A low-bandgap acceptor (ITIC) was added to a binary system composed of a wide-bandgap polymer (PBT-OTT) and an acceptor (PCBM) to increase the light harvesting efficiency of the associated organic solar cells (OSCs). A ternary blend OSC with an acceptor ratio of PCBM:ITIC = 8:2 was found to exhibit a power conversion efficiency of 8.18%, which is 18% higher than that of the binary OSC without ITIC. This improvement is mainly due to the enhanced light absorption and optimized film morphology that result from ITIC addition. Furthermore, an energy level cascade forms in the blend that ensures efficient charge transfer, and bimolecular and trap-assisted recombination is suppressed. Thus the use of ternary blend systems provides an effective strategy for the development of efficient single-junction OSCs.