Two-dimensional transition-metal dichalcogenide–based bilayer heterojunctions for efficient solar cells and photocatalytic applications

We present a first-principles investigation of the optoelectronic properties of vertically stacked bilayer heterostructures composed of 2D transition-metal dichalcogenides (TMDs). The calculations are performed with density-functional theory as well as many-body perturbation theory within the ${G}_{0}{W}_{0}$--Bethe-Salpeter-equation method. Our aim is to propose these TMD heterostructures for potential applications in solar cells. The TMD monolayers constituting the heterojunctions considered in this research are ${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$ monolayers due to their favorable band gaps, high carrier mobility, robust absorption in the visible region, and excellent stability. These four TMD monolayers provide the basis for a total of six potential heterostructures (${\mathrm{WS}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{WS}}_{2}$, ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{Se}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{WS}}_{2}$) whose structural, electronic, and optical properties have been studied in this work. At the density-functional-theory level, all six TMD heterostructures considered meet the essential criterion of type-II band alignment, a critical factor in extending carrier lifetime. However, according to ${G}_{0}{W}_{0}$ results, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{WS}}_{2}$ does not exhibit type-II band alignment; instead it shows type-I band alignment. The significantly large quasiparticle band gaps obtained from the ${G}_{0}{W}_{0}$ approximation suggest the presence of strong electron-correlation effects. The heterostructures studied exhibit superior optoelectronic properties compared with their respective isolated monolayers. Quite-significant values of the intrinsic electric fields that arise due to the asymmetric geometry of the heterostructures are obtained. Additionally, the small and nearly equal electron and hole effective masses obtained indicate high mobility and efficient charge-carrier separation, resulting in low recombination losses. The quality of these heterojunction solar cells is estimated by computing their power-conversion efficiencies (PCEs). The PCEs are calculated at both the HSE06 level and the ${G}_{0}{W}_{0}$ level, and the maximum PCE predicted by HSE06 calculations on our designed solar cells is 19.25% for the ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{WS}}_{2}$ heterojunction. In addition, all six TMD heterostructures are examined for their potential applications in photocatalysis for the hydrogen-evolution reaction, and three of them---namely, ${\mathrm{WS}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$ heterostructures---qualify as photocatalysts.