Importance of solubilizing group and backbone planarity in low band gap polymers for high performance ambipolar field-effect transistors

We investigated the performance of ambipolar field-effect transistors based on a series of alternating low band gap polymers of oligothiophene and diketopyrrolopyrrole (DPP). The polymers contain oligothiophene units of terthiophene [T3] and thiophenethienothiophene-thiophene [T2TT] and DPP units carrying branched alkyl chains of 2-hexyldecyl [HD] or 2-octyldodecyl [OD]. The structural variation allows us to do a systematic study on the relationship between the interchain stacking/ordering of semiconducting polymers and their resulting device performance. On the basis of synchrotron X-ray diffraction and atomic force microscopy measurements on polymer films, we found that longer branched alkyl side chains, i.e., OD, and longer and more planar oligothiophene, i.e., T2TT, generate the more crystalline structures. Upon thermal annealing, the crystallinity of the polymers was largely improved, and polymers containing a longer branched alkyl chain responded faster because longer alkyl chains have larger cohesive forces than shorter chains. For all the polymers, excellent ambipolar behavior was observed with a maximum hole and electron mobility of 2.2 and 0.2 cm² V^-1 ss, respectively.