Tuning the Solution Aggregate Structure of a PM7-Based Conjugated Polymer to Enable Green Solvent Processing of Organic Solar Cells

Understanding how solvents influence the solution-state aggregation of conjugated polymers and film morphology is crucial for solution-processed organic solar cells (OSCs). Herein, using the conjugated donor polymer PM7 D2, a more processable derivative of PM7 that readily dissolves in a range of halogenated and nonhalogenated (green) solvents, we find that solvent affinity toward the polymer backbone, as opposed to the alkyl side chains, drastically tunes the polymer solution aggregate structures and the resultant blend film morphologies. Our findings reveal that using a poor side-chain solvent forms semiflexible amorphous networks with strong side-chain associations, whereas using a poor backbone solvent leads to semicrystalline fiber aggregates. On the other hand, mutual solvents, which balance the polymer backbone and side-chain solubility, result in rigid amorphous networks with weak/no side-chain interactions. Upon film deposition via blade coating, both semicrystalline fibers and flexible amorphous network aggregates of PM7 D2 yield highly crystalline films with large domains. In contrast, rigid amorphous aggregates with weak or no side-chain associations prevented excessive crystallization of PM7 D2. We found that the green solvent studied in this work (anisole) exhibits poor affinity toward the backbone but is selective to the alkyl side chains, thus forming semicrystalline fibers in solution. Consequently, PM7 D2:ITIC-4F-based OSCs cast from the green solvent only exhibit 5.8% efficiency. Interestingly, similar to using mutual solvents, utilizing hot solution processing converts the semicrystalline fibers to rigid amorphous network aggregates by increasing the backbone solubility, thereby suppressing the excessive crystallization of PM7 D2. Ultimately, the devices exhibited a significant power conversion efficiency (PCE) improvement from 5.8 to 9%, where they were fully processed and tested under ambient conditions in a glovebox-free environment (without additives).