Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase-Transition Temperature and Introducing Resonant Energy Doping

Abstract

GeTe with rhombohedral-to-cubic phase transition is a promising lead-free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1−xSb_xTe with optimized carrier concentration, a record-high figure-of-merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase-transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high-density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high-performance Pb-free thermoelectric materials.

An ultrahigh figure-of-merit of 2.3 is achieved in Ge_0.89Sb_0.1In_0.01Te through enhancing power-factor and decreasing thermal conductivity. The enhanced power-factor is caused by the optimized carrier concentration, reduced phase-transition temperature, and introduced resonant-energy doping. The decreased thermal conductivity is due to the enhanced phonon scatterings by the intrinsically deformed phonon transport and the externally induced phonon scattering sources.

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