Asymmetric Alloy Acceptor Strategy Guided by Similarity Principle Enables Highly Efficient and Stable Organic Solar Cells

Abstract Introducing the guest materials into binary active layer to construct ternary organic solar cells (OSCs) is widely used to improve device performance. Nevertheless, designing the guest materials is a challenging task. Herein, asymmetric alloy acceptor strategy guided by similarity principle to design the guest materials is employed. Two small molecular acceptors (ZH1 with symmetric end groups and ZH2 with asymmetric end groups) with the same skeleton to the host acceptor are synthesized and compared. Compared to symmetric ZH1, asymmetric ZH2 delivers a remarkably higher efficiency (3.86% vs 13.03%) when paired with PM6, benefiting from the larger dipole moment to facilitate charge dynamics and more favorable morphology. More importantly, by introducing ZH1 and ZH2 as the guest materials into the PM6:BTP‐eC9 blend, both ZH1 and ZH2 well alloy with acceptor BTP‐eC9 due to the similar skeleton, not only providing a complementary absorption, but also optimizing and stabilizing the blend morphology. Notably, the asymmetric alloy acceptor distinctly outperforms symmetric alloy acceptor, PM6:BTP‐eC9:ZH2‐based device achieves an outstanding efficiency of 18.75% with better stability and reduced non‐radiative energy loss. Therefore, developing asymmetric alloy acceptor is an effective strategy to develop high‐performance and stable OSCs.