First-principles calculations of electronic and optical properties of AgGa1-xTlxS2 alloys: Analyses and design for solar cell applications

Abstract

The thallium substitution effect on structural, electronic, and optical properties of chalcopyrite-type material AgGaS2 is investigated via first-principles approaches. The computed band gap energy of pure AgGaS2 is about 2.59 ​eV using Tran-Blaha (TB)-modified Becke Johnson (mBJ) exchange potential, which is in good accord with experimental measurements. This band gap value was observed to decrease to be 1.91 ​eV, 1.61 ​eV, and 1.28 ​eV when Tl is substituted into Ga site, respectively, for 25%, 50%, and 75% concentrations. Besides, the AgTlS2 band gap was also calculated and found to be 0.61 ​eV. This investigation establishes that Tl substitution increases both hole and electron carrier mobility of the pure AgGaS2 compound. By analyzing the band alignment diagram, it was observed that the Tl substitution increases the valence band offset (VBO) and decreases the conduction band offset (CBO), which can lead to the improvement of open-circuit voltage VOC. Moreover, the optical analysis reveals that Tl substitution enhances the optical properties of AgGaS2 by reducing the transparency and improving the refractive index and the absorption in the visible light region. Based on obtained results, it is predicted that the band gap and the optical properties of the AgGaS2 chalcopyrite can be effectively tuned by Tl substitution over the Ga sites, making AgGa1-xTlxS2 alloys promising candidates for optoelectronic and photovoltaic applications.