Bi2O2Se has been known as a promising thermoelectric material with low thermal conductivity. Detail understanding of band structure is therefore important. In this report, by employing first-principles density functional theory and using primitive unit cell, the electronic band structure of Bi2O2Se is examined. The compound is found to be a narrow band gap semiconductor with very flat bands at the valence band maximum (VBM). Nevertheless, the curvature of energy surface at VBM is directional dependent. Overall, the heavy bands at VBM do not reduce drastically electrical conductivity. It is demonstrated by utilizing the solution of Boltzmann Transport Equation to compute the transport coefficients, i.e. the Seebeck coefficient, the electrical conductivity thereby the power factor and the electronic thermal conductivity. The figure of merit of the compound is also estimated and discussed. The p-type doping is suggested increasing the thermoelectric performance of the compound. All results are in good agreement with experiments and calculations reported earlier.

Bi\(_{2}\)Te\(_{3}\) and its alloys are the well-known state-of-the-art thermoelectric materials operating at around room temperature. With lead substituted, the newly formed quasi-binary compound PbBi\(_{4}\)Te\(_{7}\), shows relatively high electrical conductivity and Seebeck coefficient. In this report, we employed the solution of the Boltzmann Transport Equation in a constant relaxation-time approximation within a first-principles density-functional-theory calculation to explore the role of the electronic thermal conductivity, \(\kappa _{e}\), on the thermoelectric performance of the compound with p-type doping. Results show that \(\kappa _{e}\) increases drastically with the increases of both temperature and carrier concentration. Even the power factor has been found to be markedly improved with the increase of the carrier concentration, a rapid increase of \(\kappa _{e}\) emerges as a big hindrance to improve the dimensionless figure of merit, ZT, of the compound. This is responsible for the limit of ZT. The larger ZT is found in low temperatures and carrier concentrations. The highest ZT of about 0.48 occurs at 223 K and at the carrier concentration of \(6\times 10^{17}\)cm\(^{ - 3}\). At room temperature the maximum ZT is slightly smaller. We demonstrated that at a particular temperature to maximize the thermoelectric performance of the compound, the carrier concentration must be optimized. Results show that the compound with p-type doping is a promising thermoelectric materials operating at around room temperature.

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Physical properties of the Si1-xGex alloys (x being the composition of Ge) can be understood and predicted from their electronic band structures. In this paper, electronic band structures of the Si1-xGex alloys are calculated using the first-principles density functional theory. The supper cell approach employed in our calculations leads to the folding of electronic bands into the smaller Brillouin zone of the supercell, especially at the Γ point. This often leads to the misinterpretation that the materials have direct band gap. The problem can be resolved by the band unfolding technique which allows one to recover the primitive cell picture of band structure of Si1-xGex. As a result, unfolded electronic bands correctly show an indirect band gap with the valence band maximum (VBM) at the Γ point and the conduction band minimum (CBM) shifted away from Γ. The CBM is gradually shifted from a point along ΓX path (associated with Si) to the L point (associated with Ge) with the increased Ge composition x and the switching occurs at x in the range of 0.6~0.8 which is in accordance with the calculation using kp method. Moreover, the additional electron pockets appear and develop at Γ and L. This provides more comprehensive understanding for our recent experimental observations on the shift of the absorption energy assigned to E1 direct transitions within L and Γ points in the Brillouin zone of Si1-xGex alloy nanocrystals.