Synthesis and characterization of a low band gap quinoxaline based D–A copolymer and its application as a donor for bulk heterojunction polymer solar cells

A new alternating copolymer P comprising of benzo[1,2-b;4,5,b′]dithiophene (BDT) derivative and 4,9-bis-(5-bromothiophene-2-yl)-6,7-di-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DTQx) derivative electron, donating and electron withdrawing units, respectively, has been synthesized by Stille reaction. The copolymer was characterized by TGA, UV-visible absorption and cyclic voltammetry. The optical band gap of P was calculated from the onset wavelength of absorption to be about 1.38 eV. The copolymer P was used as electron donor along with PC60BM or PC70BM as electron acceptors for the fabrication of bulk heterojunction solar cells with configurations of ITO/PEDOT:PSS/P:PC60BM or PC70BM/Al. The power conversion efficiencies (PCE) of the copolymer solar cells blended with PC60BM and PC70BM as electron acceptors, spin cast from THF solvent, were 2.10% and 3.26%, respectively. The PCE device based on the P:PC70BM blend processed from DIO/THF was enhanced up to 4.47%. After optimizing the device parameters, such as blend ratio of P to PCBM and the choice of processing solvent, power conversion efficiency reaches as high as 5.12% for P:PC70BM blend, when processed from DIO/THF solvent, a blend ratio of 1 : 2 w/w and DMSO doped PEDOT:PSS buffer layer is used.