Ultralow Lattice Thermal Conductivity and Superhigh Thermoelectric Figure-of-Merit in (Mg, Bi) Co-Doped GeTe

High-efficiency thermoelectric (TE) technology is determined by the performance of TE materials. Doping is a routine approach in TEs to achieve optimized electrical properties and lowered thermal conductivity. However, how to choose appropriate dopants with desirable solution content to realize high TE figure-of-merit (zT) is very tough work. In this study, via the use of large mass and strain field fluctuations as indicators for low lattice thermal conductivity, the combination of (Mg, Bi) in GeTe is screened as very effective dopants for potentially high zTs. In experiments, a series of (Mg, Bi) co-doped GeTe compounds are prepared and the electrical and thermal transport properties are systematically investigated. Ultralow lattice thermal conductivity, about 0.3 W m^-1 K^-1 at 600 K, is obtained in Ge_0.9 Mg_0.04 Bi_0.06 Te due to the introduced large mass and strain field fluctuations by (Mg, Bi) co-doping. In addition, (Mg, Bi) co-doping can introduce extra electrons for optimal carrier concentration and diminish the energy offset at the top of the valence band for high density-of-states effective mass. Via these synthetic effects, a superhigh zT of ≈2.5 at 700 K is achieved for Ge_0.9 Mg_0.04 Bi_0.06 Te. This study sheds light on the rational design of effective dopants in other TE materials.