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 pi-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 alpha,alpha'-bis(trimethylstannyl)-bithiophene and alpha,alpha'-bis(trimethylstannyl)-terthiophene, respectively. The impressive high mobility of 3.94 cm(2) V-1 s(-1) for the polymer with sextetthiophene (6T) and of 2.82 cm(2)V(-1)s(-1) for polymer with septetthiophene (7T) is acquired. It is found that the introduction of longer beta-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 beta-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 pi-pi stacking of intermolecular chains and molecular weight as well as solubility of the rigid main chain.