The shift current photovoltaic effect response in monolayer MX2 (M=Mo, W; X=S, Se, Te) via first-principles calculation

Abstract

Recently, the two-dimensional (2D) bulk photovoltaic effect (BPVE) in non-centrosymmetric materials has garnered significant attention due to its potential for highly efficient 2D solar cells and optoelectronics. The shift current, a key mechanism underlying the BPVE, is crucial for designing the enhanced photovoltaic performance. However, the intrinsic shift current in these materials has not yet been comprehensively and unambiguously studied. Here, we investigate the shift current in monolayers MX₂ (where M = Mo, W, and X = S, Se, Te) materials, which lack spontaneous polarization, using both tight-binding model analysis and first-principles calculations. We construct a tight-binding model based on the local wannier90 functions, incorporating three (d_z², d_x²_−y², and d_xy), five, seven, nine, and eleven bands (without or with spin-orbit coupling) to explore the band structure of monolayer MX₂. Our findings indicate that increasing the fitted number of bands improves the accuracy of the model, although fewer bands can still yield meaningful approximations. Specifically, the shift current of the S and Se compounds exhibits a distinct double-peak structure within the visible light range, whereas the Te compounds display no obvious peak structure. By analyzing the partial density of states, we find that the double-peak structure corresponds to the peak positions of the Mo atom's d_x²_−y² and d_xy orbitals, as well as the S atom's p_y and p_x orbitals. Additionally, the location of the double peak coincides with the discontinuity point of the joint density of states. Our study elucidates the energy density of states to understand the origin of the shift current peak and offers a theoretical basis for the design of 2D bulk photovoltaic effects.