Enhanced Thermoelectric Properties of p-Type Bi_0.5Sb_1.5Te₃-Cu₈GeSe₆ Composite Materials

Bismuth-telluride-based thermoelectric materials have been applied in active room-temperature cooling, but the mediocre ZT value of ∼1.0 limits the thermoelectric (TE) device's conversion efficiency and determines its application. In this work, we show the obviously improved thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ by the Cu₈GeSe₆ composite. The addition of Cu₈GeSe₆ effectively boosts the carrier concentration and thus limits the bipolar thermal conductivity as the temperature is elevated. With the Cu₈GeSe₆ content of 0.08 wt %, the hole concentration reaches 5.0 × 10¹⁹ cm^-3 and the corresponding carrier mobility is over 160 cm² V^-1 s^-1, resulting in an optimized power factor of over 42 μW cm^-1 K^-2 at 300 K. Moreover, the Cu₈GeSe₆ composite introduces multiple phonon-scattering centers by increasing dislocations and element and strain field inhomogeneities, which reduce the thermal conductivity consisting of a lattice contribution and a bipolar contribution to 0.51 W m^-1 K^-1 at 350 K. As a consequence, the peak ZT of the Bi_0.5Sb_1.5Te₃-0.08 wt % Cu₈GeSe₆ composite reaches 1.30 at 375 K and the average ZT between 300 and 500 K is improved to 1.13. A thermoelectric module comprised of this composite and commercial Bi₂Te_2.5Se_0.5 exhibits a conversion efficiency of 5.3% with a temperature difference of 250 K, demonstrating the promising applications in low-grade energy recovery.