Designing high-performance thermoelectrics in two-dimensional tetradymites

The search for new thermoelectric materials has been of great interest in recent years because thermoelectrics offers useful applications in next-generation vehicles that can directly convert waste heat to electricity. Two-dimensional (2D) tetradymites with M ₂ X ₃ compounds, in which M (Bi) and X (Te, Se, S) are a group-V metal and group-VI anion, respectivety, are theoretically investigated in this study. Their energy bands are characterized by small energy gaps, high group velocities, small effective masses, nonparabolic bands and multi-valleys convergence at near the center of the Brillouin zone, which are favorable conditions for high power factor with the optimum power factor values can be up to 0.20–0.25 W/mK ² at room temperature. Moreover, the 2D M ₂ X ₃ contains heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as ∼1.5–2.0 W/mK at room temperature. We find that by mixtures of M and X atoms, such as Bi ₂ Te ₂ Se, the power factor further increases whereas the lattice thermal conductivity decreases. This design gives a high figure of merit of the p-type 2D Bi ₂ Te ₂ Se from 1.4 to 2.0 at operating temperature within 300−500 K.