AgGaTe₂ – The thermoelectric and solar cell material: Structure, electronic, optical, elastic and vibrational features

Silver gallium telluride AgGaTe₂ is a promising and one of few multifunctional energy convertor crystal for processing solar energy or waste heat into electricity (by the thermoelectric, photovoltaic and recently intensively studied thermophotovoltaic systems). Its electronic structure, optical, elastic and vibrational properties were consistently and systematically studied in the framework of the first principles calculations. It should be emphasized that the results of our investigations were verified by comparison with the collected available experimental and vestigial theoretical data. The main spectral features of the electronic band structure and origin of the electronic levels affecting the physical and chemical properties are analyzed in detail. Spectral dependencies of both real ε₁ and imaginary ε₂ components of dielectric function ε(ω) have been calculated showing a good agreement with spectral ellipsometry measurements. The anisotropy of optical and elastic properties, which is important for possible applications, has been highlighted by means of the 2D and 3D graphs. Phonon dispersion curves and corresponding density of states are studied in the frame of linear response approach based on the density functional perturbation theory. Unified approach focused on the analysis of a wide-range physical properties of anisotropic crystals not only results in good agreement with the experimental data, but also allows to explain the observed optical effects at the microscopic level. Therefore, the presented results should be very useful for future possible improvements in the desired properties of the base material both by changing the chemical composition and/or introducing stresses by epitaxial growth of thin films.