Hot‐Casting Strategy Empowers High‐Boiling Solvent‐Processed Organic Solar Cells with Over 18.5% Efficiency

Abstract Most top‐rank organic solar cells (OSCs) are manufactured by the halogenated solvent chloroform, which possesses a narrow processing window due to its low‐boiling point. Herein, based on two high‐boiling solvents, halogenated solvent chlorobenzene (CB) and non‐halogenated green solvent ortho‐xylene (OX), preparing active layers with the hot solution is put forward to enhance the performance of the OSCs. In situ test and morphological characterization clarify that the hot‐casting strategy assists in the fast and synchronous molecular assembly of both donor and acceptor in the active layer, contributing to preferable donor/acceptor ratio, vertical phase separation, and molecular stacking, which is beneficial to charge generation and extraction. Based on the PM6:BO‐4Cl, the hot‐casting OSCs with a wide processing window achieve efficiencies of 18.03% in CB and 18.12% in OX, which are much higher than the devices processed with room temperature solution. Moreover, the hot‐casting devices with PM6:BTP‐eC9 deliver a remarkable fill factor of 80.31% and efficiency of 18.52% in OX, representing the record value among binary devices with green solvent. This work demonstrates a facile strategy to manipulate the molecular distribution and arrangement for boosting the efficiency of OSCs with high‐boiling solvents.