| This thesis presents the design, synthesis and thin-film-transistor performance of a novel series of polythiophenes. The work can be divided into two parts: (1) study of crystal packing of the alkyl side chains in single crystals of model oligothiophene compounds for soluble polythiophenes; (2) exploration of a new series of polythiophenes to achieve favorable crystal packing and hense high mobility for use in organic thin-film-transistors.; The first part is based on the crystal packing of a series of compounds derived from the monomer of a high-performance semiconductor, poly(3,3"didodecylquarter-thiophene), PQT-12. A unique conformational polymorphism arising from side chains was observed when the conjugation of backbone of PQT-monomer was extended with phenyl, methyl-phenyl, trifluoromethyl-phenyl. The alkyl side chains preferred tilting towards the middle and then being parallel with the backbones when crystallized from a poor solvent, whereas, the side chains extended out vertically to the backbones when crystallized from a good solvent. The conformational polymorphism of the side chains in the dip-coated film was also studied.; The following chapters focus on the design, characterization and TFT performance test of novel polymer semiconductors. A novel and symmetrical poly(4,8-didodecylbenzo[1,2-b:4,5-b']dithiophene) with alkyl side chains tethered to the middle of the large fused backbone was synthesized from 2,6-dibromo-4,8-didodecylbenzo[1,2-b:4,5-b']dithiophene by a dehalogenative coupling polymerization. The thin-film transistors made from this polymer as a semiconductor produced a field-effect mobility of 0.012 cm2V-1s-1 and current on/off ratio ∼2.5x105 after thermal annealing at 140°C. The performance was greatly enhanced (field-effect mobility ∼0.15 cm 2V-1s-1 and current on/off ratio x10 6) when two 3-methyl-thienylenes were incorporated into the backbone, poly(4,8-dodecyl-2,6-bis-(3-methylthiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene). Such good performance came from well-spaced side chains, which were favorable for the large fused polymer backbone to self-assemble readily into highly structured thin films. When all the side chains were replaced by hexyl chains, poly(4,8-dihexyl-2,6-bis-(3-hexylthiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene), as a P3HT analog, the performance was further improved to have a mobility up to 0.25 cm2V-1s-1. The last optimized two polymers could achieve their good performance in as-prepared films. The high-temperature annealing showed little effect on the performance, which was consistent with the fact that no melting point was found in their DSC profiles. |
