Selenium-Functionalized Main-Chain Twisted Molecules with High Melting Points Enable Efficient and Stable Ternary Organic Solar Cells

Although organic solar cells (OSCs) based on Y6 and its derivatives have achieved rapid development, their intrinsically low open-circuit voltage (V_OC), which limits the power conversion efficiency (PCE), and the poor long-term stability remain two major obstacles to their application. To address these challenges, two selenium-functionalized main-chain twisted small molecules, i-DA and i-EA, were designed and synthesized. These molecules exhibit strong absorption in the 500−700 nm region and exhibit up-shifted LUMO energy levels, which facilitate cascading energy alignment and enhanced V_OC in ternary devices. Notably, they exhibit intrinsically high melting points (T_m) of 312.1 °C (i-DA) and 344.9 °C (i-EA) with large melting enthalpies (ΔH_m of 50.02 and 95.61 J g⁻¹), reflecting strong intermolecular cohesion. When incorporated into the PM6:Y6 as the third component, both i-DA and i-EA function as crystallization modulators that optimize the molecular packing and film morphology, leading to improved and more simultaneously balanced hole/electron mobilities. Consequently, the PM6:Y6:i-DA and PM6:Y6:i-EA ternary devices achieve promising PCEs of 19.09% and 19.35%, respectively, with simultaneously enhanced V_OC, short-circuit current density (J_SC), and fill factor (FF), surpassing the binary device (18.44%). Moreover, the PM6:Y6:i-DA and PM6:Y6:i-EA ternary devices exhibit significantly improved thermal stability (T₈₀ of 840 h and 910 h) compared with the PM6:Y6 control device (T₈₀ of 300 h). The effectiveness of these two molecules is further validated in the D18:L8-BO system, realizing excellent PCEs of 20.33% and 20.50% with i-DA- and i-EA-based ternary devices, respectively. This work demonstrates that high T_m, backbone-twisted small molecular acceptors represent a potential strategy for simultaneously optimizing morphology and charge transport toward high-performance and stable ternary OSCs.