High thermoelectric efficiency in co-doped degenerate p-type PbTe
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1267-DD04-03
High thermoelectric efficiency in co-doped degenerate p-type PbTe John Androulakis1,Ilyia Todorov2, Duck-Young Chung2, Sedat Ballikaya 3, Guoyu Wang3, Ctirad Uher3 and Mercouri Kanatzidis1, 2 1
Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, USA Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA 3 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA 2
ABSTRACT We explored the effect of K and K-Na substitution for Pb atoms in the lattice of PbTe, in an effort to test a hypothesis for the development of a resonant state that may enhance the thermoelectric power. At 300K the data can adequately be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range 1-15 x 1019 cm-3. A change in scattering mechanism was observed in the temperature dependence of the electrical conductivity, σ, for samples concurrently doped with K and Na which results in significantly enhanced σ at elevated temperatures and hence power factors. Thermal conductivity data provide evidence of a strong interaction between the light- and the heavy-hole valence bands at least up to 500K. Figure of merits as high as 1.3 at 700K were measured as a result of the enhanced power factors. INTRODUCTION Energy conservation and its effective management have given an increased impetus in the search for materials that convert waste heat to electricity. Since the efficiency of thermoelectric materials (expressed through the dimensionless quantity ZT=S2σT/κ, S being the Seebeck coefficient, σ the electrical conductivity and κ the total thermal conductivity of the lattice and electrons) depends on a combination of physical properties that strongly depend on temperature, different materials are considered for different temperature regimes. In the so called intermediate temperature regime, spanning the region between 600-800 K, the rock-salt type chalcogenide semiconductor PbTe has attracted most of the attention. For example, extensive nano-structuring of the PbTe matrix through the addition of small fractions of Ag-Sb, Na-Sb, Ag-Sn-Sb, and S has proven successful in achieving ZTs as high as 1.6 through a substantial reduction in the lattice thermal conductivity of the matrix.1-4 Although there are numerous intriguing questions associated with the role of nanofeatures in PbTe, from the practical point of view it seems that the lattice thermal conductivity, κlat, of PbTe is reaching its low limit. Attention must thus be paid to increasing the efficiency of transport of charge carriers that is quantified by the product S2σ, also known as the power factor. A possible way to enhance the power factor is the introduction of resonating levels in the density of states of PbTe through chemical substitutions.5 Since PbTe is a narrow band gap semiconductor, impurity states usually form outside the band gap (deep defect states) and interact with the lattice in a way that perturbs the band structure
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