Mitigating Over-Selenization in AgInS2(Se) via Cationic Stoichiometry Engineering for High-Performance Near Infrared Photodetectors

Gradient-bandgap photodetectors are promising for their superior carrier separation and broadband spectral sensitivity, enabled by the continuous band alignment within the device. However, the controlled formation of gradient bandgap structures in AgInS 2 (Se) is hindered by its rapid selenization, which readily leads to heavily selenized products and thus poor compositional gradation. To address this challenge, this study introduces a non-stoichiometric engineering strategy by precisely modulating the Ag/In ratio. Slightly reducing the Ag/In ratio effectively slows the selenization kinetics due to an increased reaction barrier and film densification, thereby suppressing Kirkendall effect-related defects and strengthening the built-in electric field and breakdown strength. This leads to a dramatic enhancement in the device's responsivity and response speed. The optimized AgInS 2 (Se) thin-film photodetector with Ag/In ratio = 0.9/1.1 achieves high responsivities of 13.64   A·W⁻¹ at 760   nm and 1.075   A·W⁻¹ at 1050   nm under a –2   V bias, with external quantum efficiencies reaching 2226% and 127%, respectively. The device also exhibits a 3   dB bandwidth of 0.33   MHz and fast response times of 0.79 μs (rise) and 3.18 μs (decay). This work offers a novel material design and process control strategy for the scalable fabrication of high-performance gradient-bandgap photodetectors.