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 (-C[triple bond]C-) vs. olefinic (-C=C-) spacers in thiophene-phenylene derivatives on electronic structure, physical properties, and device efficiencies. Substituting olefinic for acetylenic pi-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 degrees 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.