Small Molecules with Asymmetric 4-Alkyl-8-alkoxybenzo[1,2-b:4,5-b′]dithiophene as the Central Unit for High-Performance Solar Cells with High Fill Factors

A small molecule named DRBDTCO based on benzo[1,2-b:4,5-b′]dithiophene (BDT) with an asymmetric side chain and its dimer, dDRBDTCO, with the octamethylene connector were designed and synthesized as donor materials for solution-processed bulk heterojunction solar cells. The optimized power conversion efficiency of a DRBDTCO-based device was 8.18% under AM 1.5 G irradiation (100 mW cm–2), which is higher than that of its dDRBDTCO-based device. Both molecule-based devices exhibited high fill factors of >73%, which are attributed to their optimized morphologies with a feature size of 15–20 nm, highly efficient charge collection, less bimolecular recombination, and well-balanced charge transport properties. The results demonstrate that the asymmetric BDT unit could be a promising building block for donor materials with high efficiencies and high fill factors.