Polymer/PCBM (D/A) ratio is an important parameter for bulk heterojunction (BHJ) polymer solar cells (PSCs). To realize efficient PSC device with low PCBM content is an important but untouched topic in molecular structure design of photovoltaic polymers as high polymer content improves photon absorption in thin films. In this work, a new polythiophene derivative named as PBT1 was designed (see Scheme 1), synthesized and applied in PSCs. The photovoltaic and morphological results indicate that in the PBT1:PC71BM system, efficient morphologies can be formed in the D/A blend under varied PC71BM content (ca. 20-90%, corresponding to the D/A ratio of 4:1 to 1:9), including low PCBM content. The lamellar spacing of PBT1 in the pure polymer film and in the blend film of PBT1:PC71BM is as small as 15.7 Å, which is equal to the width of the rigid segments of the polymer, indicating that the flexible alkyl side groups are all lying prone upon the conjugated backbones. Hence, based on small /unchanged lamellar spacing and chemical structure geometry, intercalation can be effectively prevented. This work provides the first successful example of using molecular structure as a tool to realize efficient photovoltaic performance with low PC71BM content. These results are helpful for understanding the formation of bicontinuous phase separation in PSCs and also can be seen as a useful guidance to molecular design of the blending materials for other applications.