Performance comparison of hybrid functionals for describing narrow-gap semiconductors: A study on low-temperature thermoelectric material alpha-SrSi2

Narrow-bandgap thermoelectric semiconductors with precisely designed electronic structures are required for highly efficient energy-recycling societies. As a solution to the underestimation of the bandgap value, which is the most serious problem of first-principles calculations based on the density functional theory (DFT) especially for narrow-gap materials, we consider the PBEO functional, Heyd-Scuseria-Ernzerhof screening hybrid functional (HSE06), and recently developed Gaussian-Perdew-Burke-Ernzerhof (Gau-PBE) hybrid functional. The convergence of the energy and bandgap of extremely narrow-gap semiconductor alpha-SrSi2 with respect to the mesh parameters, which are the number of k-point divisions in DFT and Hartree-Fock (HF) exchange integral calculations, was investigated using each functional, and the electronic structure and transport properties were then calculated. Consequently, Gau-PBE provided similar electronic and transport properties to those obtained using the other methods while lowering the calculation costs, i.e., with >80% less computation time and a smaller number of HF mesh points. These results provide insights to the low-cost practical calculation of the precise transport properties of low-temperature thermoelectric materials.