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 (V_OC) 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 V_OC 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 V_OC, 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.