Effects
of Cyano-Substituents on the Molecular Packing Structures of Conjugated
Polymers for Bulk-Heterojunction Solar Cells
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Abstract
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:PC<sub>71</sub>BM blend exhibits enhanced exciton dissociation to PC<sub>71</sub>BM. Moreover, the electron-withdrawing 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 (<i>V</i><sub>OC</sub>) and
a lower energy loss during electron transfer from the donor to the
acceptor. A bulk-heterojunction device fabricated with the cyano-substituted
polymer:PC<sub>71</sub>BM blend has a higher <i>V</i><sub>OC</sub> (0.89 V), a higher fill factor (FF) (51.4%), and a lower
short circuit current (<i>J</i><sub>SC</sub>) (7.4 mA/cm<sup>2</sup>) than that of the noncyano-substituted polymer:PC<sub>71</sub>BM blend under AM 1.5G illumination with an intensity of 100 mW cm<sup>–2</sup>. Thus, the cyano-substitution of conjugated polymers
may be an effective strategy for optimizing the domain size and crystallinity
of the polymer:PC<sub>71</sub>BM blend, and for increasing <i>V</i><sub>OC</sub> by tuning the HOMO and LUMO energy levels
of the conjugated polymer