Largely enhanced bulk photovoltaic effects in a two-dimensional MoSi2N4 monolayer photodetector by vacancy-doping and bending-increased device asymmetry†

Two-dimensional MoSi₂N₄ monolayer semiconductors have garnered significant research attention for ultraviolet (UV) photodetection due to their outstanding performance, including ultrafast response, high responsivity, and low dark current. Using quantum transport simulations, we proposed a kind of self-powered, highly ultraviolet-sensitive polarized photodetector driven by the bulk photovoltaic effect (BPVE) based on α₁- and α₂-MoSi₂N₄ monolayer semiconductors. Here, we systematically investigated the photoelectronic properties of the MoSi₂N₄ monolayer with bending angles θ of 10°, 20°, and 30°, as well as vacancies in Mo, Si, and N atoms. The BPVE photocurrent can be generated in the MoSi₂N₄ monolayer under vertical illumination with linearly polarized light. Our results indicate that both bending stress and vacancies can reduce the symmetry of the MoSi₂N₄ monolayer photodetectors, which will result in an enhanced bulk photovoltaic effect and an increase in the photocurrent. Besides, a large and highly polarization-sensitive photocurrent is generated at zero bias voltage, which exhibits a remarkably high extinction ratio (ER) of up to 449 in the photodetector with an Si atom vacancy. Moreover, by applying an appropriate bending stress on MoSi₂N₄, the photocurrent can be substantially enhanced by up to 2 orders of magnitude, which is primarily due to the largely increased device asymmetry. Our findings not only highlight the dependence of the BPVE photocurrent on the device asymmetry during the transport process through a device, but also demonstrate the potential of the BPVE for self-powered flexible optoelectronics and photodetection with high photoresponsivity.