Effect of the Longer β‑Unsubstituted Oliogothiophene Unit (6T and 7T) on the Organic Thin-Film Transistor Performances of Diketopyrrolopyrrole-Oliogothiophene Copolymers

Two new diketopyrrolopyrrole-based π-conjugated copolymers (PDPP6T and PDPP7T) have been synthesized by Stille coupling polymerization of 3,6-bis(5′-bromo-[2,2′-bithiophen]-5-yl)-2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione with α,α′-bis(trimethylstannyl)-bithiophene and α,α′-bis(trimethylstannyl)-terthiophene, respectively. The impressive high mobility of 3.94 cm2 V–1 s–1 for the polymer with sextetthiophene (6T) and of 2.82 cm2 V–1s–1 for polymer with septetthiophene (7T) is acquired. It is found that the introduction of longer β-unsubstituted oliogothiophene unit in DPP-based copolymers has a great influence on the molecular weight and solubility of the DPP-oligothiophene copolymers that finally affects the organic thin-film transistor (OTFT) performances, indicating that a suitable number of thiophene group in β-unsubstituted oligothiophene exists for such a kind of copolymer to exhibit the best OTFT performances. This work also reveals the significance in the design of D–A copolymers for OTFTs through regulating the balance between π–π stacking of intermolecular chains and molecular weight as well as solubility of the rigid main chain.