Asymmetric π-Bridge Strategy Enables High-Sensitivity Flexible NIR Organic Photodetectors for In Situ Ultra-Trace...

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 × 10¹³ 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.