Molecular‐Weight‐Regulated Aggregation Kinetics Enables Additive‐Free Morphology Control for High‐Efficiency and Scalable Organic Solar Cells

ABSTRACT Controlling active‐layer morphology without processing additives remains a core challenge for high‐efficiency organic solar cells (OSCs), particularly for molecular‐weight‐sensitive polymer donors. Here, we report an additive‐free morphology control strategy based on molecular‐weight‐mediated aggregation kinetics using the benchmark donor polymer D18. We show that both low‐ and high‐molecular‐weight D18 exhibit aggregation behavior mismatched to the nonfullerene acceptor L8‐BO, causing to suboptimal film formation. By blending D18 with different molecular weights, the donor aggregation time window is broadened and moderated, enabling kinetically synchronized film formation without additive assistance. As a result, additive‐free D18‐mix:L8‐BO devices deliver a high power conversion efficiency of 20.0% with balanced charge transport and suppressed recombination, while maintaining efficiencies above 19% over a wide blending range. Moreover, this intrinsic kinetic regulation strategy is compatible with advanced device architectures and scalable fabrication: ternary D18‐mix:L8‐BO:AITC devices achieve an enhanced efficiency of 20.5%, and large‐area modules (17.14 cm 2 ) retain an efficiency of 17.2%. This work establishes molecular weight as an intrinsic kinetic handle for additive‐free morphology control, offering a robust and scalable materials strategy for high‐performance OSCs.