Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

The good co-existence of midgap state and valence band degeneracy is realized in Bi-alloyed GeTe through the In-Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid-high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m(0), and the corresponding carrier mobility still reaches 34.3 cm(2) V-1 s(-1) at room temperature. Moreover, the Bi-In-Cd co-alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m(-1) K-1 at 600 K. As a result, the Ge0.89Bi0.06In0.01Cd0.04Te sample obtains excellent thermoelectric properties of zT(max) approximate to 2.12 at 650 K and zT(avg) approximate to 1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.

Automated summary

The coexistence of midgap state and valence band degeneracy in Bi-alloyed GeTe through In-Cd codoping can modify the valence band structure. Doping induces a midgap state and improves the Seebeck coefficient near room temperature, while Cd doping increases the Seebeck coefficient in the mid-high temperature range by promoting valence band convergence. The synergistic effects enhance the density of state effective mass and yield excellent thermoelectric properties for the Ge0.89Bi0.06In0.01Cd0.04Te sample.