Benzobisthiazole-Based Donor–Acceptor Copolymer Semiconductors for Photovoltaic Cells and Highly Stable Field-Effect Transistors

Six new donor–acceptor copolymers based on benzobisthiazole and various donor moieties (dithienosilole, dithienopyrrole, cyclopentadithiophene, carbazole, benzodithiophene, and bithiophene) were synthesized, characterized, and used in field-effect transistors and solar cells. The series of polybenzobisthiazoles with donor–acceptor architecture have optical band gaps of 1.83–2.18 eV, have identical LUMO energy levels (∼−3.3 eV), and have a HOMO energy level that varied from −4.79 eV in PBTDTP to −5.71 eV in PBTHDDT. X-ray diffraction of the polybenzobisthiazole films showed a lamellar crystalline structure with an interlayer d -spacing of 1.56 nm in PBTOT to 1.83 nm in PBTDTP and 2.12 nm in PBTHDDT and a short π-stacking distance (0.353–0.378 nm). The highly crystalline nature of the polybenzobisthiazoles facilitated high field-effect carrier mobility (up to 0.011 cm 2 /(V s)), which remained very stable under ambient conditions for 2 years. Bulk heterojunction solar cells made from one of the benzobisthiazole-based copolymers gave a power conversion efficiency of up to 3.0% under 100 mW/cm 2 AM1.5 sunlight illumination in air.