Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYS_{2(1–<i>x</i>)}Te_2<i>x</i>Alloys

Abstract

New ternary and quaternary NaYS_{2(1-<i>x</i>)}Te_2<i>x</i> alloys (with <i>x</i> = 0, 0.33, 0.67, and 1) are proposed as promising candidates for photon energy conversion in photovoltaic applications. The effects of Te doping on crystal, spectral, and optical properties are studied within the framework of periodic density functional theory. Increasing Te content decreases the band gap (<i>E</i> _g) considerably (from 3.96 (<i>x</i> = 0) to 1.62 eV (<i>x</i> = 0.67)) and fits a quadratic model (<i>E</i> _g(<i>x</i>) = 3.96-6.78<i>x</i> + 4.70<i>x</i> ², (<i>r</i> ² = 0.96, <i>n</i> = 4)). The band gap of 1.62 eV makes the NaYS_0.67Te_1.33 alloy ideal for photovoltaic applications for their ability to absorb in the visible segment of the sunlight spectrum. The calculated exciton binding energies are 9.78 meV for NaYS_1.33Te_0.67 and 6.06 meV for NaYS_0.67Te_1.33. These values of the order of the thermal energy at room temperature suggest an easily dissociable hole-electron pair. The family of NaYS_{2(1-<i>x</i>)}Te_2<i>x</i> alloys are, therefore, promising candidates for visible photocatalytic devices and worthy of further experimental and theoretical investigations.