Molecular Ordering of Dithieno[2,3-d;2',3'-d]benzo[2,1-b:3,4-b']dithiophenes for Field-Effect Transistors

Four derivatives of dithieno[2,3-d;2',3'-d']benzo[1,2-b;3,4-b']dithiophene (DT) have been synthesized to investigate the correlation between molecular structure, thin-film organization, and charge-carrier transport. Phenyl or thiophene end-capped derivatives at alpha positions of the outer thiophenes of DT present vastly different optoelectronic properties in comparison with bay-position alkyl-chain-substituted DT, which was additionally confirmed by density functional theory simulations. The film morphology of the derivatives strongly depends on alkyl substituents, aromatic end-caps, and substrate temperature. Field-effect transistors based on DT derivatives with bay-substituted alkyl chains show the best performance within this studied series with a hole mobility up to 0.75 cm²/V s. Attachment of aromatic end-caps disturbs the ordering, limiting the charge-carrier transport. Higher substrate temperature during deposition of the DT derivatives with aromatic end-caps results in larger domains and improved the transistor mobilities but not beyond the alkylated DT.