Narrow bandgap difluorobenzochalcogenadiazole-based polymers for high-performance organic thin-film transistors and polymer solar cells

Abstract

A bithiophene donor unit, 3-alkoxy-3′-alkyl-bithiophene (TRTOR), was copolymerized with difluorobenzochalcogenadiazole (ffBZ) containing different heteroatoms on its diazole structure to afford a series of PffBZ copolymers (where Z = X, T, Se) with narrow optical bandgaps in the range of 1.34–1.47 eV. The effects of the ffBZ heteroatoms (O, S, and Se) on the optical properties, electrochemical characteristics and film morphologies of the polymers as well as the device performance were fully investigated. The results revealed that the highest occupied molecular orbitals (HOMOs) of the polymers gradually elevated accompanied by their increased materials solubility in common organic solvents as the size of the heteroatoms increased. The PffBZ copolymers exhibited a substantial hole mobility of 0.08–1.6 cm² V⁻¹ s⁻¹ in organic thin-film transistors (OTFTs). The PffBX, PffBT, and PffBSe-based polymers exhibited maximum power conversion efficiencies (PCEs) of 5.47%, 10.12%, and 3.65%, respectively, in polymer solar cells (PSCs). For the PffBZ copolymers, the alkyl chain exerts a great influence on the morphology of the polymer:PC₇₁BM blend films and hence affect the PCEs in PSCs. It was found that the performance of the polymers with branching on the 2nd position of the alkyl chain and the 3rd position of the alkoxy chain were the best among the PffBT and PffBSe-based polymers, different from the tetrathiophene-based benchmark polymer with branching on the 2nd position of the alkyl chain. X-ray diffraction revealed that all of the PffBZ-based polymers showed obvious face-on dominant orientation, and that chalcogen atoms and branched positions on the alkoxy chain have a great influence on the morphologies of the neat and blend films. The above results indicated that the branching positions and chalcogen atoms should be carefully optimized to maximize performance.