Using Chlorine Atoms to Fine-Tune the Intermolecular Packing and Energy Levels of Efficient Nonfullerene Acceptors

Replacing the halogen atoms on the end group was considered an efficient way to enhance the performance of organic solar cells (OSCs), such as tuning energy levels, and improving sunlight absorption and intermolecular stacking. Herein, three chlorine-substituted asymmetric nonfullerene acceptors, named ITIC-Cl-δ-Th,ITIC-Cl-γ-Th, and ITIC-2Cl–Th, were synthesized to study the impact of the chlorine atom position and numbers on the molecular properties. Theoretical calculation revealed that a single chlorine at the γ-position on the end group led to a better molecular planarity and lower dimer energy of ITIC-Cl-γ-Th, which is appropriate for intermolecular charge transfer. Although double chlorinatation of ITIC-2Cl–Th can significantly redshift the ultraviolet–visible absorption, the lower LUMO energy level would obstruct the improvement in photoelectric conversion efficiency (PCE) due to a lower open-circuit voltage. Hence, we found that single chlorination at the γ-position helps ITIC-Cl-γ-Th-based devices to achieve a PCE as high as 12.25% with higher electron and hole mobilities. In addition, more efficient exciton dissociation and weaker bimolecular recombination were also realized in ITIC-Cl-γ-Th-based devices.