Tethered Small‐Molecule Acceptor Refines Hierarchical Morphology in Ternary Polymer Solar Cells: Enhanced Stability and 19% Efficiency

Abstract Polymer solar cells (PSCs) are promising for efficient solar energy conversion, but achieving high efficiency and device longevity within a bulk‐heterojunction (BHJ) structure remains a challenge. Traditional small‐molecule acceptors (SMAs) in the BHJ blend show thermodynamic instability affecting the morphology. In contrast, tethered SMAs exhibit higher glass transition temperatures, mitigating these concerns. Yet, they might not integrate well with polymer donors, causing pronounced phase separation and overpurification of mixed domains. Herein, a novel ternary device is introduced that uses DY‐P2EH, a tethered dimeric SMA with conjugated side‐chains as host acceptor, and BTP‐ec9, a monomeric SMA as secondary acceptor, which respectively possess hypomiscibility and hypermiscibility with the polymer donor PM6. This unique combination affords a parallel‐connected ternary BHJ blend, leading to a hierarchical and stable morphology. The ternary device achieves a remarkable fill factor of 80.61% and an impressive power conversion efficiency of 19.09%. Furthermore, the ternary device exhibits exceptional stability, retaining over 85% of its initial efficiency even after enduring 1100 h of thermal stress at 85 °C. These findings highlight the potential advantage of tethered SMAs in the design of ternary devices with a refined hierarchical structure for more efficient and durable solar energy conversion technologies.