The principles of selecting green solvent additives for optimizing the phase separation structure of polymer solar cells based on PTB7:PC71BM

Processing solvent additives is an effective approach to improve the performance of polymer solar cells (PSCs). However, traditional additives, such as 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO), are halogenated solvent, which is not environmentally friendly. Here, we investigated the device performance of PTB7:PC71BM blend films processed with three aromatic ethers, CN and DIO. The results indicate that aromatic ethers have a higher device performance improvement than halogenated solvents. Using green solvent 1,2-dimethoxybenzene as the processing additive, the efficiency of PSCs was improved from 2.83% to 6.18%. We proposed two criteria to choose green solvent additives: (i) good solvent for PC71BM and nonsolvent for PTB7, (ii) higher boiling point than the host solvent, but not too high (about 70 celcius higher is enough). The X-ray diffraction results indicated that PTB7 does not crystallize in the blend films, and thus the formation of phase separation morphology is primarily through liquid-liquid (L-L) phase separation. The structure analyses of the blend films demonstrated that solvent additives with poor solubility to PTB7 are beneficial in eliminating PC71BM sea-island domains and in promoting the formation of bi-continuous phase separation morphology, while nonsolvent additives with relatively lower boiling point could prevent PC71BM from moving to the bottom layer of the blend films during the drying process and thus the resulting films have a better vertical phase separation structure. The selection principles of green solvent additives proposed in this study can be further used to optimize the active layer structure of other PSCs occurred upon L-L phase separation.