Two -conjugated acceptor-donor-acceptor-donor-acceptor-type (A-D-A-D-A) oligothiophenes, and were designed and synthesized with thienothiadiazole (TT) or benzothiadiazole (BT) as the core and dicyanovinyl (DCV) as the terminal acceptor groups for comprehensively investigating and understanding structure–property relationships. The resulting oligomers were first characterized by thermal analysis, UV-Vis spectroscopy, and cyclic voltammetry. By simply changing the BT to TT core in these two oligothiophenes, the highest occupied molecular orbital levels were varied from for to for , and the optical band gaps were varied from 1.72 eV for to 1.25 eV for , ascribed to the stronger electron accepting character of the TT core. However, the power conversion efficiency of bulk heterojunction organic solar cells (OSCs) with as donor and [6,6]-phenyl -butyric acid methyl ester () as acceptor was measured to be 0.04% only, which is much lower than that of (1.54%). Compared to the system, the based device shows smoother film surface morphology, and superior charge generation and charge carrier mobilities. Therefore, the results clearly demonstrate that in addition to modifying the alkyl side chains and -bridge lengths, the design of new small molecules for high-performance OSCs should also aim to choose suitable acceptor units.