Synthesis of diketopyrrolopyrrole based conjugated polymers containing thieno[3,2-b]thiophene flanking groups for high performance thin film transistors

New donor–acceptor (D–A) conjugate polymers based on 3,6-bis(2-bromothieno-[3,2-b]thiophen-5-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]-pyrrole-1,4-(2H,5H)-dione (Br-TTDPP) combined with three different electron-donating monomers bithiophene (BT), di(2-thienyl)ethene (TVT), and di(selenophene-2-yl)ethene (SVS) were synthesized. The optical band gap, electrochemical properties and organic field-effect transistor (OFET) device performance of the TTDPP-based polymers were systematically investigated. X-ray diffraction (XRD) was used to evaluate the molecular packing of the TTDPP-based polymers. Among three polymers, TTDPP-SVS showed the smallest d-spacing and the largest distance of 0.372 for π–π stacking. In particular, an OTFT having a TTDPP-SVS active layer showed the highest hole mobility of 0.196 cm² V⁻¹ s⁻¹, the lowest threshold voltage down to −3.6 V, and the highest I_on/I_off ratio of 6.5 × 10⁴. This indicates that π-extended SVS significantly improves charge transport properties.