We report herein three (Z)-1,2-bis(4-(tert-butyl)phenyl)ethane based alternating copolymers synthesized with thiophene (PBPT), thieno[3,2-b]thiophene (PBPTT) and dithieno[3,2-b:2′,3′-d]thiophene (PBPDT) moieties. Their optical characteristics, morphology, orientation, and application in flexible organic field effect transistor (OFET) devices were studied in this research. Copolymer PBPTT shows interesting properties, such as an almost planar molecular structure with extended π-conjugation, an ordered edge-on orientation in a fiber-like film, and the thieno[3,2-b]thiophene moiety located in the middle of the polymer backbone for a symmetric and strong π-stacking effect. Moreover, PBPTT was successfully used to fabricate ambient stable and high performance flexible OFET devices, with a charge carrier mobility of up to 0.27 cm² V⁻¹ s⁻¹ and an on/off ratio of 10⁴. The study reveals the profound influence of the fused thiophene and symmetric molecular structure on the copolymer characteristics and OFET device performance, and offers us a novel strategy to design semiconductor polymers for OFET applications.