Alkylthio groups can be used to modulate energy levels and molecular packing of organic semiconductors, which makes it important in the design of materials for organic solar cell. However, its effect has not been sufficiently exploited as most of the studies report introducing an alkylthio group to the donor unit and seldom to the acceptor unit of donor-acceptor conjugated polymers. In this report, two alkylthio-substituted polymers, namely, PBB-TSA and PBB-TSD, with benzo[1,2-d:4,5-d']bis(thiazole) (BBT) as the acceptor unit and benzo[1,2-b:4,5-b']dithiophene (BDT) as the donor unit, were rationally designed, synthesized, and applied in organic photovoltaics. An alkylthio side chain was substituted on the BBT-accepting unit for PBB-TSA, while for PBB-TSD, the alkylthio side chain was substituted on the BDT donor unit. PBB-TSA and PBB-TSD show upshifted and downshifted energy levels, respectively, compared to the nonsulfur-substituted material. Both polymers exhibit dominate face-on orientation, while PBB-TSD exhibits higher crystallinity compared to PBB-TSA. With the contribution of lower energy level and beneficial film morphology, the device based on PBB-TSD/IT-4F has much higher power conversion efficiency (PCE) of 14.6%, whereas the PBB-TSA blend had a lower PCE of 10.7%. 1,8-Diiodooctane can effectively optimize the blend film morphology, and the effect on device performance has also been demonstrated in detail. This result indicates that introducing an alkylthio side chain into the donor or acceptor moieties would result in materials with different energy levels and thus would be utilized to match with various acceptors, achieving optimized performance in organic solar cells.
Keywords: alkylthio side chain; benzobis(thiazole); conjugated polymers; energy levels; organic solar cells.