Transport properties and valence band feature of high-performance (GeTe)85(AgSbTe2)15 thermoelectric materials

Y Chen,C M Jaworski,X B Zhao,Y B Gao,G J Snyder,J P Heremans,T J Zhu,H Wang

doi:10.1088/1367-2630/16/1/013057

Abstract

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (AgxSbTex/2+1.5)15(GeTe)85 (TAGS) thermoelectric materials with 0.4 ⩽ x ⩽ 1.2. All samples exhibit good thermoelectric figures of merit (zT) which reach 1.5 at 700 K for x = 0.6. Thermoelectric and thermomagnetic transport properties (electrical resistivity, Seebeck, Hall and transverse Nernst–Ettinghausen coefficients) are measured and used to calculate the scattering factor, the Fermi energy, the density-of-states (DOS) effective mass and hole mean free path (mfp). The DOS effective mass is very high due to the large band mass of the primary valence band and the high degeneracy of pockets in the Fermi surface from the second valence band. The highly degenerate Fermi surface increased the total DOS without decreasing mobility, which is more desirable than the high DOS that comes from a single carrier pocket. The high-temperature hole mfp approaches the Ioffe–Regel limit for band-type conduction, which validates our discussion based on band transport and is also important for TAGS alloys having high zT with heavy bands. The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity.

Highlights

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (Agx SbTex/2+1.5)15(GeTe)85 (TAGS) thermoelectric materials with 0.4 x 1.2
The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity
Introduction (GeTe)y(AgSbTe2)100−y compounds, commonly referred to as ‘TAGS-y’, can be considered as pseudo-binary compounds of AgSbTe2 and GeTe, and are one of the best traditional thermoelectric materials. They have attracted much attention since they were discovered in the 1960s [1]. This material system possesses a combination of high electrical conductivity (σ ), Seebeck coefficient (S) and relatively low thermal conductivity (κ) on the GeTe-rich side, especially the compositions with 80 and 85 mol% GeTe, which are usually denoted as TAGS-80 and TAGS-85 [1,2,3]

Summary

Introduction

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (Agx SbTex/2+1.5)15(GeTe) (TAGS) thermoelectric materials with 0.4 x 1.2. In contrast to PbTe [4] and Bi2Te3 [5], TAGS materials have their optimal zT at much higher hole concentrations p(1020 p 1021 cm−3), where they maintain comparatively higher Seebeck coefficients, which contribute to their very high thermoelectric performance This investigation will shed light on the reasons why this nearly metallic carrier density optimizes zT in this system. Theoretical and experimental investigations indicate that TAGS are highly degenerate semiconductors [9], and that the valence band probably constitutes two sub-bands with different effective masses This suggests that the presence of a second, heavy valence band causes the dependence of the Seebeck coefficient on hole density S(p), known as the Pisarenko relation, to undergo a deviation when the carrier concentration increases and the Fermi level moves further into the heavy band [10,11,12].

Objectives

Methods

Results

Discussion

Conclusion