The use of conjugated polymers as the active layer in transistor and photovoltaic devices offers many advantages over inorganic materials, such as the potential for low cost, large-scale, solution processed, flexible devices. There has been considerable progress in this area in recent years due to innovative material design, development of new and exciting device architectures, and a deeper understanding of the underlying operating principles, and morphological requirements for optimal performance. Nevertheless, new materials with even higher performance and stable device characteristics are currently needed if commercialisation is to be achieved.

This thesis focuses on the design and synthesis of a range of ring fused conjugated polymers for use in organic electronics. Ring fused polymers are particularly promising for this application since they offer the potential to help planarise the backbone, enhance intermolecular π-π interactions, and thus improve charge transport. In the first chapter a series of five polymers based on a five-membered, dithienogermolodithiophene (DTTG) monomer (an extended version of dithienogermole) is reported. Through careful consideration of the choice of acceptor co-monomer and device optimisation, a peak saturated hole mobility of 0.22 cm2/Vs was obtained in devices spin-coated from a non-chlorinated solvent mixture, which at the time of writing was the highest reported charge carrier mobility of any germanium based polymer.

The second chapter focuses on the synthesis of a novel bisthieno[2’,3’:4,5]thieno[3,2-c:2’,3’-e]azepine-4,6(5H)-dione (BTTA) monomer in which two thieno[3,2-b]thiophene units are bridged by an azepine-2,7-dione ring. The choice of co-monomer on the optoelectronic and surface morphology of the polymers are reported, using a combination of experimental techniques. The photovoltaic and electrical performance of the polymers investigated in inverted BHJ OPV devices and TG-BC field effect transistors was high, with peak values of 6.78% and 0.027 cm2/Vs observed under optimised conditions.

The third chapter explores the use of side-chain engineering to improve the performance of one of the most promising ring fused electron accepting materials; poly[4,9(benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetrone)-alt-5,5’(2,2’-bithiophene)] (pNDI-BT). In particular, the use of alkyl side-chain branching position as an effective strategy for improving charge carrier mobility was demonstrated, and for the first time, the influence of the branch-point on the performance of all-polymer OPV devices was examined, with a gradual decrease in power conversion efficiency observed as the branch-point was moved further from the polymer backbone. The final chapter builds on this work, and uses a range of fluorinated and non-fluorinated, thiophene and selenophene containing monomers, to further improve the electrical performance of a series of NDI based polymers.