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Title: Optoelectronic properties of new conjugated materials
Author: Casey, Abby
ISNI:       0000 0004 6346 892X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Next-generation electronic devices which are cheap, lightweight and flexible could be realised through the use of solution processable organic polymer and small molecule semiconductors. Unlike inorganic semiconductors such as silicon, soluble organic semiconductors could be processed using traditional high through-put printing techniques such as roll-to-roll processing and ink-jet printing, which would dramatically reduce manufacturing costs. Whilst organic semiconductors are not expected to be as high performance as inorganic semiconductors, improvements in performance are still required before commercialisation is possible. One way to help improve performance is to exploit the chemical versatility of organic materials. Many different structures can be synthesised through chemical modification, allowing the optoelectronic properties (such as the optical band gap and energy levels) and physical properties (such as solid state structure) of the material to be tuned. Materials can therefore be chemically designed to optimise their performance in organic electronic devices. This work is focused on exploring the relationship between chemical structure, material properties and device performance, through the design and synthesis of new materials for organic field-effect transistors (OFET) and organic photovoltaics (OPV). The majority of the new materials synthesised in this thesis are new donor-acceptor polymers (Chapters 2-6), in which an electron donating monomer and electron accepting monomer are co-polymerised. Whilst there is a vast wealth of different donor monomer structures available, there has been less focus on the synthesis of new electron accepting monomers. In this work the common electron acceptor monomer 2,1,3-benzothiadiazole (BT) is chemically modified to either increase the solubility (Chapter 2) or increase the electron accepting strength (Chapters 3 and 4). Increasing the strength of the electron accepting unit in donor-acceptor polymers was found to induce N-type (electron conducting) behaviour in OFET devices (Chapter 3) or improve OPV performance by reducing the optical band gap and increasing light absorption (Chapter 4). Power conversion efficiencies of ~6.5% in OPV devices were achieved. In chapter 6 a novel BT based acceptor monomer is designed to maximise polymer backbone planarity which resulted in promising hole mobilities of up to 0.5 cm2/Vs when tested in OFET devices. In Chapter 5 the strength of the common electron accepting unit benzo[d][1,2,3]thiadiazole (BTz) is also increased through chemical modification. Similarly to Chapter 4, we find that increasing the strength of the electron accepting unit of the donor-acceptor polymer improves the OPV performance through increased light absorption, resulting in efficiencies of ~6.5% in OPV devices. Finally in Chapter 7, new electron rich conjugated small molecules are synthesised and the optoelectronic properties investigated.
Supervisor: Heeney, Martin Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral