Novel wide-bandgap copolymer bearing alkylthio-thiophene-substituted benzodithiophene and methyl thiophene-3-carboxylate for highly stable fullerene-free simple polymer solar cells

We successfully synthesized a wide-bandgap p-type copolymer, 3MT-Th-S, bearing alkylthio-substituted benzodithiophene (BDT) as a donor and methyl-3-thiophenecarboxylate (3MT) as a weak electron acceptor. The electron affinity of the sulfur atom in the 2-ethylhexylthio side chains tethered to the thiophene units induces a low-lying highest occupied molecular orbital (HOMO) energy level in the corresponding polymer. 3MT-Th-S displays a strong absorption band from 400 to 600 nm, resulting in a relatively wide optical bandgap. Thin films of 3MT-Th-S exhibit a prominent face-on orientation in the out-of-plane profile, which might be a favorable orientation for moving charge carriers in the vertical direction in polymer solar cells (PSCs). Non-fullerene PSCs based on 3MT-Th-S as the donor and an n-type small molecule (ITIC, ITIC-Me, or IMIDT) were fabricated, and their performance was investigated in detail. Among the PSC devices bearing three different acceptors, the 3MT-Th-S:ITIC-based PSC exhibits the highest power conversion efficiency of 7.86% with a high short-circuit current density of 16.11 mA cm⁻² and an open circuit voltage of 0.96 V. The high PCE values of PSC using the as-cast blend films of 3MT-Th-S and ITIC as an active layer without solvent additive are attributed to the low-lying HOMO level of the donor polymer, the complementary absorption behavior of the blend film, the face-on arrangement of polymer chains on the substrate, and the well-formed morphology of the binary blend film. These results clearly demonstrate that 3MT-Th-S is a promising wide-bandgap p-type polymer for high-performance fullerene-free simple PSCs.