Abstract
Significant bipolar conduction of the carriers in Bi2Te3-based alloys occurs at high temperatures due to their narrow bandgaps. Therefore, at high temperatures, their Seebeck coefficients decrease, the bipolar thermal conductivities rapidly increase, and the thermoelectric figure of merit, zT, rapidly decreases. In this study, band modification of n-type Cu0.008Bi2(Te,Se)3 alloys by sulfur (S) doping, which could widen the bandgap, is investigated regarding carrier transport properties and bipolar thermal conductivity. The increase in bandgap by S doping is demonstrated by the Goldsmid–Sharp estimation. The bipolar conduction reduction is shown in the carrier transport characteristics and thermal conductivity. In addition, S doping induces an additional point-defect scattering of phonons, which decreases the lattice thermal conductivity. Thus, the total thermal conductivity of the S-doped sample is reduced. Despite the reduced power factor due to the unfavorable change in the conduction band, zT at high temperatures is increased by S doping with simultaneous reductions in bipolar and lattice thermal conductivity.
Highlights
BiTe-based alloys are thermoelectric materials widely used for room-temperature applications such as solid-state cooling
We investigated S-doped Bi2 (Te,Se)3 alloys to evaluate the effects of S doping on the Eg and thermoelectric properties, while the Cu-doped Bi2 (Te,Se)3 alloys were utilized due to their stability and higher zT than Bi2 (Te,Se)3 alloys
The bandgap calculated using the Goldsmid–Sharp estimation increased with doping
Summary
BiTe-based alloys are thermoelectric materials widely used for room-temperature applications such as solid-state cooling. The broader use of BiTe-based alloys is limited by their low thermoelectric conversion performance [1,2]. The thermoelectric performance has often been evaluated by the dimensionless figure of merit—thermoelectric figure of merit (zT) = σS2 T/κtot , where σ is the electrical conductivity, S is the Seebeck coefficient, κtot is the total thermal conductivity, and T is the absolute temperature. The maximum zT of Bi2 (Te,Se) n-type alloys is below one, while different studies on (Bi,Sb) Te3 p-type alloys have demonstrated values higher than one. The zT of polycrystalline Bi2 (Te,Se) n-type alloys has been improved by doping with the Cu element [3], but it is still lower than the p-type alloys. Considering the fact that the efficiency of a thermoelectric module is closely related to the average material zT between p- and n-type thermoelectric materials, the zT of p- and n-type alloys needs to be high and similar
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