A new benzodithiophene based donor-acceptor π-conjugated polymer (P1) is designed and explored as the photoactive donor for organic solar cells (OSCs). The synthesized donor polymer, P1 displays a wide absorption profile ranging from 300–750 nm with optical band gap of 1.61 eV and moderate ionization potential of −5.30 eV. It has good solubility in non-halogenated and halogenated organic solvents. Next, we fabricated OSCs with P1 by blending with PC71BM, the pristine polymer processed from chlorobenzene showed PCE of 2.79%. Upon addition of external additive diphenyl ether to the blend showed a dramatic improvement in PCE with maximum of 5.33%. DPE tailored the active layer morphology and showed two times higher PCE than pristine films. These results clearly indicate that the new polymer has a great potentiality for enhancing efficiency upon solvent additives, which will provide new routes for the development of new class of polymers for high-performance air-stable OSCs. [Figure not available: see fulltext.].