Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYSTe Alloys.

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