| Summary: | Multiple factors affect the overall efficiency of polymer solar cells, many of which are influenced by the physical and chemical properties of the polymers themselves. Through varying the chemical make-up of the polymers; the band gaps, HOMO and LUMO levels, charge transport properties and processibility can all be adjusted to increase the efficiency of devices. One particular area of focus of in this thesis is the often overlooked effect of the dielectric constant on device efficiencies. With this in mind, manufacture of a number of materials with permanent fixed dipole moments were synthesised due to the linear relationship variance of dipole moment has with the dielectric constant. Chapters 3 to 6 particularly focus on dipole moment influence through the design of regular and random repeating structures. Chapter 2 studied the effect of replacing benzothiadiazole units with naphtho thiadiazole units along polymer chains. It was discovered that increasing the size of the conjugated system leads to lower molecular weight materials. The extended aromatic system provides extra π-stacking ability and causes enhanced aggregation. Despite the low molecular weights, improvements were seen in the narrowing of optical band gaps and deepening of HOMO levels. In chapters 3 and 4, the effect of adapting poly 3-hexylthiophene through the copolymerisation of thieno[3,4-c]pyrrole-4,6-dione and thiophene. Regio regular and regio random polymers were synthesised with different chain lengths on both co monomers. The electron withdrawing thieno[3,4-c]pyrrole-4,6-dione had the effect of narrowing band gaps and deepening HOMO levels. Regioregular polymers in chapter 4 formed the more crystalline structures as shown by XRD, with lamellae structures visible. It is postulated that the enhanced crystallinity led to the improved properties seen for the regio regular polymers, despite the lower molecular weights. Chapter 5 explored the effect of using 1 and 2 naked thiophene units as donors alongside thieno[3,4-c]pyrrole-4,6-dione. Increased donor strength from bithiophene units yielded the best photochemical properties. Higher crystallinity was achieved by the bithiophene polymers, which was gained through the additional spacing between thieno[3,4-c]pyrrole-4,6-dione units and an alternating orientation. In chapter 6 polyether side chains were attached to thiophene molecules and copolymerised with thieno[3,4-c]pyrrole-4,6-dione. The inductive effect of the ether chain led to narrow band gaps and strong intermolecular interactions. The position of attachment was also probed, with oxygen further from the conjugated system causing a widening of the band gap.
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