as switching devices for innovative electronic applications such as flexible and stretchable electronics. [1][2][3][4] Mobilities in the early stages of OFET development were poor, with polyacetylene having a fieldeffect mobility of only ≈10 −5 cm 2 V −1 s −1 . However, since the development of thiophene-based semicrystalline polymer semiconductors (e.g., regioregular poly(3hexylthiophene-2,5-diyl) (rr-P3HT), poly(3,3'-dialkyl-quaterthiophene) (PQT), and poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT)) and then donor-acceptor (D-A) type conjugated copolymers with alternating electron-donating and electron-accepting moieties along the polymer backbone, several research groups have reported high-performance semiconducting polymers with field-effect mobilities in excess of 10 cm 2 V −1 s −1 which is comparable to that of their soluble oxide-based inorganic semiconductors counterparts. [5,6] Indeed, the D-A type conjugated copolymers are intriguing materials because they possess superior mobility values to the thiophene-based semi-crystalline homopolymers, despite exhibiting somewhat amorphous-like structural characteristics. This means that their structure-property relationships may not follow the trend of higher mobilities requiring higher degrees of semicrystalline molecular order Understanding charge transport in π-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 π-π stacking distance, producing strong aggregation even in hot dilute solutions. The enhanced mobility in CDT-PTz is attributed to this high quality π-π 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.