Synthesis of fluorene based two acceptor random copolymers for organic solar cell applications

A series of fluorene based random copolymers incorporating two acceptor units – 4,7-dithienyl-2,1,3-benzothiadiazole and N-heterocyclic moieties (2,1,3-benzothiadiazole, [2,3-b]-phenanthrenequinoxaline, [2,3-b]-acenaphthenequinoxaline) have been synthesized by Suzuki polycondensation method. The synthesized polymer films exhibit strong absorption in the range from 300 nm to 700 nm with good solubility and thermal stability. The optical band gap of the synthesized copolymers are in the range of 1.86–1.90 eV. The HOMO energy levels of the polymers calculated from cyclic voltammetry measurements have been found to be in the range from −5.45 to −5.70 eV. Using these new electron donor copolymers, solar cells were fabricated and highest efficiency was obtained with PFQx(phen)DTBT as an electron donor and PC71BM as an electron acceptor. In fact with PFQx(phen)DTBT:PC71BM in the ratio of 1:4, a power conversion efficiency (PCE) ∼0.47% has been achieved with open-circuit voltage (Voc) 0.59 V, short circuit current density (Jsc) ∼2.49 mA/cm2 and fill factor (FF) ∼ 32% under the illumination of AM1.5G, 100 mW cm−2.