Study of optoelectronic and thermoelectric spectra of Tl(Nd/Gd)S 2

Present research work is devoted to the study of optoelectronic spectra and thermoelectric properties of thallium-based neodymium and gadolinium sulphides (viz. TlNdS 2 and TlGdS 2 ) using FP-LAWP method within DFT and semi-classical Boltzmann theory. Band structure calculations show that both the compounds are indirect bandgap semiconductors with wide bandgap ~ 1.61 eV for TlNdS 2 and 1.47 eV for TlGdS 2 under TB-mBJ approximation. The appearance of most prominent peak of real dielectric constant at photon energy, E ~ 5.0 eV and ~ 4.0 eV for TlNdS 2 and TlGdS 2 respectively in ultraviolet UV regime, indicates high optical response of the materials towards dielectric polarization for (UV) radiation. The high peak of refractive index at ~ 5.0 eV indicate that both the compounds act as optically denser medium and they are more photon absorbent for ultraviolet (UV) light. Refractive index data satisfy the Penn formula for real part of dielectric function that legalizes our optical calculations. The maximum value of reflection coefficient (~ 45–60%) has been found in UV regime and the observed threshold energies are ~ 1.61 eV and 1.47 eV for TlNdS 2 and TlGdS 2 , respectively. The thermoelectric (TE) properties show that both the compounds are p-type semiconductor with figure of merit (ZT) ≥ 1 and prove that both the compounds are excellent candidates for thermoelectric devices. Also, both the compounds have maximum power factor (PF = 6 × 10 11 for TlNdS 2 and 1.1 × 10 12 for TlGdS 2 ) at high temperature (900 K) confirms that these compounds can be used for higher temperatures thermoelectric applications.