New donor–acceptor conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene for photovoltaic cells

Two new donor–acceptor (D–A) low band gap conjugated copolymers P1 and P2 containing different electron acceptor functional groups i.e. thienodithiadiazole (P1) and thienopyrazine (P2) derivatives were synthesized by Suzuki polycondensation reaction and characterized using 1H NMR and gel-permeation chromatography. The optical and electrochemical properties of these copolymers were investigated. Both copolymers have a good solubility, high thermal stability, broad absorption region from 350 to 950 nm, a relatively low HOMO energy in the range of −5.28 eV for P1 and −5.25 eV for P2 and a narrow optical band gap 1.32 and 1.46 eV, respectively. Photovoltaic devices based on a mixture of the P2: PC60BM (1:3 (w/w)) at AM1.5 illumination of 100 mW cm−2 show the over all power conversion efficiency (PCE) of 0.74% with a short-circuit current (Jsc) of 2.28 mA cm−2, open circuit voltage (Voc) of 0.72 V and fill factor (FF) of 0.45, whereas P1:PC60BM showed PCE of 0.43% with a Jsc of 1.54 mA cm−2, Voc of 0.70 V and FF of 0.40. These results suggest that the thienopyrazine functional groups are attractive electron-withdrawing building blocks in conjugated D–A copolymers and can be used as electron donors for bulk heterojunction solar cells along with fullerene derivatives as electron acceptors.