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

ABSTRACT 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.