Fluorene-based conjugated copolymers containing hexyl-thiophene derivatives for organic thin film transistors

Two fluorene-based conjugated copolymers containing hexyl-thiophene derivatives, PF-1T and PF-4T, were synthesized via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weights (M_n) of PF-1T and PF-4T were found to be 19,100 and 13,200, respectively. These polymers were soluble in common organic solvents such as chloroform, chlorobenzene, toluene, etc. The UV-vis absorption maximum peaks of PF-1T and PF-4T in the film state were found to be 410 nm and 431 nm, respectively. Electrochemical characterization revealed that these polymers have low highest occupied molecular orbital (HOMO) levels, indicating good resistance against oxidative doping. Thin film transistor devices were fabricated using the top contact geometry. PF-1T showed much better thin-film transistor performance than PF-4T. A thin film of PF-1T gave a saturation mobility of 0.001-0.003 cm ² V^-1 s^-1, an on/off ratio of 1.0 x 10 ⁵, and a small threshold voltage of -8.3 V. To support TFT performance, we carried out DSC, AFM, and XRD measurements.