Engineering Inter‐Octahedral Spacing in Bismuth Halide Perovskites: Ultra‐Low Voltage UV Photodetection with 2‐Fluorobenzylamine Bismuth Iodide

ABSTRACT Zero‐dimensional bismuth halide perovskites face a fundamental challenge: electronically isolated octahedral structures suppress dark current but impede charge transport, necessitating high operating voltages. Strategic organic cation engineering bridges this transport‐noise trade‐off through controlled inter‐octahedral spacing. 2‐Fluorobenzylamine bismuth iodide ( 2FBABI ) features face‐sharing [Bi 2 I 9 ] 3− dimers with I···I contacts of 4.17 Å (97% of van der Waals sum), placing the material in an intermediate structural regime between fully isolated and strongly coupled octahedral units. Planar FTO/ 2FBABI /FTO photodetectors operate at ultra‐low voltages (0.001–0.09 V, lowest among biased bismuth‐based devices) with 0.08 pA dark current. Under 365 nm illumination at 0.09 V, devices achieve 11.5 A/W responsivity, 3880% external quantum efficiency from photoconductive gain, 1.03 × 10 13 Jones detectivity, 125 dB linear dynamic range, and 85/150 ms response times. Controlled inter‐octahedral distance engineering represents a viable strategy for optimizing lead‐free UV photodetectors.