Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal

Abstract

Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2–2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m–1 K–1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe

“…[5] On the other hand, defects, [6] solid solutions, [7] nanostructures, [6,8] and hierarchical architectures [9] are commonly utilized to suppress heat transfer through crystal lattices (κ L ). With these strategies, high TE performance has been achieved in many known TE materials such as AgPb m SbTe 2 + m (ZT = 2.2 at 800 K), [10] PbTe-4%SrTe (ZT = 2.2 at 915 K), [9] SnSe (ZT = 2.6 at 923 K), [11] FeNbSb (ZT = 1.5 at 1200 K), [12] Cu 2 Se (ZT = 2.6 at 850 K), [13] and triple-filled skutterudite (ZT = 1.7 at 850 K). [14] Discovering new TE materials has been largely focused on finding semiconductors with intrinsically low κ L .…”

“…The commonly used strategies of minimizing κ L are to reduce τ through strong anharmonicity, [17] rattling modes, [18] liquid-like ions, [19] et al and to lower v g by lattice softening and structural complexity. [20] With these strategies, many semiconductors with ultralow κ L have been discovered, such as TlInSe 2 (0.5 W m −1 K −1 at 300 K), [21] InTe (0.4 W m −1 K −1 ), [22] SnSe, [11] CsSnBr 3 (0.64 W m −1 K −1 at 300K), [23] and Ag 8 SnSe 6 (0.2 W m −1 K −1 at 300 K) [20b] .…”

An Overview of the Strategies for Tin Selenide Advancement in Thermoelectric Application

“…They discovered that the combination of ball milling and chemical reduction process efficiently removes the tin oxide layers from the surface of powdered SnSe-based materials. As a result, the thermal conductivity of polycrystalline hole-doped SnSe alloyed with 5% lead selenide is reduced to 0.11 Wm −1 K −1 , which is even lower than that of single crystals, and the zT increases to 2.5 at 773 K. According to recent research by Zhou et al [25], hole-doped SnSe polycrystalline samples made with properly purified reagents and stripped of tin oxides, had an average zT of about 3.1 at 783 K. Its lattice thermal conductivity is about 0.07 W m -1 K -1 , far lower than that of single crystals. As a result of these findings, a new age of high-performance practical TEs may begin to emerge.…”

“…In general, better mid-temperature TE performance in Group-IV chalcogenides was observed. zT values have now surpassed 2 and have surged to 3 [25], with novel synthesis methods, optimization approaches, microstructures, and concepts have emanated, all of which encourage further advancement in the research field. However, they contain expensive germanium (Ge), rare Te, and toxic Pb, restricting their large commercial and domestic usability [3].…”

“…In 2018, a research by Chang et al demonstrated that the Br-doped n-type SnSe crystals had a zT of 2.8 at 773 K along the a-axis [71]. According to recent research by Zhou et al, hole-doped SnSe polycrystalline samples made with properly purified reagents and stripped of tin oxides had approximately zT of about 3.1 at 783 K [25].…”