Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In‐doped GeSb₂Te₄ Single Crystals

Abstract

Layer-structured GeSb 2 Te 4 is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge 1-x In x Sb 2 Te 4 single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c-axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c-axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb 2 Te 4 and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323-773 K are obtained in Ge 0.93 In 0.07 Sb 2 Te 4 along the c-axis direction, which are 54% and 79% higher than that of the pristine GeSb 2 Te 4 single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb 2 Te 4 , and shed light on the performance optimization of other layered thermoelectric materials.

Impact of Various Dopants on Thermoelectric Transport Properties of Polycrystalline GeSb2Te4

“…That is, the intensive hybridization of Te p and In s orbitals introduces the resonant level in the valence band and distorts the DOS near the Fermi level as shown in Figure 4b. [19,21,30,31] This effect has also been observed in the previously reported In-doped GST225 polycrystal and GST124 single-crystal samples. [19,21] The Ga doping also introduces the resonant level in the valence band as shown in Figure S9, Supporting Information, and the small extent of Seebeck coefficient increase may be limited by the low doping efficiency.…”

“…[19,21,30,31] This effect has also been observed in the previously reported In-doped GST225 polycrystal and GST124 single-crystal samples. [19,21] The Ga doping also introduces the resonant level in the valence band as shown in Figure S9, Supporting Information, and the small extent of Seebeck coefficient increase may be limited by the low doping efficiency.…”

“…[29] The dashed line in Figure 4a is the Data from a previous work are also given. [21,22] theoretical Pisarenko curve at the density of states (DOS) effective mass m d * = 1.32 m e (m e is the mass of the electron). The effective mass of the doped GST124 samples scarcely or slightly changes except for In and Ga dopants, indicating the merely disrupted valence band maxima.…”

“…Particularly, single-crystal GST124 has been synthesized and the maximum zT value of unity has been reached in doped/ alloyed samples. [21,22] The study on thermoelectric properties of GSTs will not only expand the material families in thermoelectrics but also offer new insight for the development of the design of low-power phase-change memory devices. [9,10,14] Despite the rapid progress, the thermoelectric performance of GST materials (including GST124, GST225, and GST147) is far from the predicted maximum value due to the stubbornly high…”

Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi2Si2X6 (X = Se, Te) monolayers

“…For GeSb 2 Te 4 , the maximum ZT was enhanced to 1.0 at 673 K by In doping. 13 The maximum in-plane ZT of CuSbSe 2 reached 0.5 at 673 K via decorating PtTe 2 . 14 Sb 2 Si 2 Te 6 exhibited an inherently low κ L of ∼0.5 W m −1 K −1 at 823 K, and its maximum ZT was up to ∼1.08 at 823 K. In a cellular nanostructured Sb 2 Si 2 Te 6 , its κ L was further reduced to ∼0.29 W m −1 K −1 at 823 K, and its ZT value reached up to ∼1.65 at 823 K. 15 Recently, single-phase polycrystalline Bi 2 Si 2 Te 6 was synthesized by Luo et al through ball-milling and annealing, which is a two-dimensional layered material isostructural to Sb 2 Si 2 Te 6 .…”