Synthesis and thin-film transistor performance of fluorene-based copolymers containing thiophene moieties having different position of alkyl side chain

Abstract

We have synthesized fluorene-based copolymers (PF-TH-TT-HT and PF-HT-TT-TH) containing thiophene moieties having different position of alkyl side chain via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weight $(M_n)s$ of PF-TH-TT-HT and PF-HT-TT-TH were respectively found to be 22,300 and 11,800. These polymers dissolve in common organic solvents such as chloroform, chlorobenzene and toluene, etc. The PL emission peak of a film of PF-TH-TT-HT is clearly red-shifted with respect to PF-HT-TT-TH, due to the less repulsion of alkyl side chains in the between fluorene and thiophene moieties. Through studies of X-ray diffraction (XRD), we could access molecular alignment. The thermal properties of these polymers were measured by TGA and DSC, and electrical properties were studied by cyclic voltammetry (CV). They exhibited different field-effect transistor (FET) properties depending on their positions of alkyl side chains. The PF-TH-TT-HT having little repulsion showed better field-effect transistor performance than PF-HT-TT-TH. A solution-processed PF-TH-TT-HT FET device with a top contact geometry was found to exhibit a hole mobility of $1.05 × 10^{-4} cm^2/Vs$ and a low threshold voltage of -4 V.