Effects of cyano-substituents on the molecular packing structures of conjugated polymers for bulk-heterojunction solar cells

The molecular packing structures of two conjugated polymers based on alkoxy naphthalene, one with cyano-substituents and one without, have been investigated to determine the effects of electron-withdrawing cyano-groups on the performance of bulk-heterojunction solar cells. The substituted cyano-groups facilitate the self-assembly of the polymer chains, and the cyano-substituted polymer:PC71BM blend exhibits enhanced exciton dissociation to PC71BM. Moreover, the electronwithdrawing cyano-groups lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the conjugated polymer, which leads to a higher open circuit voltage (VOC) and a lower energy loss during electron transfer from the donor to the acceptor. A bulk-heterojunction device fabricated with the cyano-substituted polymer:PC71BM blend has a higher VOC (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (JSC) (7.4 mA/cm²) than that of the noncyano-substituted polymer:PC71BM blend under AM 1.5G illumination with an intensity of 100 mW cm-2. Thus, the cyano-substitution of conjugated polymers may be an effective strategy for optimizing the domain size and crystallinity of the polymer:PC71BM blend, and for increasing VOC by tuning the HOMO and LUMO energy levels of the conjugated polymer.