Impact of Dopants on the PbTe Thermoelectric Efficiency
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1267-DD05-19
Impact of dopants on the PbTe thermoelectric efficiency Ka Xiong , Rahul P. Guptaa, John B. Whitec, Bruce E. Gnadea and Kyeongjae Choa,b,† a*
a
Materials Science & Engineering Dept, The University of Texas at Dallas, Richardson, TX 75080, USA b Physics Dept, The University of Texas at Dallas, Richardson, TX 75080, USA c Marlow Industries, 10451 Vista Park Road, Dallas, TX 75238, USA
*[email protected] † kjcho @utdallas.edu ABSTRACT We investigated the impact of doping group III elements (Al, Ga, In and Tl) on the electronic structure of PbTe by first principles calculations. The impurity-induced defect level changes with respect to the charge state of the impurity. We find that Tl is the good for the enhancement of thermoelectric efficiency of p-type PbTe, consistent with the experimental data. INTRODUCTION Thermoelectric (TE) materials have attracted much attention for being used in energy applications because of their ability to convert heat into electricity or vice versa. The lead chalcogenide salt PbTe, a IV-VI narrow band-gap semiconductor, is found to be one of the best thermoelectric materials at mid-temperature range (400-800K) [1-2]. However, the major obstacle for adopting these materials into real applications is that the energy conversion efficiency of these materials is rather low. The effectiveness of a TE material is linked to the dimensionless TE “figure of merit”, which can be expressed as: ZT=α2σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. The above equation clearly shows that to have a high ZT value the candidate material should have a large S, a high σ, and a low κ. This has led to considerable effort on improving the thermoelectric efficiency figure of merit (ZT) of PbTe [3-5]. Recent experimental work showed that doping Tl into bulk PbTe could double the ZT (~1.4) as compared to that of the undoped PbTe [5]. This ZT enhancement is attributed to the enhancement of the Seebeck coefficient (S) caused by the Tl-induced resonant states in the valence band of PbTe. The distortion of the density of states causes an increase of the derivative dn(E)/dE and hence increases the S, assuming that doping with low concentration of Tl does not affect the carrier mobility. The relationship between this local increase in the density of states (DOS) and the S is given by the Mott expression [6], 1 dn( E ) 1 d µ ( E ) π 2 kB π 2 kB d [ln(σ ( E ))] S= k BT = k BT + (1), µ dE E = EF 3 q dE 3 q E = EF n dE
where kB is the Boltzmann constant, n is the carrier density and µ is the mobility. This finding sheds new light on the TE efficiency improvement. However, an understanding of the mechanism of this Tl-induced enhancement of the TE efficiency, especially at an atomic level, is still missing. This motivates us to investigate the impact of Tl on the electronic structure of PbTe by first principles calculations. Doping group III elements into PbTe has shown interesting properties and has been investigated for a
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