Tuning Novel NaLaS_2(1–x)(Se or Te)_2x Alloys as Light-Absorbing Materials by Dopant-Induced Crystallographic Phase and Electronic Structure Transitions

The electronic and optical properties of a NaLaS2 compound, doped with different concentrations of Se and Te atoms, are explored using periodic density functional theory calculations. The primary objective is to identify light-absorbing materials for use in photovoltaic applications. It is found that the dopant concentration can induce a crystalline structure phase transition from cubic to trigonal, which is accompanied with drastic changes in both the nature (direct/indirect) and extent of the energy band gap. As the proportion of selenium atoms in the NaLaS2(1–x)Se2x alloys increases from 0 to 100%, the gap decreases from 2.93 to 2.41 eV, respectively. For the NaLaS2(1–x)Te2x alloys, the gap undergoes a (sharp) decrease reaching as low as 1.75 eV as the proportion of tellurium increases to 75%. The transport properties of the alloys reveal a significant drop in the effective mass and exciton binding energies, which is particularly marked for the NaLaS2(1–x)Te2x alloys with a Te concentration range of 25–100%. The exciton binding energy for NaLaS0.5Te1.5 is 4.37 meV, much less than the thermal energy at room temperature (25 meV). Therefore, the NaLaS0.5Te1.5 alloy shows potential as a light-absorbing material for photovoltaic applications that is worthy of further investigations.