An H ‐Aggregation Mediator Enhances Photoluminescence Quantum Yield and Crystallinity for Efficient Nonannealed Organic Solar Cells

ABSTRACT Organic solar cells (OSCs) hold promise for building‐integrated and wearable photovoltaics, yet excessive H ‐aggregation of non‐fullerene acceptors (NFAs) quenches luminescence and lowers the photoluminescence quantum yield (PLQY), limiting further efficiency gains. Here, we introduce NTCl‐4F, a molecular H ‐aggregation mediator, into the D18:L8‐BO‐C 4 binary system. Designed with an identical backbone to L8‐BO‐C 4 but featuring a rotatable central core that induces steric hindrance (dihedral angle of 4.87° as revealed by single‐crystal diffraction), NTCl‐4F effectively suppresses H ‐aggregation while promoting ordered J ‐aggregation (with average fibril diameters from 20.75 to 17.16 nm). This aggregation‐mode modulation enhances the PLQY from 6.13% to 7.52%. Furthermore, the ordered J ‐aggregation induced by NTCl‐4F enhances the crystallinity of the system with increased crystal coherence lengths (CCL) and decreased fractal dimension ( D f ), achieving synchronous improvement of short‐circuit current. Consequently, the NTCl‐4F‐modified device achieves a power conversion efficiency (PCE) of 20.8% under non‐annealed conditions, which stands among the highest reported efficiencies for non‐annealed OSCs. The strategy proves general, yielding efficiency improvements in PM6:Y6 and D18:Y7 systems. This work establishes aggregation‐mode engineering via an H ‐aggregation mediator as a versatile route to enhance PLQY and crystallinity for high‐performance OSCs.