Ambient‐Compatible Solvent Bath Thermal Annealing for Highly Efficient Organic Solar Cells and Large‐Area Modules

Abstract Thermal annealing is essential for optimizing the bulk‐heterojunction (BHJ) morphology of organic solar cells (OSCs), yet traditional thermal annealing (TA) under ambient air suffers from film degradation and nonuniform heating. Here, a novel post‐treatment, solvent bath thermal annealing (STA), enabling effective TA of large‐area OSC active layers under ambient conditions, is introduced. In STA, the BHJ film is immersed in a perfluorodecalin (PFD) bath during heating, ensuring uniform thermal distribution and protection against oxygen and moisture. STA‐treated films thus exhibit optimized vertical component distribution, enhanced crystallinity, and improved molecular packing with higher structural order, leading to efficient charge transport and suppressed nonradiative recombination regardless of the annealing atmosphere. As a result, STA‐treated OSCs based on PM6/L8‐BO blend deliver outstanding performance: small‐area cells achieve a power conversion efficiency (PCE) of 19.05%, while large‐area (20.25 cm 2 ) modules attain a PCE of 17.37% (certified 16.75%), among the highest PCEs for large‐area organic solar modules. It also successfully bridges the performance gap between inert and ambient processing conditions. Moreover, STA‐treated devices show excellent operational stability under continuous illumination and thermal stress. This ambient‐compatible annealing strategy thus provides a scalable route toward high‐efficiency and stable organic solar modules for practical large‐area manufacturing.