All-scale hierarchical nanostructures and superior valence band convergence lead to ultra-high thermoelectric performance in cubic GeTe

Abstract

GeTe is among the most fascinating inorganic compounds for thermoelectric (TE) conversion of waste heat into electricity. However, the TE performance in its ambient rhombohedral phase is strongly impeded by natural excessive Ge vacancies resulting in high hole concentration, and the rhombohedral to cubic phase transition at high temperature (T ∼ 700 K) deteriorates its mechanical robustness. Thus, stabilization of the high T cubic phase near ambient conditions would resolve many of these unwarranted challenges. Importantly, the higher symmetric cubic phase is beneficial for large Seebeck coefficient (S) due to its higher valence band (VB) degeneracy. Here, we show a simple innovative strategy of using high energy ball-milling (BM) and spark plasma sintering (SPS) to promote the crystal symmetry in Sb doped GeTe, which stabilizes in a near-cubic phase under ambient conditions. Consequently, the energy gap between the primary and secondary VBs drastically decreases to ∼0.06 eV and the band degeneracy enhances, leading to high S. BM followed by SPS simultaneously lead to the formation of hierarchical nano/meso architectures comprising solid solution point defects, Ge and GeSb4Te7 nanoprecipitates and nano/mesoscale grains, which efficiently scatter broad length scales (few Å–200 nm) of phonons responsible for thermal transport. As a result, the lattice thermal conductivity (κlat) is suppressed to ∼0.59 W m−1 K−1. This combined effect of VB convergence due to enhanced crystal symmetry and ultra-low κlat via hierarchical nanostructuring results in an ultra-high TE figure of merit (zT) ∼2.5 at 662 K in Ge0.9Sb0.1Te-BM + SPS. Furthermore, the fabricated double leg thermoelectric device shows promising output power density of ∼570 mW cm−2 for a ΔT of 442 K.