Solution viscosity-governed phase separation and aggregation kinetics enable high-efficiency, eco-friendly slot-die coated organic solar cells

• • A universal optimal solution viscosity of ~0.8 mPa·s is identified across three distinct donor:acceptor systems (PM6:Y6, PTQ10:Y6, D18:Y6), enabling peak device efficiency despite different optimal processing temperatures; this threshold provides a direct, transferable process control parameter for industrial slot-die coating, eliminating trial-and-error temperature optimization. • • Lower solution viscosity enhances solute diffusion, accelerating molecular aggregation while suppressing liquid-liquid phase separation (LLPS); this kinetic competition dictates the formation of finely structured, continuous nanoscale domains, which are essential for efficient charge generation and extraction in printed active layers. • • In situ characterizations confirm that at ~0.8 mPa·s, all three systems exhibit constrained LLPS and rapid aggregation, yielding morphology with domain sizes conducive to high photovoltaic performance; this mechanistic insight enables real-time process monitoring and quality control in roll-to-roll manufacturing. • • The findings establish solution viscosity as a universal governing parameter for morphology control in printed active layers, offering a fundamental framework to advance high-throughput, environmentally friendly printing techniques for high-efficiency OSCs, with potential to reduce production costs and environmental impact.