An H-Aggregation Mediator Enhances Photoluminescence Quantum Yield and Crystallinity for Efficient Nonannealed...

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-C₄ binary system. Designed with an identical backbone to L8-BO-C₄ 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.