Deciphering the Structure of Push-Pull Conjugated Polymer Aggregates in Solution and Film

Abstract

The morphology of conjugated polymer films is highly tunable, influencing their performance in organic electronics. Specifically, molecular packing or crystal structure strongly influence electronic processes such as light absorption and charge transfer. However, the unit cells of high-performance electron donor polymers remain unknown, limiting the understanding of how processing affects structure and device performance. This study characterizes the aggregate structure of PM6-type push-pull polymers using X-ray scattering, cryogenic electron microscopy, and molecular dynamics (MD) simulations. A novel forward simulation approach linking grazing-incidence wide-angle X-ray scattering (GIWAXS) with MD resolves a monoclinic unit cell that accurately describes PM6-type polymer aggregates in both thin films and casting solutions. Intimate pi-pi stacking between donor and acceptor units emerges from this unit cell. Analysis of experimental GIWAXS using this unit cell quantifies sliding disorder in these aggregates, which may impact device performance. The shape and internal structure of solution aggregates are also identified in chlorobenzene. These findings enhance our understanding of PM6-type polymer packing, outline a strategy for elucidating the crystal structure of weakly ordered materials, and provide an opportunity to control optoelectronic performance through aggregate formation in PM6 and other push-pull conjugated polymers.