or via a vinylene spacer. Hence carborane functionalised benzodithiophene monomers were prepared via a novel method of carborane C-arylation, before subsequent polymerisation with a range of electron accepting comonomers. The resultant polymers demonstrate OFET performance higher than that reported for non-carborane containing equivalents and exhibit promising solar cell performance. The final chapter details the attachment of various carboranes onto two polythiophene systems via a vinylene linker. Through careful consideration of carborane and vinyl bond stereochemistries, and pendant group spacing it was observed that optical, material, OFET and OPV properties of the resultant polymers could be controlled.
Supervisor: Heeney, Martin Sponsor: Engineering and Physical Sciences Research Council ; Atomic Weapons Establishment
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral