Ultrahigh Photo‐Responsivity and Detectivity in  2D Bismuth Sulfide Photodetector for Vis–NIR Radiation

Bismuth sulfide (Bi₂S₃) exhibits a direct energy bandgap and an exceptional optical absorption capability over a broadband radiation, thus presents a novel class of 2D photodetector material. The field effect transistor (FET) photodetector device is fabricated from 2D Bi₂S₃. An anomalous variation in the transport characteristics of 2D Bi₂S₃ is observed with the variation in temperature. The electrical resistance reduces by 99.26% at 10 K compared to the response at 300 K. Defects due to the bismuth and sulfur vacancies play a critical role in the dramatic behavior, which is confirmed using photoluminescence, time-resolved photoluminescence, Hall measurements, and energy dispersive X-ray spectroscopy. The density functional theory calculations provide a significant insight into the thermodynamic properties of intrinsic defects in Bi₂S₃. Moreover, the effect of gate bias on responsivity additionally confirms its invariance at low temperature. The Bi₂S₃ based FET photodetector achieves ultrahigh responsivity in the order of ≈10⁶ A W^-1 and detectivity of ≈10¹⁴ Jones. Moreover, the external quantum efficiency of ≈10⁷% is measured in a wide spectrum of optical illumination (532 to 1064 nm) with a noise-equivalent power of 3.5 × 10^-18 W/√Hz at a bias of 0.2 V. The extraordinary performance of Bi₂S₃ photodetector outstands 2D photodetectors.