Solution-processable field-effect transistor using a fluorene- and selenophene-based copolymer as an active layer

We have synthesized a new p-type polymer, poly(9,9′-n- dioctylfluorene-alt-biselenophene) (F8Se2), via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weight (M_n) of F8Se2 was found to be 72 600. F8Se2 dissolves in common organic solvents such as chloroform and chlorobenzene. The PL emission peak of a film of F8Se2 is clearly red-shifted with respect to that of its sulfur analogue, poly(9,9′-n- dioctylfluorene-alt-bithiophene) (F8T2), due to the electron-donating properties of selenium and the strong interactions between the biselenophene moieties in neighboring copolymer chains. We confirmed that F8Se2 is a thermotropic liquid crystalline polymer with an aligned structure by carrying out DSC, PLM, and XRD measurements. The introduction of the selenophene moiety into the liquid-crystalline polymer system results in better field-effect transistor (FET) performance than that of F8T2. A solution-processed F8Se2 FET device with a bottom contact geometry was found to exhibit a hole mobility of 0.012 cm ²/(V s) and a low threshold voltage of -4 V, which is the one of the highest solution-processable FET performances.