Promoted thermoelectric performance in cubic-phase GeTe via grain-boundary phase elimination under phase diagram guidance

Abstract

Eliminating the ferroelectric phase transition to obtain a cubic phase under ambient conditions is deemed an ultimate goal for p-type GeTe-based materials to be feasible for thermoelectric generator applications at mid-temperature range. Alloying stoichiometric AgSbTe₂ or non-stoichiometric Ag_1−δSb_1+δTe_2+δ into GeTe can regulate its cubic-rhombohedral phase transition temperature well below 300 K; nevertheless, the existence of the high-resistivity Ag₈GeTe₆ grain boundary phase impeded the further improvement of thermoelectric performance in both cases. In this work, as guided by the phase equilibrium diagram, we successfully eliminated the Ag₈GeTe₆ grain boundary phase by meticulously tailoring the alloying ratio x and regulating Ag/Sb ratio δ in (GeTe)_x(Ag_1−δSb_1+δTe_2+δ)_100−x while retaining its cubic crystalline structure. Furthermore, the formation of Ge₂Sb₂Te₅ imbedded in GeTe grains involved discrete van der Waals planar gaps, which can further help reduce the lattice thermal conductivity. As a result, a peak figure of merit ZT_max. ∼2.0 at 673 K and average ZT_ave. of ∼1.5 at 323–773 K were obtained in our cubic-phase (GeTe)₇₈(Ag_0.77Sb_1.23Te_2.23)₂₂, and the fabricated eight-pair thermoelectric power generator exhibited an outstanding conversion efficiency of ∼6.3% with output power of ∼1.32 W at a temperature difference of 480 K. This work demonstrated that eliminating the high-resistivity grain boundary phase is a facile way to realize enhanced thermoelectric performance, and could shed light on further research on other thermoelectric materials.