Oxygen‐Positional 2D Side‐Chain Engineering of <i>n</i> ‐Type Acceptors for Record &gt;90% Near‐Infrared External Quantum Efficiency in Broadband Perovskite–Organic Photodetectors

ABSTRACT Side‐chain engineering, a powerful approach to tune molecular properties and charge transport, has to the best of our knowledge never been applied to n ‐type semiconductors in perovskite‐organic heterojunction photodetectors (POH‐PDs). Herein, we report two n ‐type non‐fullerene acceptors (Y1PhO and Y2PhO), featuring 2D‐conjugated outer side chains in which a single oxygen atom is incorporated at distinct positions. The oxygen‐position‐tuned 2D‐conjugated chains afford precise control over bulk photophysics and buried‐interface energetics in POH‐PDs. Relative to the benchmark non‐fullerene acceptor Y6, both molecules exhibit larger dipole moments, higher dielectric constants, and up‐shifted frontier‐orbital energies. The relaxed backbone planarity serves to inhibit over‐aggregation, yielding smoother bulk‐heterojunction blend films and superior interfacial coupling with CsFA perovskite layer, most notably in PM6:Y2PhO blend. As a consequence, the Y2PhO‐based POH‐PD delivers a near‐infrared external quantum efficiency exceeding 90%−the highest value reported for solution‐processable broadband PDs to date− together with a high responsivity of 0.623 A W −1 , shot‐noise‐limited and noise‐based detectivities of 7.05 × 10 12 and 1.43 × 10 11 Jones, respectively, at 830 nm, and a linear dynamic range of 109.1 dB. These performance metrics significantly surpass those of the Y6‐based counterpart, establishing oxygen‐position engineering as a compelling molecular design strategy for next‐generation, ultrahigh‐sensitivity broadband photodetectors.