Recently, near-infrared (NIR) photodetectors are gaining increasing attention due to the significant development of automotive vehicles, smart phones, machine vision, augmented reality, and so on. Particularly, some high-throughput applications such as optical communication and computed axial tomography also require NIR photodetectors to response fast. In the wavelength region between 850 and 1100 nm, where LiDAR (light detection and ranging) technology is operated, silicon photodetectors dominate the current market owing to their low cost and easy integration with fabrication procedures in industry. However, silicon photodetectors are usually bulky and require high-temperature purification processes, making them less attractive for wearable devices. In this respect, emerging absorbers including organic materials, lead-halide perovskites, and chalcogenide quantum dots are perfect alternatives to silicon because they can be synthesized using low-temperature solution-processing methods. Nevertheless, these emerging absorbers are also facing different challenges. For instance, organic materials have poor carrier transport, and lead-halide perovskites are not stable in air. Moreover, almost all the efficient NIR photodetectors based on lead-halide perovskites and chalcogenide quantum dots contain Pb or Cd, which raises great concerns on toxicity as well as environmental pollutions. It is also worth noting that only a handful of NIR photodetectors based on organic materials or PbS have reported a cut-off frequency exceeding 300 kHz, indicating that a fast photo-response is still challenging to achieve in these solution-processed photodetectors, especially in the NIR region.

In this work, we aim to develop fast NIR photodetector using a perovskite-inspired material – AgBiS₂, which has an ideal bandgap (~1.2 eV) along with strong absorption coefficients (> 10⁵ cm^-1 in the visible and UV region), and is only composed of RoHS-compliant (low toxicity) elements. We found that AgBiS₂ photodetectors could exhibit high cut-off frequencies either under white light (> 1 MHz) and NIR light (approaching 500 kHz) illumination, which correspond to rise/fall times of only a few microseconds. Such fast photo-response is attributed to the short transit distances of photo-excited charge-carriers in the ultrathin AgBiS₂ layer, where drift transport could dominate. Temperature-dependent transient current measurements also revealed ion migration to play a crucial role in limiting the photo-response speed and increasing the dark currents of AgBiS₂ photodetectors, while its detrimental impacts can be effectively suppressed by carefully tuning the thickness of the AgBiS₂ layer. These outstanding characteristics enable an air-stable, real-time, and solution-processable heartbeat sensor to be realized based on our AgBiS₂ NIR photodetector, demonstrating the great potential of AgBiS₂ devices in high-throughput systems.