Developing s-Tetrazine-Based Terpolymer for High-Performance Polymer Solar Cells by Tuning Side Chains

Two terpolymers, TP-H and TP-EH, have been designed and synthesized by incorporating hexyl- or 2-ethylhexyl-substituted 3,6-bis(4-alkylthiophen-2-yl)-1,2,4,5-tetrazine derivatives as the second electron-withdrawing moieties with a 20% molar ratio into the backbone of PM6, respectively. The introduction of s-tetrazine units can efficiently depress the highest occupied molecular orbital energy levels of the terpolymers, which results in higher open-circuit voltages (Voc’s) in the polymer solar cells (PSCs) than the PM6:Y6 PSCs. TP-H with hexyl side chains possesses a stronger molecular aggregation than TP-EH with 2-ethylhexyl side chains, and the former also shows better morphology with a nanofibrous network structure than the latter in blends with the acceptor Y6, which are all favorable for charge transport and exciton dissociation. As a result, the PSC based on the TP-H:Y6 blend achieves a higher efficiency of 16.76% with a short-circuit current density of 26.78 mA cm–2, a Voc of 0.851 V, and a fill factor of 73.53%. This work provides not only an s-tetrazine-based terpolymer for high-performance PSCs but also a helpful structure–property relationship about side-chain engineering toward the design strategy of terpolymers.