Contribution to the study of the optoelectronic properties of chalcogenide-based materials for solar cells application

Abstract

This work presents a contribution to the study of the thallium substitution effect on structural, electronic, and optical properties of chalcopyrite-type materials AgGaS2 and AgGaSe2. To predict these different properties, we performed simulations by applying the Linearized Augmented Planar Wave method with local orbitals (FP-APW+lo) which is based on density functional theory (DFT) using the generalized gradient approximation (WC-GGA) to treat the exchange and correlation term. The computed band gap energy of pure AgGaS2 and AgGaSe2 is about 2.59 eV and 1.85eV using Tran-Blaha (TB)-modified Becke Johnson (mBJ) exchange potential, which is in good accord with experimental measurements. Those band gap values were observed to decrease when Tl is substituted into Ga site, respectively, for 25%, 50%, and 75% concentrations. This investigation establishes that Tl substitution increases both hole and electron carrier mobility of the pure AgGaS2 and AgGaSe2 compounds. 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 and AgGaSe2 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 and AgGaSe2 chalcopyrite can be effectively tuned by Tl substitution over the Ga sites, making AgGa1-xTlxS2 and AgGa1-xTlxSe2 alloys promising candidates for optoelectronic and photovoltaic applications.