Nitration–Chlorination Synergy Enables 20% Efficiency and Long‐Term Stability in Q‐PHJ Organic Photovoltaics

ABSTRACT Achieving both high efficiency and long‐term stability in organic solar cells (OSCs), particularly those based on non‐fullerene acceptors (NFAs) with emerging structures, remains a major challenge. Here, we propose a synergistic design strategy that combines nitration and chlorine‐mediated interactions to enhance the photovoltaic performance of quasi‐planar heterojunction (Q‐PHJ) OSCs. The nitrated NFAs, NO 2 Q‐2Cl and NO 2 Q‐2F, show blue‐shifted absorption compared to the benchmark molecule BTP‐eC9, along with improved molecular packing and crystallinity. In particular, the introduction of chlorine‐mediated strategy in NO 2 Q‐2Cl enables preferable miscibility with the donor D18. As a result, D18/NO 2 Q‐2Cl‐based devices achieve a record Q‐PHJ efficiency of 20.0%, outperforming all previously reported Q‐PHJ systems and ranking among the highest for nitrated or chlorinated NFAs to date. Moreover, these devices exhibit excellent photostability, retaining 80% of their initial efficiency ( T 80 lifetime) after 2982 h of continuous light exposure, far surpassing many of the latest systems. This work demonstrates the effectiveness of the combined strategy and offers valuable insights for advancing high‐performance OSCs.