Relationship between the liquid crystallinity and field-effect-transistor behavior of fluorene-thiophene-based conjugated copolymers

A series of fluorene-thiophene-based semiconducting materials, poly(9,9′-dioctylfluorene-αα′-bisthieno[3,2-b]thiophene) (F8TT2), poly(9,9′-di(3,6-dioxaheptyl) fluorene-alt-thieno[3,2-b] thiophene) (BDOHF8TT), poly(9,9′-di(3,6-dioxaheptyl)fluorene-alt- bithiophene) (BDOHF8T2), and poly(9,9′-dioctylfluorene-co-bithiophene-co- [4-(2-ethylhexyloxyl)phenyl]diphenylamine) (F8T2TPA), was synthesized through a palladium-catalyzed Suzuki coupling reaction. F8TT2, BDOHF8TT, BDOHF8T2, and F8T2TPA films exhibited photoluminescence maxima at 523, 550, 522, and 559 nm, respectively. Solution-processed field-effect transistors (FETs) fabricated with all the copolymers except F8T2TPA showed p-type organic FET characteristics. Studies of the differential scanning calorimetry scans and FETs of the polymers revealed that more crystalline polymers gave better FET device performance. The greater planarity and rigidity of thieno[3,2-b]thiophene in comparison with bithiophene resulted in higher crystallinity of the polymer backbone, which led to improved FET performance. On the other hand, the random incorporation of the triphenylamine moiety into F8T2TPA caused the polymer chains to lose crystallinity, resulting in an absence of FET characteristics. With this study, we could assess the liquid-crystallinity dependence of the field-effect carrier mobility on organic FETs based on liquid-crystalline copolymers.