Donor - Acceptor Conjugated Polymers For Optoelectronic Applications

The development of renewable energy is of paramount importance if scientists are to mitigate the enhanced greenhouse effect and dwindling supplies of fossil fuels. Solar energy is believed to be the largest carbon-neutral energy source available with the sun producing vast quantities of energy every second. Current inorganic photovoltaic systems that are available have numerous limitations and shortcomings associated with them. Thus, research into an alternate, cost-efficient photovoltaic device that efficiently harvest a large portion of the incident solar spectrum has resulted in scientists focussing on so-called third generation photovoltaics. Organic photovoltaic devices, which use semiconducting conjugated polymers and fullerene derivatives in the active layer, are an example of third generation devices. Organic photovoltaic devices possess numerous advantages over their inorganic counterparts including: reduced embodied energy, increased flexibility, abundant materials for fabrication and better operation at lower light intensity levels. However, currently they are not as efficient as inorganic devices.
The performance of organic photovoltaic devices is influenced by the chemical structure of the semiconducting conjugated polymer. Specifically, the optical band gap, frontier energy levels, charge carrier mobility and solubility are determined by the chemical structure. Copolymerising electron-donating monomers with electron-withdrawing substituents in a so called donor-acceptor (D-A) arrangement has proven to be an effective method in producing conjugated polymers that have high efficiencies when fabricated into organic photovoltaic devices.
Chapter II and Chapter III ascertained that attaching fluorine substituents to the benzothiadiazole moieties resulted in a decreased molecular weight. This phenomenon continued to occur when larger alkyl chains were attached to the conjugated backbone. Furthermore, fluorination of the benzothiadiazole unit yielded blue shifted absorption maxima. However, it did deepen the HOMO level of the polymer resulting in increased oxidative stability. Additionally, changing the fluorene donor for the planar, fully aromatic carbazole moiety was successful in lowering the optical band gap.
In chapter IV, the donor portion of the conjugated polymer was changed for a triisopropylsilylacetylene functionalised anthracene unit. Previous work within the Iraqi group has shown that the anthracene unit to be a weak donor. Incorporation of the anthracene units resulted in a narrower optical band gap despite the low molecular weights of the polymers. Furthermore, the weak donating properties of the anthracene donor unit yielded deep HOMO levels.
In Chapter V, the benzothiadiazole moieties used as acceptor units in Chapter IV were substituted for a thieno[3,4-c]pyrrole-4,6-dione acceptor; an extremely planar compound with strong electron withdrawing properties. Various solubilising groups were attached to the thieno[3,4-c]pyrrole-4,6-dione acceptor to ascertain the impact solubilising chains has upon the physical properties of the polymer. However, it was discovered the steric clash between the bulky triisopropylsilylacetylene groups and the chains attached to the thieno[3,4-c]pyrrole-4,6-dione acceptor resulted in unfavourable backbone twisting. Thus, the polymers displayed wide optical and electrochemical band gaps despite having a high molecular weight. The amorphous nature of these polymers was confirmed with power X-ray diffraction.
In chapter VI, the thieno[3,4-c]pyrrole-4,6-dione acceptor unit was changed for diketopyrrolopyrrole acceptor unit. It was hypothesised that the increased planarity of this molecular, relative to thieno[3,4-c]pyrrole-4,6-dione would minimse steric clash between alkyl chains and increase planarity. This hypothesis was proved correct when all polymers sysnthesised in this chapter displayed extremely narrow optical and electrochemical band gaps. Furthermore, powder X-ray diffraction showed the polymers adopted a semi-crystalline conformation, which should, in theory, improve charge transportation.
Finally, in Chapter VII the triisopropylsilylacetylene functionalised donor was polymerised with either a fluorinated or non-fluorinated quinoxaline acceptor unit. Relative to the non-fluorinated polymer, the fluorinated polymer displayed a blue-shifted absorption maximum and wider optical and electrochemical band gaps. However, the fluorinated polymer adopted a more crystalline structure, a consequence of enhanced π-π stacking and intermolecular interactions brought about by the incorporation of fluorine.