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. By systematically varying the properties of conjugated polymers, fullerenes, and small molecules in polymer:fullerene and polymer:small molecule blends used in organic solar cells, their effects on molecular mixing are studied. It is shown that many polymer:fullerene blends exhibit molecular mixing, and the factors that govern molecular mixing are discussed. The discovery of a bimolecular crystal composed of a semiconducting polymer and a non‐fullerene acceptor is also reported. Finally, differential scanning calorimetry and molecular simulations are used to probe the strength and nature of the interaction responsible for polymer–fullerene mixing.