A new highly conjugated crossed benzodithiophene and its donor–acceptor copolymers for high open circuit voltages polymer solar cells

A novel benzodithiophene-substituted benzodithiophene (crossed-BDT) with an enlarged conjugated skeleton was designed and synthesized as an electron-rich unit for the construction of donor–acceptor copolymers. Different acceptors, including 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) and 4,7-di(thiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) were used as electron-deficient units for the target copolymers to investigate the effect of the inclusion of two fluorine atoms on the acceptor unit of the polymer. The crossed-BDT and DTBT based polymers show high open circuit voltages (Voc) of over 0.9 V with the decent power conversion efficiency (PCE) of 3.74%. The crossed-BDT and DTffBT based polymers show nearly 1 V (0.95 V) Voc due to the fluorine induced low-lying HOMO energy level. The 0.9–0.95 V is an impressive Voc result for a 1.7 eV band gap (Eg) polymer. To the best of our knowledge, this is the first report about a crossed conjugated benzodithiophene building block design, and this building block is effective in improving the Voc of low band gap DBT polymers.