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′′′‐quaterthiophene (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 cm2 V−1 s−1 for DtT, 1.6×10−2 cm2 V−1 s−1 for D3TN, and 3.7×10−2 cm2 V−1 s−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. Asymmetric organic semiconductors based on thiophene–naphthalene oligomers were prepared: 5‐decyl‐2,2′:5′,2′′:5′′,2′′′‐quaterthiophene (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). UV/Vis absorption measurements suggested that DtT, D3TN, and DP2TN form H‐type aggregates. These oligomers were used as the active layers of organic field‐effect transistors (see figure), and the resulting devices showed field‐effect mobilities of 3.3×10−3, 1.6×10, and 3.7×10−2 cm2 V−1 s−1 for DtT, D3TN, and DP2TN, respectively.