A DFT Investigation of the structural and optoelectronic properties of the tetragonal monochalcogenide TlSe for photovoltaics application

In this paper, we examine the application of density functional theory (DFT) to determine the structural and optoelectronic properties of the tetragonal monochalcogenide TlSe, in order to assess its suitability for use in optoelectronic devices, photovoltaics, etc. These calculations are carried out using the full-potential linearized augmented plane wave (FP-LAPW) method, implemented in Wien2k software. The monochalcogenide compound TlSe adopts a tetragonal structure with I4/mcm space group symmetry (No. 140). We determined the ground-state values by calculating the total energy as a function of volume, relaxing the atomic positions for each volume, in order to minimize both the strength and the c/a ratio. Equilibrium structural parameters are derived from the internal structure parameters by fitting the total energy versus volume results with the Birch-Murnaghan equation of state. We studied the electronic properties using two approaches, GGA and TB-mbj. The latter approach gave an energy gap of 0.49 eV, close to the experimental value, which led us to adopt the TB-mbj approach in calculating optical properties such as the complex dielectric function \(\mathrm\varepsilon\left(\mathrm\omega\right)\) $\epsilon \left(\omega \right)$ , complex refractive index \(\mathrm N\left(\mathrm\omega\right)\) $\mathrm{N}\left(\omega \right)$ , optical reflectivity \(\mathrm R\left(\mathrm\omega\right)\) $\mathrm{R}\left(\omega \right)$ , energy loss function \(\mathrm L\left(\mathrm\omega\right)\) $\mathrm{L}\left(\omega \right)$ , optical absorption \(\mathrm\alpha\left(\mathrm\omega\right)\) $\alpha \left(\omega \right)$ and optical conductivity \(\mathrm\sigma\left(\mathrm\omega\right)\) $\sigma \left(\omega \right)$ .