Owing to their lightweight, flexibility, and transparency, organic photovoltaics have emerged as a promising renewable energy technology that can be used in a variety of novel applications including smart windows and powering portable devices or IoTs. Although the device performance of OPVs has now reached a promising 19% efficiency, their morphology control and device optimization process remain challenging, often requiring time-consuming trial-and-error optimization of processing parameters. Recent studies have shown that the solution-state aggregation of conjugated polymers plays a critical role in the morphology and device performance of bulk heterojunction (BHJ) organic solar cells (OSCs). Despite this, the detailed structures of polymer solution-state aggregates and their impact on the morphology and device properties of OSCs remain largely unexplored. To address this research gap, this study uses a benzodithiophene-based donor polymer with ester-substituted long alkyl sidechains (PM7 D2) as a model system to investigate how the polymer solution-state aggregate structures change with varying solvent quality. Using small angle X-ray scattering, we reveal that PM7 D2 forms amorphous network-like aggregates of single polymer chains in a good solvent, whereas in intermediate solvents, PM7 D2 forms rigid-rod like polymer chains, eventually leading to semicrystalline fiber aggregates with strong lamellar and Ï-Ï stacking with decreasing solvent quality. We further demonstrate that the initial solution-state aggregate structures determine the morphology of the neat and blend films. Using both grazing incident X-ray diffraction and optical microscopy, we reveal that the amorphous network-like aggregates of PM7 D2 formed in good solvent led to highly aligned neat films with predominantly edge-on molecular orientation, whereas polymer solutions cast from poorer solvents result in face-on orientation without in-plane alignment. When these polymer solutions are blended with a non-fullerene acceptor, ITIC-4F, the resulting blend films also show drastically different BHJ morphologies as evidenced by photo-induced force microscopy imaging. More importantly, we show that organic solar cell devices fabricated from the amorphous network-like aggregates showed a three-fold increase in the photostability compared to devices fabricated from poorer solvents. We attribute this enhanced stability to the thermodynamically stable morphology originating from the amorphous network-like polymer solution. To generalize the relationship between polymer aggregate structure, morphology, and device properties, we further investigate the solution-state aggregation of various donor polymers used in high performing OSCs and their resulting morphology and device properties.
