Band alignments and heterostructures of monolayer transition metal trichalcogenides MX₃ (M = Zr, Hf; X = S, Se) and dichalcogenides MX₂ (M = Tc, Re; X=S, Se) for solar applications

Abstract

Knowledge of band alignments and heterostructure formations is fundamental for a new generation of optoelectronics based on two-dimensional layered materials. Herein, band alignments and heterostructures of IVB-VIA monolayer MX₃ (M = Zr, Hf; X = S, Se) and VIIB-VIA monolayer MX₂ (M = Tc, Re; X = S, Se) are calculated by density functional theory with hybrid functionals. The results indicate that for monolayer MX₃, the valence bands mainly depend on the p state of the chalcogens and the conduction bands mainly depend on the d state of the transition metals. In contrast, for monolayer MX₂, both valence and conduction bands depend on the d state of the transition metals. This suggests that their work functions are obviously different. Meanwhile, the characteristics of the band alignments and the planar-averaged local density of states show that ZrS₃, HfS₃, TcSe₂ and ReS₂ could be favorable candidates for photocatalytic water splitting. ZrS₃, HfS₃ and MX₂ with the same structures are able to form type II heterostructures at their interfaces, which could be used for solar energy conversion. The power-conversion efficiency of an MX₃ thin-film solar cell is approximately 16-18%, which is higher than those of MX₂ thin-film solar cells. In addition, for heterostructures composed of MX₃, both of the two kinds of material (M and X) play an important role in every band formation. Meanwhile, for MX₂ heterostructures, almost every band depends only on a single material. The charge density difference of the heterostructures demonstrates a higher charge accumulation at the interface of MX₃ heterostructures than that of MX₂ heterostructures. These phenomena show that type II heterostructures formed of MX₃ are more stable than those of MX₂.