The influence of electron-deficient comonomer on chain alignment and OTFT characteristics of polythiophenes

The structure–property relations of polythiophene derivatives have been the subject of research for both materials and electronic device applications. We have designed and prepared two kinds of polythiophene derivatives with similar chemical structures but different electronic properties by Stille polymerization. One is composed of only electron-rich thiophenes (PQT2T), while the other is composed of both electron-rich thiophenes and electron-deficient thiazoles (PTZT2T). The number-average molecular weights of PQT2T and PTZT2T were 23,000 (PDI = 1.5) and 13,000 (PDI = 1.7), respectively. PTZT2T showed a large bathochromic shift of absorption maximum from solution to film by 57 nm. Further investigation by differential scanning calorimetry and X-ray diffraction revealed the formation of well-ordered interdigitated structure of PTZT2T in the solid state (d1 = 20.7 Å). From absorption onset, optical band-gap of PQT2T and PTZT2T was calculated to be 2.22 eV and 1.92 eV, respectively. A mobility of 1.1 × 10−3 cm2/Vs, an on/off ratio of 4.7 × 104 have been obtained from organic thin-film transistors (OTFTs) using PTZT2T as a channel. By comparison with PQT2T, the mobility was 18 times and the on/off ratio was 52 times higher for PTZT2T, which shows the usefulness of electron-deficient thiazoles as comonomer units of polythiophene derivatives for OTFT applications.