Type-II GeS/Ta ₂ NiSe ₅ Heterojunction Photodetector with Effectively Suppressed Dark Current

The emergent two-dimensional (2D) narrow bandgap ( E g ) semiconductors are able to absorb at all visible wavelengths, possessing unique photoelectric properties with great potential in the field of photodetection and optical imaging. However, materials with narrow E g usually suffer from a large internal dark current, which deteriorates their photoelectric performance. The built-in electric field ( E b ) in the heterostructure can be effectively utilized to suppress the dark current of the device. Herein, we report the study of a type-II heterojunction device designed by vertically stacking low-dark-current GeS and high-photosensitivity Ta 2 NiSe 5 flakes. The GeS/Ta 2 NiSe 5 heterojunction is found to exhibit largely suppressed ultralow dark current of down to 10 pA compared to that of the single Ta 2 NiSe 5 device and significantly enhanced photocurrent and photoresponse speed compared to that of the single GeS devices. Particularly, under illumination with a 405 nm laser, the device demonstrates a responsivity of up to 92.16 A/W, a detectivity of 2.23 × 10 11 Jones, and a photocurrent on/off ratio around 10 3 . Besides, the device can operate effectively in self-powered mode as well, with a responsivity of 11 mA/W. Indeed, the GeS/Ta 2 NiSe 5 heterojunction demonstrates optical imaging with clear contrast in the visible range, showing excellent fatigue reliability and humid resistivity, highlighting its potential for future high-sensitivity, high-stability photodetecting systems. This work emphasizes band engineering as a strategy for optimizing the photoelectronic properties of 2D heterostructures while maintaining the advantages of single material, serving as a reliable reference for the design and development of future high-performance photoelectronics.