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Thermally-reactive conjugated polymers

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Thesis type
(Thesis) Ph.D.
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Author: Han, Xu
тт-Conjugated polymers (ттCPs) are promising active materials in application areas of organic microelectronics such as light-emitting devices (LEDs), thin-film transistors, and photovoltaic (PV) cells. A crucial requirement to implement this emerging technology is the reproducible deposition of active material in a spatially-controlled fashion. This thesis research concerns the study of thermally-reactive ттCPs and 2D deposition of these materials. The technique of choice depends on the material under consideration, the substrate, and the intended application or research objective. Chemically-amplified soft lithography and direct thermal lithography were used to pattern polyfluorenes (PFs) to investigate their potential application as LEDs. Topographical control of active materials is also demonstrated using polymer blends for the application of PV cells. Thermally-reactive PFs containing tetrahydropyran (THP) functional groups have been synthesized. The removal of the THP groups causes the polymers to become insoluble in normal organic solvent. Chemically-amplified soft lithography was used to pattern the luminescent polymers. However, these PFs may suffer from a degradation of fluorescence colour purity due to fluorenone defects, which are introduced by photo-oxidation and/or thermal oxidation process, or during device fabrication. Therefore, a series of thermally-reactive PFs exhibiting enhanced oxidative stability were synthesized. These PFs, bearing aromatic groups directly attached to the 9-site carbon, eliminate the formation of fluorenone defects. Direct thermal lithography was used to pattern these PFs. A facile method for preparing nano/micro-sized ттCPs from polymers bearing thermally-cleavable THP groups is investigated. The feature size of the ттCP formed is dependent on parameters such as choice of solvent, solution concentration, polymer blends composition, and the nature of the polymer(s) used. Features as small as 150 nm are demonstrated, and much smaller features are possible with further optimization of conditions. The insoluble nano-structured ттCP has been exploited as a donor layerin polymeric PV cells following spin-casting an electron acceptor layer on top of the donor layer to form a nano-sized donor-acceptor heterojunction. The nano-structured ттCP donor-acceptor layers provide an advantage over molecularly blended films in that electron acceptor is not isolated in the donor film, and vice versa, which may circumvent charge trapping and thus improve the device efficiency.
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