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 H-1 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 (J(sc)) of 2.28 mA cm(-2), open circuit voltage (V-oc) of 0.72 V and fill factor (FF) of 0.45, whereas P1:PC60BM showed PCE of 0.43% with a J(sc) of 1.54 mA cm(-2), V-oc, 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. (C) 2013 Elsevier B.V. All rights reserved.