High-performance polymer field-effect transistors fabricated with low-bandgap DPP-based semiconducting materials

A series of new pi-conjugated D-A copolymers PDMOTT-n (n = 118, 122, 320, and 420) containing a diketopyrrolopyrrole unit and a 3,6-dimethoxythieno[3,2-b]thiophene moiety was designed and synthesized, and their field-effect properties were characterized. The polymers PDMOTT-n have a low bandgap of 1.27 eV and exhibit a broad absorption. Solution-processed field-effect transistors based on these polymers were fabricated with a bottom-gate/bottom-contact configuration and demonstrated typical p-type charge transporting properties. Of these, PDMOTT-420 with the longest alkyl side chains and having the longest distance between the branching point of the alkyl side chain and pi-conjugated backbone of the polymer, exhibited the best carrier transporting performance with a hole mobility of 2.01 cm(2) V-1 s(-1) and with an on/off current ratio of 10(4)-10(5). The characterization results of grazing incidence X-ray diffraction and tapping-mode atomic force microscopy showed that the thin film of the polymer PDMOTT-420 forms larger grains and more useful nanofibrillar intercalated structures and owns shorter pi-pi stacking distance and better crystallization than those of another three polymers. These results could help us to better understand the structure-property relationship of polymer semiconductors and to design novel pi-conjugated polymers for high-performance field-effect transistors.