Doctor of Philosophy
Department of Chemistry
Cutler, Charlotte, Electrochemical and photoelectrochemical studies of functionalised polythiophenes, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2000. http://ro.uow.edu.au/theses/1140
Thiophene is a widely exploited precursor for the synthesis of environmentally stable, conducting electroactive polymers. The ease of substitution at the C3 position of the thiophene allows the incorporation of a wide range of substituents that are designed to tailor the electronic properties of the ensuing polymer for a variety of applications. Substituents are predominantly attached to the thiophene via a saturated alkyl or alkoxy linker and the effect of the substituent upon the polymer is often of a steric, rather than an electronic nature.
In this thesis the effect of attaching ρ-substituted phenyl substituents via an alkene linker to the C3 position of thiophene is explored. The purpose of the alkene linker was to promote the inclusion of the substituent into the extended conjugation of the subsequent polythiophene backbone. The alkene may be considered to act as a bridge, that will allow greater electronic communication between the substituent and the thiophene moiety.
A range of systematically substituted thiophene derivatives has been synthesised through the implementation of the Wittig reaction. The substituents range from electron donating to electron withdrawing. The homopolymerisation by electrochemical growth methods of the styryl thiophene derivatives did not produce electroactive, conductive homopolymers. Alternative methods for the incorporation of the styryl substituents into conductiving electroactive polymers were investigated. These included the copolymerisation of the styryl thiophene derivatives with bithiophene, and alternatively the homopolymerisation of a styryl substituted terthiophene monomer. Both options allowed the incorporation of the styryl substituent into polythiophene, while minimising the steric constraints of a planar pendant side chain.
The electroactive polythiophene derivatives were characterised by a range of methods including electrochemical characterisation techniques, UV-visible and IR spectroscopy, SEM and AFM, as well as conductivity measurements.
It was found that the substituents have an effect on the electrochemical properties of the polythiophene polymers. The presence of electron-withdrawing groups shifted the observed redox responses to more anodic potentials and increased the conductivity of the polymer. Substituted poly(terthiophene)s were found to exhibit different mechanical and electronic properties to those of the substituted thiophene copolymers. The growth method was also found to have a greater effect on the properties of substituted poly(terthiophene)s than the styryl substituted copolymers.
The effects of substituents on the photovoltaic response of polythiophenes in photoelectrochemical devices were explored; since substituted polymers may enhance charge separation within the device. The results showed the substituents influenced the photovoltaic properties of the polymers. Thin films of poly(terthiophene)s were found to produce a larger photovoltaic response than polybithiophene and the styryl thiophene copolymers. However, poly(bithiophene) and poly(terthiophene) gave an improved photovoltaic performance in comparison to the substituted analogs.
Finally, the effect of the doping level of the polymer upon the photovoltaic response was also systematically investigated, to meet the need to improve charge transport properties in photovoltaic devices that incorporate conducting polymers as the photoactive component. It was found that partially doped polymers produced photovoltaic responses comparable to those of the fully reduced polymers. In some instances, the partially oxidised polymers gave better photovoltaic responses than the fully reduced form; for example the dimethylamino styryl substituted thiophene/bithiophene copolymer was most efficient when poised at a potential of 0.75 V.