Synthesis and transistor properties of asymmetric oligothiophenes: Relationship between molecular structure and device performance

A series of three thiophene-naphthalene-based asymmetric oligomers-5-decyl-2, 2′:5′,2″:5″,2′″- quaterthio-phene (DtT), 5-decyl-5″-(naphthalen-2-yl)-2, 2′:5′,2″-terthiophene (D3TN), and 5-(4-decylphenyl)-5′- (naphthalen-2-yl)-2, 2′-bithiophene (DP2TN)-was synthesized by Suzuki cross-coupling reactions. The long alkyl side chains improved both the solubility of the oligomers in solvents and their tendency to self-assemble. UV/Vis absorption measurements suggested that DtT, D3TN, and DP2TN form H-type aggregates with a face-to-face packing structure. In addition, the three oligomers were found to adopt vertically aligned crystalline structures in films deposited on substrates, as revealed by grazing-incidence wide-angle X-ray scattering. These oligomers were used as the active layers of p-type organic field-effect transistors, and the resulting devices showed field-effect mobilities of 3.3×10^-3 cm²V^-1s ^-1 for DtT, 1.6 × 10^-2cm²V ^-1s^-1 for D3TN, and 3.7 × 10^-2cm ²V^-1s^-1 for DP2TN. The differences in transistor performances were attributed to the degree of π overlap and the morphological differences determined by the molecular structures.