Designing high-performance thermoelectrics in two-dimensional tetradymites

Abstract 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 M2X3 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/mK2 at room temperature. Moreover, the 2D M2X3 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 Bi2Te2Se, the power factor further increases whereas the lattice thermal conductivity decreases. This design gives a high figure of merit of the p-type 2D Bi2Te2Se from 1.4 to 2.0 at operating temperature within 300 − 500  K.