Realization of High Thermoelectric Figure of Merit in GeTe by Complementary Co-doping of Bi and In

GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of similar to 2.1 at 723 K in In and Bi co-doped GeTe along with an extremely high TE conversion efficiency of similar to 12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness (similar to 200 H-v, H-v = kgf/mm(2)) reveals excellent mechanical stability.