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

Abstract

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_π of 3.57 Å, 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_π of 3.49 Å, but its film formed optimal nanoscale phase separation, leading to a good performance with PCE of up to 5.2%.