Modification on the Quinoxaline Unit to Achieve High Open-Circuit Voltage and Morphology Optimization for Organic Solar Cells

Abstract

Core engineering plays a vital role in the construction of efficient non-fullerene acceptors. Here, we synthesized a molecule named QX, based on the core of quinoxaline, and by replacing the H atoms with 2-ethylhexyl, 2-(ethylhexyl)thiophene, and 2-(2-ethylhexyl)-3-fluorothiophene groups, we obtained other three non-fullerene acceptors, named QX-EH, QX-TH, and QX-THF, respectively. Compared with QX, the introduction of substitution groups ameliorated the molecular solubility, and their lowest unoccupied molecular orbital levels were elevated. Also, the introduction of a thiophene ring in QX-TH and QX-THF enhanced their miscibility with PM6 compared to QX-EH. Furthermore, the introduction of a fluorine atom in QX-THF greatly optimized the blend morphology, leading to efficient charge transport, less bimolecular recombination, and suitable nanophase aggregation in devices. Eventually, PM6:QX-THF-based devices exhibited an impressive power conversion efficiency of 17.45% with a fill factor of 78.99%. This work reveals that modification on the quinoxaline core is effective in tuning the morphology and open-circuit voltage for high-efficiency organic photovoltaics.