(151k) Understanding the Solution-State Aggregate Structures of Donor Polymers and Their Impact on the Morphology and Device Properties of Organic Solar Cells

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 π-π 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.