Crystallography, Packing Mode, and Aggregation State of Chlorinated Isomers for Efficient Organic Solar Cells

Revealing the molecular packing, intermolecular interactions, and aggregation behaviors in the nanocrystalline bulk heterojunction (BHJ) domains undertake the tasks for future materials design for efficient solar cells, especially in understanding the structure–property relationship of isomeric non-fullerene acceptors (NFAs). Theoretical calculations reveal that 2ClIC-βδ, with β- and δ-chlorine-substituted terminal groups, achieves a relatively higher dipole moment for enhanced intermolecular interactions. More importantly, when comparing the single-crystal X-ray diffraction patterns of three isomeric NFAs, BTIC-BO4Cl-βδ, BTIC-BO4Cl-βγ, and BTIC-BO4Cl, the synergistic effect of chlorine atoms at the β- and δ-positions endows BTIC-BO4Cl-βδ better molecular planarity with a dihedral angle of 1.14°. In turn, this creates the shortest π∙∙∙π distance (3.28 Å) and smallest binding energies (−51.66 kcal mol−1 ) of the three NFAs, resulting in the tightest three-dimensional network packing structure with a framework of Lx =14.0 Å and Ly =13.6 Å. Such a structure has multiple intermolecular interactions for better charge transfer. However, the chlorine atom at the γ-position in the other two isomers contributes to non-intermolecular interactions with subordinate packing arrangements. Subsequently, the red-shifted UV-absorption and higher electron mobility observed in neat films of BTIC-BO4Cl-βδ agree well with its more ordered crystallinity. This leads to a more suitable fiber-like phase separation in the corresponding active blend, ultimately improving the device performance with superior charge transport. As a result, the highest power conversion efficiency of 17.04% with a current density of 26.07 mA cm−2 was obtained with the BTIC-BO4Cl-βδ-based device. The carrier dynamics test and grazing incidence wide-angle X-ray scattering measurement indicate that the packing arrangement of molecules in the nanocrystalline BHJ domains is consistent with their crystallinity. This work investigates the structure–property differences in three acceptors and emphasizes the effect of isomeric chlorine substitution, which suggests that changes in the crystal packing arrangement, especially the size of the framework, have a considerable influence on charge carrier transport and ultimately are reflected on the device efficiency elevation.