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

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 [Bi2I9]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 × 1013 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.