Oligofluorene-thiophene derivatives as high-performance semiconductors for organic thin film transistors

A series of alkyl chain end-capped oligofluorene-thiophenes have been prepared with high yields using Suzuki or Stille coupling reactions. The electronic and optical properties of the thin films deposited at different substrate temperatures have been investigated. Morphological studies using transmission electron microscopy (TEM) revealed well-interconnected microcrystalline domains in these thin films. X-ray diffraction measurements of the vacuum-evaporated films showed enhanced crystalline order with increasing substrate deposition temperature. Thermal analysis as well as electrochemical measurements of the materials indicated that the new oligomers have high thermal and oxidative stability. Highly ordered polycrystalline vacuum-evaporated films with charge carrier mobility as high as 0.12 cm(2) V-1 s(-1) have been achieved with 5,5'-bis(7-hexyl-9H-fluoren-2-yl)-2,2'-bithiophene (DHFTTF). Thin film field-effect transistor (TFT) devices made from these materials showed remarkable stability even after LTV (366 nm) irradiation for more than 48 h in air. The semiconductors exhibit high on/off ratios (up to 105) and no significant decrease in mobility and on/off ratio over several months in air with exposure to ambient light. Finally, bright emission colors from greenish yellow to orange-red were observed in this new series of oligomer solid films excited with LTV light (366 nm). In addition, a comparative study of the newly synthesized oligomers with alpha,alpha'-dihexylsexithiophene (DH6T), one of the most widely investigated oligothiophenes, is presented. The current approach to the molecular design can be applied toward the rational design of new TFT materials.