Altering the Positions of Chlorine and Bromine Substitution on the End Group Enables High‐Performance Acceptor and Efficient Organic Solar Cells

Abstract It is widely recognized that subtle changes in the chemical structure of organic semiconductors can induce dramatic variations in their optoelectronic properties and device performance, especially for the nonfullerene small‐molecule acceptors (SMAs). For instance, halogenation of the end groups in the acceptor–donor–acceptor‐type SMAs is an effective strategy to modulate the properties of the end group and thus the entire SMA. While previous position modulations have focused on only one substituent, this study shows the development of three isomeric SMAs (BTP‐ClBr, BTP‐ClBr1, and BTP‐ClBr2) via manipulating the position of two halogen substituents (chlorine and bromine) on the terminal unit. BTP‐ClBr exhibits a blueshifted absorption, a shallower lowest unoccupied molecular orbital energy level, and a weaker crystallization tendency relative to BTP‐ClBr1 and BTP‐ClBr2. A power conversion efficiency (16.82%) and an excellent fill factor (FF) (0.79) are realized in the optimal PM6:BTP‐ClBr organic solar cell device. The higher FF is consistent with the results of the characterization including a longer charge recombination lifetime, a faster photocurrent decay, a weaker bimolecular recombination, and a more favorable domain size for PM6:BTP‐ClBr, which all originate from a subtle change in the substitution sites that strongly influences the physicochemical properties of the SMA.