Erosion Sequential Deposition with Halogen‐Free Solvent Enables Organic Solar Cells With 21.05% Efficiency and 83.19% Fill Factor

Halogen‐free solvent processed organic solar cells (OSCs) usually show much lower power conversion efficiencies (PCEs) if compared with halogenated solvents, especially for sequential deposition (SD) process. Herein, erosion‐SD processing with halogen‐free solvents was demonstrated, in which an eroded material was pre‐inserted into a donor (D18) bottom layer. Subsequent deposition of acceptor (L8‐BO) solution swells the bottom layer more easily, enabling deeper acceptor penetration and optimizing vertical phase separation. Judicious selections of eroded materials and halogen‐free solvents open chances to re‐organize SD morphology. With toluene:CS2/toluene as halogen‐free solvents and partially removable polymer acceptor PYDT as eroded material, OSCs based on D18:PYDT/L8‐BO achieved a remarkable PCE of 21.05% (certified 20.71%), greatly higher than 20.53% achieved with chloroform/chloroform solvent system. Moreover, the erosion‐SD processed 300 and 500 nm thick active layers exhibited superior PCEs of 19.64% and 18.75%, respectively, showing promising ability to re‐organize morphology toward halogen‐free solution printing.