19.5%‐Efficient and Ultrastable Quasiplanar Heterojunction Organic Solar Cells Enabled by Highly Crystalline 2D Acceptors

Abstract The limited operational stability of organic solar cells (OSCs) remains a major barrier to their practical application. In this work, two highly crystalline 2D acceptors, PhIC‐BO and AnIC‐BO are reported, which are incorporated as a third component into bilayer‐dominated quasiplanar heterojunction (Q‐PHJ) OSCs to simultaneously enhance efficiency and stability. PhIC‐BO, featuring a phenanthrene extension, forms a typical 3D network crystal with an elliptical framework, while the anthracene‐based AnIC‐BO adopts a linear packing motif, resulting in a quasi‐3D network structure. Owing to their high compatibility with host materials, both acceptors preferentially accumulate within the narrow bulk heterojunction (BHJ) region of the Q‐PHJ architecture, facilitating charge generation and simultaneously acting as a diffusion barrier to suppress molecular intermixing. These synergistic effects lead to significant improvements in both device performance and long‐term stability. The resulting PhIC‐BO‐based ternary OSCs exhibit a high power conversion efficiency (PCE) of 19.44% and retain 99% of their initial PCE after 6245 h of storage (with an extrapolated T 80 lifetime of ≈58 600 h), ranking among the most stable high‐performance OSCs reported to date. This study demonstrates a molecular design strategy that bridges high efficiency and operational stability, offering practical guidelines for the development of commercially viable OSCs.