Manipulation of Band Structure and Interstitial Defects for Improving Thermoelectric SnTe

Abstract

Many efforts are recently devoted on improving thermoelectric SnTe as an environment-friendly alternative to conventional PbTe and successful approaches include valence band convergence, nanostructuring, and substantial/interstitial defects. Among these strategies, alloying SnTe with MnTe enables the most effective reduction in the valence band offset (between L and Σ) for a convergence due to its high solubility of ≈15%, yet there is no indication that the solubility of MnTe is high enough for fully optimizing the valence band structure and thus for maximizing the electronic performance. Here, a strategy is shown to increase the MnTe solubility up to ≈25% by alloying with 5% GeTe, which successfully locates the composition (20% MnTe) to optimize the valence band structure by converging a more degenerated Λ (as compared with band L) and Σ valence bands. Through a further alloying with Cu 2 Te, the resultant Cu-interstitial defects enable a sufficient reduction in lattice thermal conductivity to its amorphous limit (0.4 W m −1 K −1 ). These electronic and thermal effects successfully realize a record-high thermoelectric figure of merit, zT of 1.8, strongly competing with that of PbTe. This work demonstrates the validity of band manipulation and interstitial defects for realizing extraordinary thermoelectric performance in SnTe.

Solute manipulation enabled band and defect engineering for thermoelectric enhancements of SnTe

“…This approaches the amorphous limit of SnTe estimated by the Debye‐Cahill model (gray line in Figure C), but there is still available room for a further reduction according to a recently developed model taking into account the periodic boundary conditions . It is worth noticing that the lowest κ L in this work is comparable to that of literature high‐ zT Sn 0.75+ δ Ge 0.05 Mn 0.2 Te(Cu 2 Te) 0.05 alloys . In addition to the cubic structure of SnTe alloys involved in this work, the thermoelectric properties are therefore believed to be isotropic.…”

“…Composition within the miscibility gap leads to the formation of MnTe impurities as verified by the SEM observations (Figure S2). This work focuses on solid solutions of Sn 0.8‐ x Mn 0.2 Pb x Te (0.1 ≤ x ≤ 0.3), of which a MnTe concentration of 20 at% is found to maximize the valence band degeneracy according to the literature work . The lattice parameter for Sn 0.8‐ x Mn 0.2 Pb x Te is found to increase linearly with increasing x (Figure B), which can be well understood by the larger atomic size of Pb than that of Sn.…”

“…Temperature‐dependent Hall coefficient ( R H ) (A) and Hall mobility ( m H ) (B) and room‐temperature Hall carrier concentration–dependent Seebeck coefficient (C) and Hall mobility (D) for Sn 0.8‐ x Mn 0.2 Pb x Te, with a comparison to literature modeling and measurements …”

“…Greatly inspired by the successful strategies for zT enhancements realized in PbTe with alloying, monotelluride solutes such as MnTe, MgTe, CdTe, CaTe, and HgTe are found to successfully minimize the ∆ E L‐∑ for an electronic improvement of SnTe. Among these solutes, MnTe is found to be one of the most effective for maximizing the valence band degeneracy ( N v ) due to its high solubility of ~15 at% .…”

“…Among these solutes, MnTe is found to be one of the most effective for maximizing the valence band degeneracy ( N v ) due to its high solubility of ~15 at% . Fortunately, the solubility of MnTe in SnTe is found to be further increased up to 25 at% with an additional 5 at% GeTe alloying (Sn 0.7 Ge 0.05 Mn 0.25 Te) . Such an increase in MnTe solubility enables a further N v maximization through an involvement of new highly degenerated transporting band ( Λ ) .…”