Investigation of halogen-free solvents towards high-performance additive-free non-fullerene organic solar cells

Environmental sustainability and low toxicity of processing are vital for next-generation organic solar cells (OSCs). Herein, two halogen-free solvents carbon disulfide (CS2) and ortho-xylene (o-XY) were used to illustrate the promising potential in additive-free non-fullerene OSCs with PM7 as the donor and IT-4F and IT-4Cl as the acceptors. The CS2-processed pristine films show higher crystallinity than those processed by o-XY and the corresponding blend films exhibit higher and more balanced charge mobilities. Relative to o-XY as the solvent, the CS2-processed blend films display lower absorption coefficients. As such, the CS2-processed devices show higher open-circuit voltage and fill factor, but with lower short-circuit current density if compared with those processed by o-XY. Specially, the PM7:IT-4Cl based device processed by CS2 solvent can obtain the highest power conversion efficiency (PCE) of 12.52% for conventional device, which is the highest PCE value in CS2 casted single-junction OSCs so far. Furthermore, a high PCE of 12.31% for PM7:IT-4Cl is also achieved in inverted structure. This work implies that it is of great potential for the halogen-free and non-aromatic green solvent towards mass production.