Achieving a direct band gap and high power conversion efficiency in an SbI/BiI type-II vdW heterostructure via interlayer compression and electric field application.

Type-II van der Waals (vdW) heterostructures are considered as a class of competitive candidates of high-efficiency photovoltaic materials, due to their spontaneous electron-hole separation. However, most of the vdW heterostructures possess an indirect gap and a large band offset, which would lead to low photon-to-electron conversion efficiency. Taking an SbI3/BiI3 vdW heterostructure as an illustrative example, we propose interlayer compression and vertical electric field application as two effective strategies to modulate the electronic and photovoltaic properties of type-II vdW heterostructures. Our results reveal that a lattice-matched SbI3/BiI3 vdW heterostructure has an indirect band gap of 1.34 eV with the conduction band minimum (CBM) at the Γ point and the valence band maximum (VBM) between the Γ and M points. The power conversion efficiency (PCE) of an SbI3/BiI3-based excitonic solar cell (XSC) is predicted to be about 14.42%. When compressing the heterostructure along the vdW gap direction, the highest valence band state at the Γ point is lifted significantly and the VBM gradually approaches the Γ point, implying an indirect-direct gap transition. This interesting evolution can be attributed to the increasing k-dependent electronic hybridization of the pz orbitals of interlayer adjacent I atoms with a reduced interlayer distance. Moreover, the interlayer compression also enhances the PCE of the system monotonically. When applying a vertical electric field, the band alignment of the heterostructure undergoes a transition from type-II to type-I and then returns to type-II between 0.1 and 0.6 V Å-1. Meanwhile, the PCE of the SbI3/BiI3 XSC could be enhanced up to 21.63%. This work provides guidance for improving the electronic and photovoltaic properties of type-II vdW heterostructures.