Electronic, optical and thermoelectric properties of sodium pnictogen chalcogenides: A first principles study

Ternary chalcogenides have been of recent investigation for applications in photovoltaic and thermoelectric devices. We study the structural, electronic, optical, and thermoelectric properties of nine ternary chalcogenides, NaAX$_{2}$, where A represents pnictogens (As, Sb, and Te) and X represents chalcogens (S, Se, and Te). Calculations based on density functional theory yield the following results: (i) phonon dispersion curves predict three of the compounds, NaAsS$_{2}$, NaSbS$_{2}$, and NaSbSe$_{2}$, to be dynamically stable in the monoclinic, C2/c, structure, (ii) the layered atomistic configuration causes the corresponding electronic and optical properties to display a high degree of anisotropy, (iii) A-X electronic bonding features vary significantly with structural distortions arising from atomic size mismatch, therefore directly influencing stability, (iv) strong absorption is observed in the stable compounds, with coefficients ranging from 10$^{4}$ to 10$^{5}$ cm$^{-1}$ in the visible-UV range, and (v) remarkably high Seebeck coefficients exceeding 500 ${\mu}$V/K at carrier concentrations commonly achieved in such materials. From these results, we conclude that NaAsS$_{2}$, NaSbS$_{2}$, and NaSbSe$_{2}$ are suitable candidates for both photovoltaic, particularly in tandem solar cells, and thermoelectric applications. Experimental synthesis and verification are suggested.