Enhancing singlet excitons delocalization via selective asymmetric fluorination of electron acceptors for efficient organic solar cells

Abstract

Exciton dissociation in organic solar cells (OSCs) is primarily achieved through interfacial charge-transfer (CT) states, leading to a trade-off between open-circuit voltage (<i>V</i>_OC) and short-circuit current (<i>J</i>_SC). Spatially dispersed delocalized singlet excitons (DSEs) in nonfullerene acceptors (NFAs) provide an alternative channel to promote charge generation without interfacial CT state. Here, we manipulate intermolecular interactions, carrier dynamics, and photovoltaic properties through selective asymmetric fluorination. Two asymmetric molecules, Z12 and Z13, were synthesized by substituting the terminal group with different fluorine atoms compared with the symmetrical molecule, Z11. Z12 showed enhanced molecular interactions, promoting to more compact and ordered stacking, which in turn promotes the DSE formation, benefiting the synergistic enhancement of <i>V</i>_OC and <i>J</i>_SC. The D18:Z12-based device achieved a remarkable power conversion efficiency of 19.5%, notably outperforming the other two devices. Our study indicates that controlling the molecular configuration by selective fluorination to enhance the DSE formation in NFAs is an effective strategy to achieve efficient OSCs.