Sodium-based di-chalcogenide: A promising material for tandem solar cells

Compounds based on chalcogen elements are widely studied currently due to their many interesting applications for electronic devices. The sodium-based dichalcogenide (NaNbS$_2$) is a fascinating material with storage and conversion energy applications. In this paper, we conduct a first-principles investigation of the structural and thermodynamic stability and electronic properties of this material. We analyze a total of four structures to find the ground state using a fourth-order Birch-Murnaghan equation of state: the $\alpha$ and $\eta$ related to the A-phase and the $\zeta_{1}$ and $\zeta_{2}$ related to the B-phase. We carefully address the exchange-correlation effects using the semi- local GGA-PBEsol targeted for solids. To analyze the electronic structure with higher accuracy, we implement the quasi-particle G${\textup{o}}$W${\textup{o}}$ approximation. \textcolor{red}{Our results for the fourth-order Birch-Murnaghan equation show that the most thermodynamically stable phase at zero temperature is $\alpha$.} To provide experimentalists insights about the possible routes to grow these materials, we calculated the convex hull of the $\alpha$-model and $\zeta_{1}$-model, finding that both are energetically stable. Finally, the calculated band gap with quasiparticle corrections for the $\alpha$-model is 1.03 eV, which suggests possible applications of this material as a bottom cell in modern solar cells.