Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYS2(1-x)Te2x Alloys

New ternary and quaternary NaYS2(1-x)Te2x alloys (with x = 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 (E-g) considerably (from 3.96 (x = 0) to 1.62 eV (x = 0.67)) and fits a quadratic model (E-g(x) = 3.96-6.78x + 4.70x(2), (r(2) = 0.96, n = 4)). The band gap of 1.62 eV makes the NaYS0.67Te1.33 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 NaYS1.33Te0.67 and 6.06 meV for NaYS0.67Te1.33. These values of the order of the thermal energy at room temperature suggest an easily dissociable hole-electron pair. The family of NaYS2(1-x)Te2x alloys are, therefore, promising candidates for visible photocatalytic devices and worthy of further experimental and theoretical investigations.