Asymmetric π‐Bridge Strategy Enables High‐Sensitivity Flexible NIR Organic Photodetectors for In Situ Ultra‐Trace Water Pollutant Detection

The in situ monitoring of water pollutants demands mechanically compliant, highly sensitive, real‐time monitoring platforms that operate reliably under practical conditions, yet conventional analytical techniques are bulky, rigid, and unsuitable for on‑site deployment. Herein, we design a flexible near‐infrared organic photodetector (NIR‐OPD) operating over 300–1100 nm, enabled by an asymmetric π‐bridge molecular engineering strategy. Incorporating an asymmetric π‐bridge into the non‐fullerene acceptor backbone affords acceptor ZCH‐12, which exhibits enhanced molecular planarity, improved intermolecular π‐π interactions, and reduced energetic disorder relative to its symmetric analogue. The optimized device exhibits a specific detectivity of 5.52 × 10 13 Jones, a responsivity of 0.51 A W −1 at 908 nm, and an ultralow dark current of 2.21 × 10 −12 A at 0 V. Benefitting from the solution processability and mechanical compliance of organic semiconductors, the flexible device can conform to curved surfaces for on‐site water analysis. The resulting device achieves detection limits of 0.2 mg L − 1 for plastic particles, and 0.2, 0.1, and 0.03 mg L −1 for Congo red, chromium ions, and Malachite green, respectively, surpassing relevant environmental standards. This work establishes molecular asymmetry as an effective design principle for high‐sensitivity NIR OPDs and offers a versatile platform for real‐time ecological monitoring.