Imidazole‐Extended Non‐Fullerene Acceptors With Tunable Crystallinity for Ultrasensitive, Ultrafast Near‐Infrared Organic Photodetectors

ABSTRACT Emerging optoelectronic tasks such as night‐vision imaging and wearable health monitoring call for near‐infrared (NIR) photodetectors that operate without external bias, exhibit minimal dark current, and respond within nanoseconds. Organic photodetectors (OPDs) have yet to meet these benchmarks, hindered by the scarcity of NIR‐absorbing molecules and poor control over active‐layer crystallinity. Here we report a unified approach that couples molecular engineering, morphology tuning, and trap‐state suppression to accelerate charge dynamics in OPDs. Three non‐fullerene acceptors‐CA1, CA2, and CA3 were constructed by extending the π‐backbone of an imidazole‐centered core. DFT calculations and single‐crystal analyses show that each additional fused ring systematically modulates backbone planarity and intermolecular packing, which in turn governs charge mobility and defect density. CA2 strikes an optimal balance: its moderate rigidity and favorable surface energy match the donor polymer, producing a bicontinuous fibrillar network with low energetic disorder. Conversely, the highly crystalline CA1 readily self‐aggregates, introducing grain boundaries that act as recombination centers. Consequently, the PM6:CA2 blends deliver outstanding OPD performance, with a record‐low dark current of 2.54 × 10 −12 A cm −2 at zero bias. They also feature a total noise current of 8.51 × 10 −15 A·Hz −1/2 , yielding a detectivity peak of 1.08 × 10 13 Jones (specific detectivity from real noise) at 840 nm. In addition, the devices exhibit a wide linear dynamic range of 167.8 dB and fast transient responses of 307 ns (rise) and 319 ns (fall). The detector enables cuff‐free deep‐tissue photoplethysmography and a 10 Mbps optical data link, underscoring the potential of imidazole‐extended acceptors for sensitive, ultrafast NIR OPDs.