Effect of Solution-State Aggregation on Solid-State Crystal Orientation in Donor–Acceptor Conjugated Copolymers

Despite recent progress in tailoring the molecular packing of donor–acceptor (D–A) copolymers in the solid state, studies on their solution-state aggregation and the dependence of the solid-state packing on solution aggregates remain to be clearly understood. Herein, we outline the relationship between the solution aggregates and solid-state crystal orentation in a specific class of D–A copolymers, i.e., 5,6-difluoro-4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (FBTT) units connected to bithiophene (2T) or thienothiophene (TT), by adjusting both the intrinsic (i.e., backbone planarity) and extrinsic factors (i.e., solvent) and investigate the influence of these structures with different crystal orientations on the organic field-effect transistor (OFET) performance. Remarkably, both PFBTT-2T and PFBTT-TT weakly aggregate in good solvents, forming face-on crystals in solid-state films. In contrast, they aggregate more strongly in relatively poor solvents and form edge-on crystals in films. When thin films are produced from the same solvent chlorobenzene (CB), PFBTT-2T with reduced planarity adopts face-on orientation, whereas PFBTT-TT with greater planarity adopts edge-on orientation. Moreover, the charge transport properties of both polymers strongly correlate with their various crystalline orientations. This work comprehensively elucidates how FBT-based polymers aggregate and pack in the solution and solid states and how these processes and electrical properties can be controlled by tuning the solvent quality and molecular structure.