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 Bi0.5Sb1.5Te3 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 Bi0.5Sb1.5Te3-0.08 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 Bi2Te2.5Se0.5 exhibits a conversion efficiency of 5.3% with a temperature difference of 250 K, demonstrating the promising applications in low-grade energy recovery.
Keywords: Cu8GeSe6; bipolar effect; bismuth telluride; thermoelectric; thermoelectric module.