Synthesis and characterization of novel low band gap semiconducting polymers for organic photovoltaic and organic field effect transistor applications

Abstract

This thesis describes the synthesis, characterization and device properties of a range of conjugated polymers incorporating 3,6-dilakylthieno[3,2-b]thiophene. We report a new and facile synthesis for the preparation of 3,6-dialkylthieno[3,2-b]thiophene, which is readily scaled up to the multi-gram scale. With this synthesis in hand, we initially investigated the properties of poly(thienothiophene-alt-vinylene) polymers incorporating both straight and branched side-chains. Two different polymerization methods were investigated to synthesise the conjugated polymers, namely Stille and Gilch polymerization. The Gilch route was found to lead to high molecular-weight polymers with less cis-defects in the backbone. The polymers were found to be largely amorphous by X-ray diffraction measurements, although there were clear signs of aggregation by optical investigations. Field-effect transistors fabricated with these polymers exhibited charge carrier mobilities up to 0.02 cm2 V-1 s-1 for the straight chain analogue, with the branched polymer displaying lower mobilities. Blends with PC71BM were found to exhibit solar cell device efficiencies up to 2.5 %, with significant differences observed for polymers containing two different side-chains. In the third chapter we investigated the properties of ethynylene-linked 3,6-dialkylthieno[3,2-b]thiophene polymers. The simple homo-polymers were found to exhibit much worse device performance than the analogous vinylene-containing polymers in transistor devices. Co-polymers with a range of electron accepting monomers were also synthesized. These displayed low optical energy gaps and signs of aggregation in the solid state. Transistors were fabricated and their performance examined. In the final part of this thesis, co-polymers bearing 3,6-dialkylthieno[3,2-b]thiophene donor and squaraine acceptor units were synthesized. These zwitterionic conjugated polymers displayed band gaps as low as 1.0 eV. The influence of the nature of the side-chains and co-monomer was investigated with regard to their optoelectronic properties.