As an ending acceptor unit (A) within acceptor–donor–acceptor (A–D–A)-type small molecule acceptors (SMAs), monobrominated 1,1-dicyanomethylene-3-indanone (IC-Br) plays a critical role on developing high-performance SMAs and polymer acceptors from polymerizing SMAs. IC-Br is usually a mixture (IC-Br-m) consisting of positional isomeric IC-Br-γ and IC-Br-δ (bromine substituted on the γ and δ positions, respectively). The positional isomeric effect of these monobrominated ending groups has been witnessed to take an important role on regulating the photovoltaic performance. Fully investigating this isomeric effect of monobromination would be of great value for SMAs and even polymer acceptors. In this study, benefitting from the separation of IC-Br-γ and IC-Br-δ from IC-Br-m with high yields, bis(thieno[3,2-b]cyclopenta)benzo[1,2-b:4,5-b′]diselenophene (BDSeT) was chosen as the D unit and combined with IC-Br-γ, IC-Br-δ and IC-Br-m as A units, respectively. Three A–D–A type SMAs (BDSeTICBr-γ, BDSeTICBr-δ and BDSeTICBr-m) have thus been obtained. When blended with the representative donor polymer of PBDB-T-2Cl to construct bulk heterojunction (BHJ) polymer solar cells (PSCs), BDSeTICBr-γ, BDSeTICBr-δ and BDSeTICBr-m devices offered power conversion efficiencies (PCEs) of 9.42, 10.63, and 11.54% respectively. The result indicated the superior photovoltaic performance of the isomer mixture over the pure isomers, which was contrary to the reported ones that the pure isomers of SMAs used to give a better performance. The superior performance of the BDSeTICBr-m devices was mainly reflected in the improved carrier generation and transport as well as the carrier recombination suppression. In the three PBDB-T-2Cl:SMA BHJ films, a comparable intermixing phase and acceptor domain sizes were observed. Compared with BDSeTICBr-γ and BDSeTICBr-δ, BDSeTICBr-m showed a preferential face-on orientated packing with the closest π–π stacking in its BHJ film, probably accounting for its higher photovoltaic performance than those of the pure isomers. This study provides an alternative sight to develop efficient SMAs with suitably monobrominated IC ending groups for the strategy of polymerizing SMAs.