Side chains and backbone structures influence on 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based low-bandgap conjugated copolymers for organic photovoltaics

Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density (J _sc) and PCE value due to its better intermolecular stacking.