Environmentally-friendly solvent processed fullerenefree organic solar cells enabled by screening halogen-free solvent additives

Though the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been boosted to 12%, the use of highly pollutive halogenated solvents as the processing solvent significantly hinders the mass production of OSCs. It is thus necessary to achieve high-efficiency OSCs by utilizing the halogen-free and environmentally-friendly solvents. Herein, we applied a halogen-free solvent system (oxylene/1-phenylnaphthalene, XY/PN) for fabricating fullerene-free OSCs, and a high PCE of 11.6% with a notable fill factor (FF) of 72% was achieved based on the PBDB-T: IT-M blend, which is among the top efficiencies of halogen-free solvent processed OSCs. In addition, the influence of different halogen-free solvent additives on the blend morphology and device performance metrics was studied by synchrotron-based tools and other complementary methods. Morphological results indicate the highly ordered molecular packing and highest average domain purity obtained in the blend films prepared by using XY/PN co-solvent are favorable for achieving increased FFs and thus higher PCEs in the devices. Moreover, a lower interaction parameter (chi) of the IT-M: PN pair provides a good explanation for the more favorable morphology and performance in devices with PN as the solvent additive, relative to those with diphenyl ether and Nmethylpyrrolidone. Our study demonstrates that carefully screening the non-halogenated solvent additive plays a vital role in realizing the efficient and environmentally-friendly solvent processed OSCs.