A simple and effective modification of phenyl-C70-butyric acid methyl ester (PC70BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified PC70BM, namely CN-PC70BM, showed broader and stronger absorption in the visible region (350–550 nm) of the solar spectrum than PC70BM because of the presence of a cyanovinylene 4-nitrophenyl segment. The lowest unoccupied molecular energy level (LUMO) of CN-PC70BM is higher than that of PC70BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN-PC70BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC70BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN-PC70BM is related to the increase in both the short circuit current (Jsc) and the open circuit voltage (Voc). The increase in Jsc is related to the stronger light absorption of CN-PC70BM in the visible region of the solar spectrum as compared to that of PC70BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN-PC70BM and the HOMO of P3HT causes an enhancement in the Voc. The addition of 2% (v/v) 1-chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.