High Uniformity and High Thermal Stability of Solution-Processed Polycrystalline Thin Films by Utilizing Highly Ordered Smectic Liquid Crystals

Abstract

We have investigated the fabrication of polycrystalline thin films of a liquid crystalline benzothienobenzothiophene derivative, i.e., 2-octylthienyl-benzothienobenzothiophene having a highly ordered smectic liquid crystal phase (SmE), the thermal stability of the films, and the performance of their field effect transistors (FETs). The polycrystalline thin films spin-coated at a temperature of SmE phase exhibited uniform textures and were molecularly flat. The FETs after solvent vapor anneal showed the high FET mobility of 1.1 cm2 V-1 s-1, while as-fabricated FETs did the low FET mobility of ca. 10-3 cm2 V-1 s-1. In addition, the thermal stability of the films improved up to 180 °C without melting. After thermal stress at 150 °C for 5 min, the FETs exhibited low mobility as in the as-fabricated FETs but they recovered to the high FET mobility of 1.0 cm2 V-1 s-1 after additional anneals. We compared these results with those of dialkyl benzothienobenzothiophene derivatives and discussed the role of highly ordered liquid crystal phase.