Doping a Main-Chain Twisted Wide Bandgap Bifunctional Molecule Enables Efficient Ternary Organic Solar Cells

The rapid development of organic solar cells (OSCs) has been particularly remarkable following the introduction of the high-performance small molecule electron acceptor Y6 and its derivatives. Despite the advances in state-of-the-art OSCs, challenges remain, notably the relatively low open circuit voltage (VOC) and charge mobility imbalance, which continue to hinder further improvements in OSCs’ performance. To address these issues, in this work, we synthesized a main-chain twisted wide bandgap small molecule i-IEDTB, which not only possesses superior hole mobility over electron mobility but also has higher lowest unoccupied molecular orbital (LUMO) energy levels, and adopted it as a bifunctional third compound to couple with a high-performance terpolymer Z10 and Y6 based OSC system. The integration of i-IEDTB in the Z10:Y6 blend, either as an acceptor or as a donor, significantly reduces the excessive aggregation of Y6 and facilitates a harmonized distribution of electron and hole mobilities by modulating the crystallization properties of the materials. This strategic intervention leads to a marked improvement in VOC and fill factor (FF). Consequently, the power conversion efficiency (PCE) of the optimized Z10:(Y6:i-IEDTB) ternary device is elevated to an impressive 17.70%, surpassing the 16.50% of binary Z10:Y6 OSC. Besides, after doping 5% weight i-IEDTB replaced that of Z10, the device’s VOC, and FF were also increased to 0.846 V and 78.08% respectively, resulting in an enhanced PCE of 17.47%. Further investigation demonstrates the universality of this molecule in PM6:BTP-eC9-based and PM6:L8-BO-based OSCs, achieving champion efficiencies of 18.14 and 18.08%, respectively. This work highlights the key role of i-IEDTB as a bifunctional complementary component in modulating the crystallographic order and carrier balance within the Y-type acceptor-based OSCs.