Robust type-II band alignment in Janus-MoSSe bilayer with extremely long carrier lifetime induced by the intrinsic electric field

van der Waals (vdW) bilayers have many attractive novel properties that offer an ideal platform for various electronic and optoelectronic applications. However, due to the strong interlayer overlapping, the electron-hole recombination lifetime in vdW bilayers is usually very short, which limits their applications. Here, based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that the Janus-MoSSe bilayer can have a staggered band gap, in which the maximum valence and minimum conduction bands are spatially separated on different layers. Such type-II band alignment is surprisingly robust against external perturbations, such as twisting or stacking modes, which can be ascribed to the intrinsic out-of-plane electric field. Further analysis indicates that due to the existence of such a built-in electric field, the overlapping within the interlayer is suppressed, leading to the completely spacial detachment of the electrons and holes. We then evaluate the photogenerated electron and hole recombination dynamics. The predicted recombination time is extremely long, with a value up to 16.5 ns, which is even longer than that of the ${\mathrm{MoS}}_{2}/{\mathrm{WS}}_{2}$ vdW heterostructure. Our findings propose that introducing an intrinsic electric field is an efficient way to tune the electronic structures of bilayers, which can significantly extend the recombination time of a photoexcited electron and hole, and thus facilitate the design of advanced two-dimensional semiconducting devices.