Decoupling Processing‐Morphology‐Stability Relationships Enables 19.65% Organic Solar Cells With Exceptional Photostability

The commercial viability of organic solar cells (OSCs) hinges critically on the simultaneous achievement of high efficiency and long-term operational stability. Yet, a quantitative mechanistic understanding of how multiple processing strategies collectively control film formation kinetics and device longevity remains scarce. In this work, a mechanism-guided multivariate framework is introduced to systematically deconvolve the individual and synergistic effects of solvent additives, solid additives, multi‑component blending, and thermal annealing. Using in situ time-resolved absorption spectroscopy, three distinct stages of film formation are identified, and the crystallization dynamics governed by different processing pathways are quantitatively resolved. A rationally designed synergistic combination of these strategies yields a highly ordered, thermodynamically robust bulk heterojunction (BHJ) morphology, enabling optimized OSCs that achieve a champion power conversion efficiency of 19.65% and outstanding stability under continuous 1‑sun-equivalent illumination (ISOS-L-1I). This work establishes a clear quantitative processing‑morphology‑stability correlation, providing a universal platform for the rational design of high‑performance, long‑lifetime OSCs.