Factors Governing Intercalation of Fullerenes and Other Small Molecules Between the Side Chains of Semiconducting Polymers Used in Solar Cells

While recent reports have established significant miscibility in polymer:fullerene blends used in organic solar cells, little is actually known about why polymers and fullerenes mix and how their mixing can be controlled. Here, X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and molecular simulations are used to study mixing in a variety of polymer:molecule blends by systematically varying the polymer and small‐molecule properties. It is found that a variety of polymer:fullerene blends mix by forming bimolecular crystals provided there is sufficient space between the polymer side chains to accommodate a fullerene. Polymer:tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) bimolecular crystals were also observed, although bimolecular crystals did not form in the other studied polymer:non‐fullerene blends, including those with both conjugated and non‐conjugated small molecules. DSC and molecular simulations demonstrate that strong polymer–fullerene interactions can exist, and the calculations point to van der Waals interactions as a significant driving force for molecular mixing.