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.