A >25 kDa Star‐Shaped Dodecamer Acceptor as a Heterogeneous Nucleation Agent Enables 20.50%‐Efficient Organic Solar Cells

Star-shaped molecules possess strong cohesive energy and welldefined molecular weights compared to polymers, making them promising additives for regulating microphase separation in bulk-heterojunction blends. Herein, we report the synthesis of an ultrahigh-molecular-weight star-shaped electron acceptor that serves as a heterogeneous nucleation agent to precisely control donor/acceptor blend morphology for high-performance organic solar cells (OSCs). The star-shaped acceptor, denoted SP12, was synthesized via an arm-first strategy and comprises 12 Y-type acceptor arms, affording a molecular weight exceeding 25 kDa-the highest reported to date for star-shaped electron acceptors. Owing to its pronounced pre-aggregation behavior in solution and three-dimensional (3D) architecture, SP12 acts as an effective heterogeneous nucleation center during film formation. This leads to bulk-heterojunction thin films with enlarged fiber diameters and enhanced crystallinity, as confirmed by multiple advanced characterization techniques. Consequently, OSCs incorporating SP12 as an additive deliver a high-power conversion efficiency of 20.50% in green-solvent-processed devices. Moreover, the high glass transition temperature and 3D topology of SP12 kinetically stabilize the optimized morphology by suppressing acceptor diffusion and re-aggregation, resulting in markedly improved morphological and operational stability. This work establishes a topology-driven molecular design and morphology-engineering strategy for achieving efficient and stable organic photovoltaic devices.