High Thermoelectric Performance of p‑Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach

SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit <i>ZT</i>. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficient, and (b) enlarging the energy band gap. Thus, alloying with Cd atoms enables a form of valence band engineering that improves the high-temperature thermoelectric performance, where p-type samples of SnCd_0.03Te exhibit <i>ZT</i> values of ∼0.96 at 823 K, a 60% improvement over the Cd-free sample. Finally, we introduce endotaxial CdS or ZnS nanoscale precipitates that reduce the lattice thermal conductivity of SnCd_0.03Te with no effect on the power factor. We report that SnCd_0.03Te that are endotaxially nanostructured with CdS and ZnS have a maximum <i>ZT</i>s of ∼1.3 and ∼1.1 at 873 K, respectively. Therefore, SnTe-based materials could be ideal alternatives for p-type lead chalcogenides for high temperature thermoelectric power generation

Role of Sodium Doping in Lead Chalcogenide Thermoelectrics

The solubility of sodium and its effects on phonon scattering in lead chalcogenide PbQ (Q = Te, Se, S) family of thermoelectric materials was investigated by means of transmission electron microscopy and density functional calculations. Among these three systems, Na has the highest solubility limit (∼2 mol %) in PbS and the lowest ∼0.5 mol %) in PbTe. First-principles electronic structure calculations support the observations, indicating that Na defects have the lowest formation energy in PbS and the highest in PbTe. It was also found that in addition to providing charge carriers (holes) for PbQ, Na introduces point defects (solid solution formation) and nanoscale precipitates; both reduce the lattice thermal conductivity by scattering heat-carrying phonons. These results explain the recent reports of high thermoelectric performance in p-type PbQ materials and may lead to further advances in this class of materials

Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi₂Te₃ Doping

As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (<i>ZT</i>) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high <i>ZT</i> of ∼1.9 can be achieved at 773 K in Ge_0.87Pb_0.13Te upon the introduction of 3 mol % Bi₂Te₃. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge_0.87Pb_0.13Te + 3 mol % Bi₂Te₃ was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well

High <i>ZT</i> in p‑Type (PbTe)_{1–2<i>x</i>}(PbSe)_<i>x</i>­(PbS)_<i>x</i> Thermoelectric Materials

Lead chalcogenide thermoelectric systems have been shown to reach record high figure of merit values via modification of the band structure to increase the power factor or via nanostructuring to reduce the thermal conductivity. Recently, (PbTe)_1–<i>x</i>(PbSe)_<i>x</i> was reported to reach high power factors via a delayed onset of interband crossing. Conversely, the (PbTe)_1–<i>x</i>(PbS)_<i>x</i> was reported to achieve low thermal conductivities arising from extensive nanostructuring. Here we report the thermoelectric properties of the pseudoternary 2% Na-doped (PbTe)_{1–2<i>x</i>}(PbSe)_<i>x</i>(PbS)_<i>x</i> system. The (PbTe)_{1–2<i>x</i>}(PbSe)_<i>x</i>(PbS)_<i>x</i> system is an excellent platform to study phase competition between entropically driven atomic mixing (solid solution behavior) and enthalpy-driven phase separation. We observe that the thermoelectric properties of the PbTe–PbSe–PbS 2% Na doped are superior to those of 2% Na-doped PbTe–PbSe and PbTe–PbS, respectively, achieving a <i>ZT</i> ≈2.0 at 800 K. The material exhibits an increased the power factor by virtue of valence band modification combined with a very reduced lattice thermal conductivity deriving from alloy scattering and point defects. The presence of sulfide ions in the rock-salt structure alters the band structure and creates a plateau in the electrical conductivity and thermopower from 600 to 800 K giving a power factor of 27 μW/cmK². The very low total thermal conductivity values of 1.1 W/m·K of the <i>x</i> = 0.07 composition is accounted for essentially by phonon scattering from solid solution defects rather than the assistance of endotaxial nanostructures