Post-polymerisation modification of conjugated polymers: towards novel organic semiconductor materials
Creators: name: Rapley, Charlotte Louise
Publisher: name: Imperial College London
Year: 2022

description: Organic conjugated polymer semiconductor materials have grasped the attention of many researchers, particularly in the last 10 years, for their use in electronic devices. In order to engineer materials for specific applications, it is essential to consider the polymer design and chemical structures. With thoughtful polymer design, it is possible to modify and manipulate physical and optoelectronic properties to optimise semiconductor materials for intended applications. Post-polymerisation techniques have been widely explored as a useful tool to modify polymers with desired properties or specialised functionality. Nucleophilic aromatic substitution (SNAr) has been identified as a useful reaction to facilitate the modification of conjugated polymers without introducing unwanted defects within the polymer backbone. In this thesis, SNAr was used as technique to introduce conjugated polymer sidechains and various other functional groups along conjugated polymer backbones. Polymers were designed and synthesised for use in electronic devices such as; organic thin-film transistors (OTFTs) and organic solar cell (OSC) devices, where physical and optoelectronic properties were evaluated for all polymers. The work presented (Chapter 2-6) looks at polymers designed with a monofluorinated benzothiadiazole as a repeat unit within the backbone of a conjugated polymer. A range of nucleophiles were substituted onto the polymer backbones via SNAr, directly onto the monofluorinated benzothiadiazole repeat unit, highlighting the versatility of this post-polymerisation modification method. In chapters 2 and 3, this method was used to develop graft co-polymers with an n-type conjugated polymer backbone and p-type conjugated polymer sidechains. In chapter 3, single-component graft co-polymer structures were shown to provide a comparatively similar solar cell device performance to the blended polymers, however, with higher recorded Voc values. Further exploration of the versatility of post-polymerisation modification requires the development of new conjugated backbone polymers that can readily undergo modification. Hence, in Chapter 4, the direct modification of two novel n-type conjugated polymer backbones via SNAr was demonstrated, including the quantitative incorporation of substituents along the polymer backbone. Unsubstituted polymers achieved an electron mobility of 0.16 cm2 V-1 s-1 in OTFT devices. After substitution, mobilities reduced by around one order of magnitude. Further developments looked at azide functionalised conjugated polymers, where polymer thin films were successfully thermally cross-linked or photo-patterned (high azide content polymer). Finally in Chapter 5, polymers were modified with increasing anionic content, for use as interlayers between active polymer layers and electrodes within OSCs, for example. Increasing the anionic content did not largely affect the electronic properties of the polymer, however, did modify the work function of indium tin oxide (ITO).