Synthesis and characterization of highly soluble phenanthro[1,10,9,8-c,d,e,f,g]carbazole-based copolymer: Effects of thermal treatment on crystalline order and charge carrier mobility

In this study, a novel donor-acceptor type semiconducting polymer was designed and synthesized based on phenanthro[1,10,9,8-c,d,e,f,g]carbazole backbone. The resulting polymer, poly[ 4-(5-(6-(2-decyltetradecyl)-6H-phenanthro[1,10,9,8-c,d,e,f,g]carbazol-4-yl)thiophen-2-yl)-5,6-bis(octyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PPTBTT), exhibited high solubility in various solvents which enables facile fabrication of semiconducting devices such as thin-film transistors, and also opens up possibilities for large scale solution processing. Absorption spectra of PPTBTT showed large bathochromic shift when the polymer was prepared as a thin-film, which can be explained by the high solubility and molecular packing of the synthesized polymer. X-ray diffraction measurements showed that the crystalline structure of PPTBTT polymer film is largely influenced by the thermal treatment temperature. Together with the atomic force microscopy data, it was found that the optimal thermal treatment temperature of PPTBTT film is 170 °C. The charge transfer characteristics were confirmed by fabricating PPTBTT thin-film transistor. In accordance with the polycrystalline structure, PPTBTT film transistor showed the highest mobility of 0.0092 cm2 V−1 s−1 and on/off ratio of 104 when the device was annealed at 170 °C.