Efficient Ternary Organic Solar Cells Enabled by Synergistic Morphology Regulation via Wide-Bandgap Dimers

Rapid progress has been achieved in organic solar cells (OSCs) since the emergence of Y6 and its derivatives. However, the open-circuit voltages (VOC) of efficient OSCs remain relatively low, which poses a potential obstacle to further performance improvements. Another drawback is the over aggregation of Y-type molecules in the active layer, leading to an unbalanced charge transportation. To address these issues, two wide-bandgap dimeric acceptors, BETh4O and BETh4S, were synthesized and used as second acceptors in the PM6:BTP-eC9-based binary device. Benefiting from their up-shifted LUMO energy levels and wide bandgaps, these two dimers constructed cascade energy levels and complementary absorption with PM6 and BTP-eC9. In addition, the two dimers exhibit distinct cold crystallization temperature (TCC) and melting characteristics, which are beneficial to use to adjust the crystallization characteristics of BTP-eC9. Unlike BETh4S with a 1,3-diethyl-2-thiobarbituric acid end group, BETh4O sealed by 1,3-dimethylbarbituric acid is characterized by a higher melting point and a lower melting enthalpy, indicating stronger intrinsic crystallinity and structural order. This inherent order enables BETh4O to serve as an effective template for guiding the crystallization of host acceptor BTP-eC9 into highly ordered domains. Consequently, the resulting PM6:BTP-eC9:BETh4O ternary OSCs demonstrate enhanced and balanced charge transport, suppressed bimolecular and trap-assisted recombination, and improved charge collection. These advantages collectively contribute to simultaneous increases in VOC and fill factor (FF) without sacrificing the short-circuit current density (JSC), yielding a power conversion efficiency (PCE) of 19.00%, which surpasses that of the binary PM6:BTP-eC9 device (18.40%) and the PM6:BTP-eC9:BETh4S ternary device (18.45%). This work demonstrates the efficacy of wide-band gap dimers in controlling active-layer crystallinity for high-efficiency OSCs.