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

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 resonant-like behavior, Fermi level tuning, and miscibility gap exploration can significantly improve the energy conversion performance of GeTe. The resonant-like hump in GeTe is induced by Ga doping, while Bi provides the optimized carrier concentration and induces band convergence. 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, resulting in a very low lattice thermal conductivity of approximately 0.5 Wm-1K-1 at 600 K for the triple-doped samples. The improved power factor and reduced lattice thermal conductivity resulted in a significant increase in the maximum thermoelectric figure of merit ZT, reaching 2.1 at 600 K for the Ge0.87Pb0.05Bi0.06Ga0.02Te sample. Additionally, the average figure of merit ZTave for this sample achieved an outstanding value of 1.4 for Ge0.87Pb0.05Bi0.06Ga0.02Te at a temperature gradient of 475 K (Tc = 298 K). The developed material shows great potential for use in energy conversion devices, with an estimated energy conversion efficiency exceeding 17%.