title: Fluorinated benzothiadiazole (BT) groups as a powerful unit for high-performance electron-transporting polymers.
abstract: Over the past few years, one of the most remarkable advances in the field of polymer solar cells (PSCs) has been the development of fluorinated 2,1,3-benzothiadiazole (BT)-based polymers that lack the solid working principles of previous designs, but boost the power conversion efficiency. To assess a rich data set for the influence of the fluorinated BT units on the charge-transport characteristics in organic field-effect transistors (OFETs), we synthesized two new polymers (PDPP-FBT and PDPP-2FBT) incorporating diketopyrrolopyrrole (DPP) and either single- or double-fluorinated BT and thoroughly investigated them via a range of techniques. Unlike the small differences in the absorption properties of PDPP-FBT and its nonfluorinated analogue (PDPP-BT), the introduction of doubly fluorinated BT into the polymer backbone induces a noticeable change in its optical profiles and energy levels, which results in a slightly wider bandgap and deeper HOMO for PDPP-2FBT, relative to the others. Grazing incidence X-ray diffraction (GIXD) analysis reveals that both fluorinated polymer films have long-range orders along the out-of-plane direction, and π-π stacking in the in-plane direction, implying semicrystalline lamellar structures with edge-on orientations in the solid state. Thanks to the strong intermolecular interactions and highly electron-deficient π-systems driven by the inclusion of F atoms, the polymers exhibit electron mobilities of up to 0.42 and 0.30 cm2 V(-1) s(-1) for PDPP-FBT and PDPP-2FBT, respectively, while maintaining hole mobilities higher than 0.1 cm2 V(-1) s(-1). Our results highlight that the use of fluorinated BT blocks in the polymers is a promising molecular design strategy for improving electron transporting performance without sacrificing their original hole mobility values.
authors.affiliation: School of Energy and Chemical Engineering, KIER-UNIST Advanced Center for Energy, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST) , Ulsan 689-798, South Korea.
authors.forename: Junghoon
authors.initials: J
authors.lastname: Lee
authors.forename: Moonjeong
authors.initials: M
authors.lastname: Jang
authors.forename: Sang Myeon
authors.initials: SM
authors.lastname: Lee
authors.forename: Dohyuk
authors.initials: D
authors.lastname: Yoo
authors.forename: Tae Joo
authors.initials: TJ
authors.lastname: Shin
authors.forename: Joon Hak
authors.initials: JH
authors.lastname: Oh
authors.forename: Changduk
authors.initials: C
authors.lastname: Yang
keywords: diketopyrrolopyrrole; fluorinated benzothiadiazole; fluorine; n-channel dominant FET; organic field-effect transistors (OFET)