Multiple defect states engineering towards high thermoelectric performance in GeTe-based materials

• The thermoelectric properties of Ga, Bi and Pb co-doped GeTe are investigated. • Ga and Bi tune the electronic transport properties for high power factor. • Pb induces the miscibility gap for low lattice thermal conductivity. • The very high ZT of 2.1 is achieved for the GeTe based material. GeTe is a promising material for thermoelectric (TE) applications. However, its low Seebeck coefficient and high thermal conductivity in the mid-temperature range of 298–550 K have hindered its widespread use in TE devices. In this work, we show that a combination of band engineering, Fermi level tuning, and miscibility gap exploration can significantly improve the energy conversion performance of GeTe. Bi provides the optimized carrier concentration and induces band convergence, while Ga possibly forms a resonant state. The synergistic effects of Bi and Ga significantly improve the Seebeck coefficient from 38 µVK −1 for undoped GeTe to over 120 µVK −1 for Bi and Ga-containing materials at 298 K and provide a high power factor over a wide temperature range. Phonon scattering is strengthened by optimizing the microstructure through Pb-induced spinodal decomposition and enhanced point defect scattering due to increased dopant solubility, resulting in a very low lattice thermal conductivity of about 0.5 Wm -1 K −1 at 600 K for the triple-doped samples. The maximum thermoelectric figure of merit ZT was significantly increased to 2.1 at 600 K for the Ge 0.87 Pb 0.05 Bi 0.06 Ga 0.02 Te sample due to the improved power factor and reduced lattice thermal conductivity. Additionally, the average figure of merit ZT ave for this sample achieved a very high value of 1.4 for Ge 0.87 Pb 0.05 Bi 0.06 Ga 0.02 Te at a temperature gradient of 475 K ( T c = 298 K). The developed material shows great potential for use in energy conversion devices, with an estimated energy conversion efficiency exceeding 17 %.