Integrating band engineering with point defect scattering for high thermoelectric performance in Bi₂Si₂Te₆

A layer-structure compound Bi₂Si₂Te₆ is introduced as a promising thermoelectric material. The intrinsic low lattice thermal conductivity of Bi₂Si₂Te₆ is because of the low phonon group velocities near the Γ point and the overlap of optical and acoustic phonon branches. A further decreased thermal conductivity is obtained by alloying Sb at the Bi site for the additional point-defect scattering of phonons. This Sb alloying also enhances the density-of-states near the valence band maximum for a larger carrier effective mass, and suppresses the bipolar effect by increasing the carrier concentration and widening the band gap. Benefitting from both the enhancement of power factor and the decrease of lattice thermal conductivity, the peak zT value is improved from ∼ 0.45 at 573 K for Bi₂Si₂Te₆ to ∼ 1.2 at 773 K for BiSbSi₂Te₆.