Ab initio investigation of the structural, optoelectronic, mechanical, vibrational, and thermoelectric properties of the Si_xSn_1−xSe alloys

Single-crystal SnSe holds the world record for high thermoelectric performance. However, the efficiency of polycrystalline SnSe is limited due to high thermal conductivity (κ) and low electronic conductivity (σ) compared to single-crystal SnSe. Here, we report the low κ and high σ on SnSe with Si doping using semi-classical Boltzmann transport theory and first-principles calculations. Our calculated phonon dispersion curves and elastic constants of the Si_xSn_1−xSe alloys (x varied from 0 to 0.25) show the thermodynamic and mechanical stability. Our density functional theory (DFT)-calculated elastic moduli of the α-SnSe match well with theoretical and experiment reports. The electronic properties are calculated using diversity of exchange-correlation functionals. The band gap for pristine α-SnSe is estimated to be 0.88 eV using the hybrid functional (HSE06), which is in excellent agreement with the experimental value 0.86 eV. The G₀W₀+Bethe–Salpeter Equation, which incorporates combined electron–electron (e–e) and electron–hole (e–h) interactions, is used to calculate the optical characteristics of pure and Si-alloyed α-SnSe. Our DFT-calculated Seebeck coefficient for Si_xSn_1−xSe alloys for all Si doping concentrations revealed p-type behavior. At 770 K, for x = 0.25, Si_xSn_1−xSe is found to have the lowest thermal conductivity of ~ 0.16 Wm-1K-1 attributed to the high lattice anharmonicity. Our results propose a novel strategy to enhance the optical and thermoelectric properties in the SnSe-based thermoelectric materials.