Donor-acceptor small molecules and donor-acceptor polymers containing benzo[1,2-b:4,5-b']dithiophene (BDT) and benzo[1,2-b:4,5-b']difuran (BDF) were synthesized and tested in bulk heterojunction solar cells. A systematic study was performed to determine the influence of the combinations of different heteroatoms in the donor-acceptor small molecules and copolymers. In bulk heterojunction solar cells, the polymer with all furan building blocks in the electron donating units, poly[(4,8-bis(5-dodecyl-2-furanyl)benzo[1,2-b:4,5-b’]difuran-2-yl)-alt-(2-eth yl-1-(3-fluorothieno[3,4-b]thiophen-2-yl)-1-hexanone)] achieved a power conversion efficiency of ~5.23%.