One‐Dimensional 3‐Fluorobenzylamine Bismuth Iodide with Enhanced Inter‐Chain Coupling: Achieving High External Quantum Efficiency in Ultra‐Low Voltage UVC Photodetectors

ABSTRACT Developing efficient, lead‐free materials for deep‐ultraviolet (UVC) photodetection remains a critical challenge for energy‐efficient applications. Here, we report bis(3‐fluorobenzylammonium)pentaiodobismuthate(III) ( 3FBABI ), a one‐dimensional hybrid bismuth halide that achieves high photodetector performance through optimized structural design. Single‐crystal x‐ray diffraction reveals highly ordered zig‐zag [BiI 5 ] 2− chains with short inter‐chain I···I contacts (3.87 Å), enabling enhanced electronic coupling and a direct bandgap of 2.09 eV. Photodetectors fabricated in planar FTO/ 3FBABI /FTO configuration operate at ultra‐low voltages starting from 0.01 V, significantly lower than typical perovskite photodetectors. At 0.2 V bias, the device delivers notable performance metrics: responsivity of 9.65 A/W, specific detectivity of 2.3 × 10 1 2 Jones, and external quantum efficiency of 4710% at 254 nm. The device demonstrates robust environmental stability, maintaining performance after prolonged exposure to air, water, and continuous UV illumination. Comparative analysis with literature shows 3FBABI among the top‐performing lead‐free UVC photodetectors, with operating voltages 50–100 times lower than conventional devices. These results establish 3FBABI as a promising lead‐free alternative for next‐generation low‐power optoelectronic systems, addressing both performance and sustainability requirements for practical UV sensing applications.