Optimized electronic properties and nano-structural features for securing high thermoelectric performance in doped GeTe

Recently, thermoelectric (TE) materials have been attracting great attention due to their improved capability to convert heat directly into electricity. PbTe-based TE materials are among the most competitive ones; however, lead toxicity limits their potential applications. Thus, the current focus in the field is on the discovery of lead-free analogues. GeTe is considered to be a promising candidate, however, its thermoelectric performance is limited by a non-ideal band structure and intrinsic Ge vacancies. In this work, GeTe was co-doped with Bi, Zn, and In. Initial doping with Bi enhances the performance by tuning the electronic properties and bringing down the thermal conductivity. Subsequent Zn doping permits to maintain the high power factor by increasing carrier mobility and reducing carrier concentration. Additionally, Zn incorporation lowers thermal conductivity and, thus, increases the performance. Subsequent In doping in (Ge_0.97Zn_0.02In_0.01Te)_0.97(Bi₂Te₃)_0.03 reduces thermal conductivity even further and makes this material the best performing one. Scanning transmission electron microscopy shows the presence of nano twinning, defect layers, and dislocation bands that contribute to the suppression of the lattice thermal conductivity. A peak zT value of 2.06 and an average zT value of 1.30 have been achieved in (Ge_0.97Zn_0.02In_0.01Te)_0.97(Bi₂Te₃)_0.03. These results are among the best state-of-the-art thermoelectric materials.