20.64% Efficient and Stable Binary Organic Solar Cells via Thermodynamic‐Engineered Interlayer Diffusion and Exciton Generation

Abstract Despite thermodynamics playing a central role in active‐layer optimization, unresolved temperature‐dependent mechanisms hinder further efficiency improvements in organic solar cell. Herein, real‐time thermal imaging is employed to unravel the temperature‐controlled assembly dynamics during sequential processing (SqP) of active‐layer films on a hot‐substrate (HS). The HS process provides higher temperature and prolonged heating time for the active layer during SqP compared to the widely adopted hot‐solution technique, enabling accelerated liquid‐phase reorganization and nucleation in the bottom layer. The HS‐induced interfacial energy difference promotes layer interpenetration and achieves suitable donor content in the bottom region of the active layer while boosting exciton generation. The highly crystalline fibrous structure improves hole mobility and suppresses non‐radiative recombination (0.214 eV), yielding a high fill factor (81.00%) and open‐circuit voltage (0.868 V). The 100 nm‐thick D18 HS/eC9 device achieves an efficiency of 19.75% (vs 18.89% for the control) and retains 90% of its initial efficiency after 270 h under ≈1 sun illumination (vs 84% for the control). With 2PACZ as the hole transport layer, over 20% efficiency is demonstrated in three systems: 20.02% (D18/eC9‐4F), 20.25 (D18/eC9), and 20.64% (D18/L8‐BO, certified 20.10%). Notably, HS‐processed 300 nm‐thick binary devices achieve over 18.12% efficiency—among the highest reported.