Realizing compact three-dimensional charge transport networks of asymmetric electron acceptors for efficient organic solar cells

Asymmetry has been demonstrated an effective approach in recent years to tune the structural and energetic orders of non-fullerene electron acceptors (NFAs) to prepare efficient organic solar cells (OSCs). In this article, five asymmetric NFAs, namely C9BTP-BO-ThCl-2F, C9BTP-BO-Cl-2F, C9BTP-BO-2Cl-2F, C7BTP-BO-2Cl-2F and C5BTP-BO-2Cl-2F possessing varied asymmetric end-groups and alkyl chains are synthesized to tune the charge transport networks formed within these NFAs. We found that the enhanced planarity in the asymmetric NFA can facilitate closer π-π stacking distance in either the A-to-A or A-to-D type NFA dimers, whilst the larger dipole moment can promote the formation of three-dimensional (3D) charge transport networks among NFAs. Taking those advantages, C7BTP-BO-2Cl-2F exhibit a compact 3D honeycomb network with a high packing coefficient of 72.1% and molecular packing density of 0.48 g/cm3, contributing to a superior power conversion efficiency of 18.0% when employing PM6 as the donor, with an open-circuit voltage of 0.85 V, short-circuit current of 26.7 mA cm−2 and fill factor of 79.3%. Our work provides guidelines in engineering the end group and side chains of asymmetric NFAs to achieve compact charge transport networks for high efficiency OSCs.