Role of crystallinity of non-fullerene acceptors in bulk heterojunctions

Understanding the solid-state structure of the bulk heterojunction provides insight into how to improve the performance of nonfullerene acceptors in organic solar cells. We have characterized the self-assembly of three functionalized pentacene acceptors in single crystals, neat films and bulk heterojunctions formed by blending with a diketopyrrolopyrrole-based molecular donor. Atomic force microscopy, grazing incidence wide-angle X-ray scattering and optical spectroscopy indicate that the presence of the donor perturbs the packing and texture of acceptors with smaller substituents. The structural characterization explains the differences in performance among the three acceptors studied and suggests that, unlike fullerenes, disordered domains of molecular acceptors with planar molecular structure have inefficient electron transport in BHJ thin films.