Improved thermal stability in organic FET fabricated with a soluble BTBT derivative

We have synthesized a new benzothienobenzothiophene (BTBT) derivative, 2-octylthienylbenzothienobenzothiophene having a highly ordered liquid crystal phase of smectic E (SmE) in the temperature range next to its crystal phase, fabricated its polycrystalline thin films by spin-coating technique, and applied them to field effect transistors (FETs). Thanks to the liquid crystal phase as a precursor of polycrystalline thin films, the resulting films were molecularly flat and uniform. The thermal stability of the thin films was very much improved compared with dialkyl BTBT derivatives because of the solid-like nature of the SmE phase up to 180 °C. The FETs fabricated with films after solvent vapor annealing with toluene showed high FET mobility of 1.4 ± 0.3 cm2/V s. As expected, the FETs were not destroyed by thermal load up to 150 °C. The FET mobility was recovered to 0.54 cm2/V s by the solvent vapor annealing, although the FET mobility was decreased by the thermal load. Highly ordered liquid crystalline phases provide us with easy fabrication of quality polycrystalline thin films and high thermal stability of the polycrystalline thin films.