Benzo[1,2-b:4,5-b′] difuran and furan substituted diketopyrrolopyrrole alternating copolymer for organic photovoltaics with high fill factor

In comparison to the conjugated polymers synthesized from thiophene or thiophene derivatives, furan and its derivatives are promising alternative building units due to their desirable properties such as smaller heteroatom size, more electronegative heteroatom, and larger dipole moment. Considering the advantages of furan units, conjugated polymers synthesized from furan and its derivatives show a higher degree of conjugation with reduced twisting between adjacent units, smaller pi-stacking distance, and improved solubility. To date, despite research on polymers constructed from furan derivatives gaining attention, conjugated polymers made up of only furan or its derivatives are still rare. Herein, we report a new conjugated polymer, poly(4,8-bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5-b']difuran-alt-2,5-didodecyl-3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), P(BDF-FDPP), for organic solar cells. The smaller oxygen atom in furan of P(BDF-FDPP) results in a planar conjugated backbone with negligible torsion (dihedral angle < 0.1 degrees) determined by density functional theory. P(BDF-FDPP) exhibits broad absorption up to 940 nm with HOMO and LUMO located at -5.19 eV and -3.63 eV, respectively. Power conversion efficiency (PCE) of 5.55% with a high fill factor (FF) of 0.73 was obtained for the devices fabricated using DPE as an additive. The substantial changes in photovoltaic performance of the device fabricated with or without additives was further investigated with grazing incident wide-angle X-ray scattering, and transmission electron microscopy experiments. Preferential face-on orientation of P(BDF-FDPP) and sophisticated interpenetrated network for P(BDF-FDPP)/PC71BM blend films enabled relatively good PCEs and high FF in solar cell devices.