Side-chain engineering of wide-bandgap polymers based on benzo[1,2-b:4,5-b′]dithiophene and [2,2′-bithiophene]-4,4′-dicarboxylate for fullerene-free organic solar cells

Six wide-bandgap copolymers, i.e., PBDTEH-M, PBDTEH-E, PBDTEH-H, PBDTO-E, PBDTO-H and PBDT-OD, were synthesized and characterized. The copolymers have identical polymer backbones in which benzo[1,2-b:4,5-b ']dithiophene (BDT) and [2,2 '-bithiophene]-4,4 '-dicarboxylate (DCBT) units are alternatively linked, but carry different side chains. The length and bulkiness of the alkyl side chains on BDT-T or BDT and DCBT units have noticeable impacts on the photophysical properties, molecular packing structures, charge transport and photovoltaic properties of the polymers. All of these polymers possessed wide optical bandgaps in the range of 1.96-2.10 eV. Polymer solar cells (PSCs) with power conversion efficiencies (PCEs) from 4.03% to 8.21% were fabricated with these polymers as donors and ITIC-Th as an acceptor. The devices based on PBDTO-H with linear side chains on both BDT-T and DCBT units exhibited the best photovoltaic performance, ascribed to the high charge carrier mobility, well-ordered molecular packing and proper blend film morphology. The lowest PCE was obtained for PBDT-OD with branched long side chains, resulting from the relatively weak intermolecular pi-pi stacking in the film and the poor morphology with severe phase separation.