Ternary bulk-heterojunction polymer solar cells have been fabricated using a polymer blend (PBDB-T:PBDTTPD) as a donor in combination with a small molecule ITIC as a non-fullerene acceptor. In addition to the similarity in their energy levels, both PBDB-T and PBDTTPD polymers demonstrated rather similar crystalline or lamellar packing structure to support an alloy-type blend model, which well-explained the progressively monotonic change in photovoltaic V-OC against the compositions. In addition, photoinduced electron-transfer (PET) between the PBDB-T/PBDTTPD polymers forming an intermediate charge separation state between the donors. A synergistic improvement in exciton generation rate, enhancement in charge carrier transport with good balance, and charge separation rate due to PET all contribute to the achieve a photovoltaic device with high efficiency.