Doping effect of solution-processed thin-film transistors based on polyfluorene

We report the fabrication of solution-processable organic thin-film transistors (OTFTs) based on poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) doped with an electron-acceptor, 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F₄TCNQ). The 8% doping of the F8T2 film was found to result in an increased hole mobility, up to 0.01 cm² V ^-1 s^-1, and in an improved on/off ratio, as well as in a low off-current, of the order of 10^-11 A. The doped F8T2 device was also found to exhibit a better threshold voltage and reduced hysteresis behavior. These improvements in performance are attributed to the formation of the F8T2-F₄TCNQ complex, which results in better hole injection and improved device stability.