Insights into Bulk-Heterojunction Organic Solar Cells Processed from Green Solvent

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so-called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular-level structures and optoelectronic properties lack in-depth characterization. Here, insights into the structure-processing-property correlation of a PPDT2FBT: PC61BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well-mixed features in the bulk morphologies of the active layer are observed for the o-XY processed BHJ blend. Furthermore, molecular-level differences in the D-A intermolecular interactions at the BHJ interfaces in o-XY and CB cast films are elucidated by 2-dimensional solid-state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green-solvent-processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor.