Enhanced photoelectric performance of MoSSe/MoS₂ van der Waals heterostructures with tunable multiple band alignment

Janus MoSSe with mirror asymmetry has recently emerged as a new two-dimensional (2D) material with a sizeable out-of-plane dipole moment. Here, based on first-principles calculations, we theoretically investigate the electronic properties of two patterns of 2D MoSSe/MoS₂ van der Waals heterostructures (vdWHs). The electronic properties of MoSSe can be tuned by the intrinsic out-of-plane dipole field. When the Se side of the Janus layer faces the MoS₂ layer, the dipole field points from the MoSSe layer towards the MoS₂ layer, and the vdWH possesses a type-I band alignment which is desirable for light emission applications. With a reversal of the Janus layer, the intrinsic field inverts accordingly, and the band alignment becomes a typical type-II band alignment, which benefits carrier separation. Moreover, it possesses superior optical absorption (∼10⁵ cm^-1), and the calculated photocurrent density under visible-light radiation is up to 0.9 mA cm^-2 in the MoSSe/MoS₂ vdWH. Meanwhile, an external electric field and vertical strain can remarkably modulate the band alignment to switch it between type-I and type-II. Thus, MoSSe/MoS₂ vdWHs have promising applications in next-generation photovoltaic devices.