Efficient Charge Transport Driven by Strong Intermolecular Interactions in Cyclopentadithiophene-Based Donor-Acceptor Type Conjugated Copolymers

Understanding charge transport in pi-conjugated polymers is critical for the design of high-mobility polymers. However, charge transport in donor-acceptor (D-A) type copolymers and the roles of D and A units are still unclear. Herein, the charge transport properties of three cyclopentadithiophene (CDT)-based D-A type copolymers with CDT as a common donor and benzothiadiazole, fluoro-2,1,3-benzothiadiazole, or pyridyl-2,1,3-thiadiazole (PTz) as an acceptor are investigated. The best charge transport is found for CDT-PTz showing 0.79 +/- 0.01 cm(2) V-1 s(-1) hole mobility in field-effect transistors, with the lowest energetic disorder by temperature-dependent mobility measurements. Using various structural probes and density functional theory simulations, it is found that CDT-PTz has the most rigid and planar backbone structure and the strongest intramolecular dipole moment thanks to the noncovalent N-S interactions and the strong electron-withdrawing PTz acceptor unit. These structural properties lead to strong intermolecular interactions in CDT-PTz, reducing its pi-pi stacking distance, producing strong aggregation even in hot dilute solutions. The enhanced mobility in CDT-PTz is attributed to this high quality pi-pi stacking, driven by increased backbone rigidity and strong molecular dipoles. This work suggests that rigidifying the copolymer backbone with enhanced push-pull strength of the D-A units is a key requirement for high-mobility D-A copolymers.