Vinylene-bridged difluorobenzo[c][1,2,5]-thiadiazole (FBTzE): a new electron-deficient building block for high-performance semiconducting polymers in organic electronics

A new class of an acceptor unit, vinylene-bridged 5,6-difluorobenzothiadiazole FBTzE, has been developed. Palladium-catalyzed Migita-Kosugi-Stille coupling reactions of 1 with 2, yielding 3 and its sequential dehydrogenative coupling with 4, readily afforded FBTzE-containing monomers 5a-5c that have lower lowest unoccupied molecular orbital (LUMO) energy level and smaller energy gap than those of 5,6-difluorobenzothiadiazole (DFBT). Subsequently, three types of FBTzE-containing copolymers 3T, 4T, and 2TTT were synthesized by Migita-Kosugi-Stille coupling of monomers 5b and 5c with distannylated thiophene, bithiophene, and thienothiophene, respectively and their physicochemical properties and solar cell performances were evaluated. As a result of cyclic voltammogram, the synthesized FBTzE-based polymers have deeper highest occupied molecular orbital (HOMO) and LUMO energy levels, and stronger intermolecular interactions than those of DFBT-based polymer PffBT4T-DT. Although 3T/PC61BM blended film formed favorable face-on orientation with short d of 3.57 angstrom, its solar cell showed poor PCE of 2.7% owing to the construction of large phase separation structure with a domain size over 100 nm. In a sharp contrast, 2TTT/PC61BM formed unsuitable edge-on orientation with short d of 3.49 angstrom, but its film formed optimal nanoscale phase separation, leading to a good performance with PCE of up to 5.2%.