Easy Isomerization Strategy for Additives Enables High-Efficiency Organic Solar Cells

The morphological features of the active layer has always been an important factor limiting the efficiency of organic solar cells (OSCs). Although halogen-based additives capable of forming non-covalent bonds with active molecules can effectively adjust the morphology of active layer, the inner mechanism of positional isomerization of additives on the crystallization kinetics of bulk heterojunction has been ignored, which hinders further development of this technique. Herein, a new additive-assisted optimization strategy for high-efficiency OSCs based on three positional isomeric additives is proposed, which have different sites for two bromine substituents on the benzene ring. The results demonstrate that symmetrically structured additives with the smallest dipole moment, the lowest steric hindrance and the most uniformly distributed electrostatic potential, can form more suitable non-covalent interactions with the acceptor, resulting in more reasonable molecular spatial distribution and better π–π stacking behavior. For other non-fullerene systems, the symmetrically structured additive also shows the best effect on optimizing molecular aggregation and stacking. This work provides guidance for screening and designing additives with excellent morphology improvement capability and is expected to have a profound influence on further increasing efficiency of OSCs without adopting the isomerization strategy for active molecules possessing complex conjugated backbones and branched chains.