Effect of fluorine–fluorine repulsive coupling on charge transport in cyclopentadithiophene-based donor–acceptor-type conjugated copolymer films

This study elucidates the effect of fluorine–fluorine interactions on the charge transport properties of semiconductor channels in polymer-based field effect transistors (PFETs) in which both a donor–acceptor (D–A)-type conjugated polymer and a polymer dielectric material contain fluorinated units. The fluorinated units on the benzothiadiazole (BTZ) block of cyclopentadithiophene-alt-benzothiadiazole (CDT-BTZ) allow conformation locking via F⋯S nonbonding interactions, which increases the planarity of cyclopentadithiophene-alt-fluoro-2,1,3-benzothiadiazole (CDT-FBTZ). Thus, the CDT-FBTZ copolymer films are highly crystalline and adopt the edge-on orientation, resulting in an eight-fold increase in the field-effect mobility for CDT-FBTZ-based PFETs coated with a poly(methyl methacrylate) (PMMA) dielectric layer compared with PMMA-coated CDT-BTZ-based PFETs. Meanwhile, the field-effect mobility of CDT-BTZ-based PFETs containing a fluorinated dielectric polymer (CYTOP) is double that of PMMA-coated CDT-BTZ-based PFETs. These enhanced electrical properties are attributed to surface polarization doping at the interface between the conjugated polymer and the fluorinated polymer dielectric. However, despite the effect of surface polarization doping, the electrical properties of CDT-FBTZ-based PFETs deteriorate when combined with the fluorinated CYTOP dielectric. Charge transport analyses based on the Gaussian disorder model reveal that, for CDT-based D–A-type conjugated copolymer PFETs, very few localized states in the semiconducting channel of PFET devices in which fluorine–fluorine interaction effects occur at the interface switch to delocalized states as the gate-bias increases. In contrast, the localized charge states in PFET channels without fluorine–fluorine interactions depend strongly on the applied gate-bias and become delocalized upon applying a gate-bias. Thus, it is inferred that the fluorine–fluorine repulsive coupling in D–A-type conjugated copolymers containing fluorinated functional moieties that interface with fluorinated polymer dielectric layers hinders the delocalization of charges induced by the gate-bias, thereby degrading the device performance.