Naphthalene‐Extended Star‐Shaped Trimer Acceptor Simultaneously Enabling Over 20% Efficiency and Enhanced Operational Stability in Organic Solar Cells

ABSTRACT Oligomeric acceptors have emerged as one of the promising alternatives for constructing high‐performance and stable organic solar cells (OSCs). Among them, star‐shaped oligomers are particularly noteworthy due to their unique 3D architecture, which facilitates multi‐dimensional molecular packing and charge transport. Nevertheless, this class of materials has received relatively limited attention to date, and molecular design strategies to optimize their photophysical and morphological properties remain underexplored. Herein, we designed and synthesized a novel star‐shaped trimeric acceptor, T‐NIC, based on a benzotrithiophene (BTT) central core and conjugated‐extended naphthalene‐based terminal groups. Compared to its benzene‐capped analogue T‐IC, T‐NIC exhibits broadened absorption, enhanced molecular packing, and a higher glass transition temperature ( T g). The binary device PM6:T‐NIC exhibits a power conversion efficiency (PCE) of 18.94% and demonstrates exceptional stability by maintaining 80% of its initial performance over 1380 h of continuous illumination. Crucially, the introduction of T‐NIC into the PM6:BTP‐eC9 host system results in a ternary device with a high PCE of 20.36%. The observed efficiency enhancement originates from the synergistic interplay of an optimized blend morphology and enhanced molecular packing. This study demonstrates that terminal group conjugation extension in trimer acceptors offers a powerful and synergistic strategy for simultaneously increasing the efficiency and device stability.