Trifluoromethylthio Engineering Enables a Balanced Electrostatic Potential for Organic Solar Cells with over 20% Efficiency and Low Energy Loss

Fluorination is widely used to regulate the molecular electrostatic potential (ESP) and packing of nonfullerene acceptors (NFAs) in organic solar cells (OSCs). Current strategies mainly involve direct fluorination or trifluoromethylation, but high-performance NFAs still suffer from large exciton binding energy (Eb) and nonradiative energy loss (ΔE3). Herein, we introduce a novel fluorinated group, trifluoromethylthio (SCF3), into the quinoxaline core of an A-DA'D-A-type NFA, combined with alkyl side-chain engineering, to construct three new acceptors, ZY-SCF3-EH, ZY-SCF3-BO, and ZY-SCF3-HD. Theoretical calculations reveal that SCF3 possesses moderate electron-withdrawing ability, affording a favorable ESP to balance exciton dissociation and ΔE3 suppression. Besides, SCF3-based acceptors exhibit smaller hole-electron overlap, larger centroid distances, and lower Eb, which are beneficial for further suppressing energy loss. Experimentally, the high lipophilicity of SCF3 enhances solubility and solution processability, while multiple noncovalent interactions induce a three-dimensional honeycomb-like packing network that improves charge mobility. Consequently, the side-chain-optimized acceptor ZY-SCF3-HD delivers a superior power conversion efficiency (PCE) of 19.4%, with simultaneously improved open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF), alongside a low ΔE3 of 0.23 eV. A ternary blend further boosts the efficiency to 20.1%. Notably, a 20.25 cm2 module fabricated at room temperature achieves an impressive efficiency of 16.4%, ranking among the highest values reported to date. This work illustrates SCF3 as an effective fluorinated unit for constructing high-performance NFAs, providing a new strategy to simultaneously achieve high VOC, JSC, FF, and low ΔE3, and offering new design insights for next-generation high-efficiency OSCs.