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)}, PFcbpDTBT, 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 °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.