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 (similar to-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 pi-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.