title: Effects of cyano-substituents on the molecular packing structures of conjugated polymers for bulk-heterojunction solar cells.
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:PC71BM blend exhibits enhanced exciton dissociation to PC71BM. 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 (V(OC)) 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 V(OC) (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (J(SC)) (7.4 mA/cm(2)) 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 V(OC) by tuning the HOMO and LUMO energy levels of the conjugated polymer.
authors.affiliation: Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH) , Pohang, 790-784, Republic of Korea.
authors.forename: Hyojung
authors.initials: H
authors.lastname: Cha
authors.forename: Hyoung Nam
authors.initials: HN
authors.lastname: Kim
authors.forename: Tae Kyu
authors.initials: TK
authors.lastname: An
authors.forename: Moon Sung
authors.initials: MS
authors.lastname: Kang
authors.forename: Soon-Ki
authors.initials: SK
authors.lastname: Kwon
authors.forename: Yun-Hi
authors.initials: YH
authors.lastname: Kim
authors.forename: Chan Eon
authors.initials: CE
authors.lastname: Park
keywords: alkoxy naphthalene derivative; conjugated polymer; electron-withdrawing cyano-groups; morphology control; organic photovoltaics; phase separation