Effect of alkyl substituents on the electrical properties of thieno[2,3-b] thiophene derivatives in solution-processed organic thin-film transistor applications

Three solution-processable small-molecule thieno[2,3-b]thiophene (2,3-bTT) derivatives-2,5-bis(5-(2-ethylhexyl)thiophen-2-yl)thieno[2,3-b]thiophene (compound 1), 2,5-bis(5-octylthiophen-2-yl)thieno[2,3-b]thiophene (compound 2), and 3,4-dimethyl-2,5-bis(5-octylthiophen-2-yl)thieno[2,3-b]thiophene (compound 3)-were synthesized with a focus on the effect of alkyl chain structure (linear vs. branched) and backbone modifications on the electrical properties of the 2,3-bTT derivatives. Their thermal and optical properties were examined for potential application in the channel layer of organic thin-film transistors (OTFTs). Atomic force microscopy (AFM) and X-ray diffraction (XRD) were employed to investigate the surface characteristics and microstructural features of each compound. Thin films based on the 2,3-bTT compounds were incorporated as a channel layer of top-contact/bottom-gate OTFTs. The compounds exhibited p-channel characteristics, with compound 2, featuring linear alkyl chains and an unmodified 2,3-bTT backbone, resulting in minimal steric hindrance and enhanced molecular packing efficiency, demonstrated the highest electrical performance, exhibiting a hole mobility of up to 0.03 cm2/V & sdot;s and a current on/off ratio exceeding 106.