Mesogens Mediated Self-Assembly in Applications of Bulk Heterojunction Solar Cells Based on a Conjugated Polymer with Narrow Band Gap

The ability to control the molecular organization of electronically active liquid-crystalline polymer provides opportunities to develop easy-to-process yet highly ordered supramolecular systems in the field of photovoltaics. Here, we report a novel donor acceptor type liquid-crystalline copolymer, poly{9,9-bis[6-(4'-cyanobiphenyloxy) hexyl]fluorene-alt-5,5,-(4',7'-di-2-thienyl-2',1,3'-benzothiadiazole)}, PF-cbpDTBT, which contains both electron-donating fluorene and electron-accepting benzothiadiazole units. Incorporating the electron withdrawing cyanobiphenyl units not only narrowed the band gap of the copolymer but also help main chain form spontaneously through self-organization. The films with structural anisotropy can endow the PFcbpDTBT with special features, including absorption band red-shift; fluorescence enhancement; lower lying LUMO level, and crystallinity improvement. When blended with PCBM, the PFcbpDTBT enables the acceptors to adopt the preferential well-oriented arrangement in both surface and inner of the bulk. Among all the thermal treatments, the mesophase annealing achieves the most obvious effect. From the device annealed at 200 degrees C, the internal quantum efficiency remains or exceeds 20% throughout the 400-650 nm spectrum and the power conversion efficiency values reaches 1.10% without extensive optimization.