Anisotropic field-effect hole mobility of liquid crystalline conjugated polymer layers formed on photoaligned polyimide films

We have fabricated organic field-effect transistors (OFETs) with a highly oriented active layer of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2), which was formed with the help of a photoaligned polyimide film. Photoalignment is an attractive technique for integrating OFETs with aligned active layers on the same substrate, because of its potential capability of two-dimensional alignment patterning. The F8T2 layer formed on the photoaligned polyimide film showed an absorption dichroic ratio greater than 15 after annealing at 285 °C. Top-gate/bottom-contact-type OFETs with a parylene gate insulating layer exhibited an enhanced hole mobility (0.016 cm2 V−1 s−1) along the alignment direction of the F8T2 backbone structure and a suppressed one (0.002 cm2 V−1 s−1) along the perpendicular direction. This result shows that the photoaligned polyimide film is an attractive alignment layer for fabricating and integrating OFETs with aligned active layers. Current-direction-dependent bias stress effect was observed for those OFETs; when the current flowed parallel to the alignment direction of the F8T2 backbone structures, a larger negative threshold voltage shift was observed. This anisotropic bias stress effect was discussed on the basis of a microstructure model of aligned F8T2 films.