Acetylenic spacers in phenylene end-substituted oligothiophene core for highly air-stable organic field-effect transistors

Abstract

Two thiophene–phenylene semiconductors, bis(2-phenylethynyl) end-substituted oligothiophenes (diPhAc-nTs, n = 2, 3), were synthesized and studied with respect to their optical, electrochemical, structural and electrical properties. The optical and electrochemical properties of the oligomers in solution were investigated by UV-vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. High vacuum evaporated thin films were investigated by optical absorption, X-ray diffraction and AFM, and implemented as p-type semiconducting layers into organic thin-film transistors (OTFTs). A comparative study in solution and in the solid state with distyryl-oligothiophenes (DSnTs, n = 2, 3) reveals the great influence of acetylenic (–CC–) vs. olefinic (–CC–) spacers in thiophene–phenylene derivatives on electronic structure, physical properties, and device efficiencies. Substituting olefinic for acetylenic π-spacers in terthiophene-based conjugated semiconductors leads to one of incontrovertible attributes of OTFTs for low cost applications, a high mobility at low substrate temperature (T_sub) i.e. typically 25 °C. Fine-tuning in the HOMO/LUMO levels by reducing the HOMO level introduces increased air-oxidation strength of thin films where OTFTs provide exactly the same hole mobility value after 100 days in air. All the results suggested that introduction of carbon–carbon triple bonds provided an efficient route to highly air-stable organic thin film transistors.