Realizing high power factor and thermoelectric performance in band engineered AgSbTe2

Abstract

AgSbTe₂ is a promising p-type thermoelectric material operating in the mid-temperature regime. To further enhance its thermoelectric performance, previous research has mainly focused on reducing lattice thermal conductivity by forming ordered nanoscale domains for instance. However, the relatively low power factor is the main limitation affecting the power density of AgSbTe₂-based thermoelectric devices. In this work, we demonstrate that hole-doped AgSbTe₂ with Sn induces the formation of a new impurity band just above the valence band maximum. This approach significantly improves the electrical transport properties, contrary to previous strategies that focused on reducing lattice thermal conductivity. As a result, we achieve a record-high power factor of 27 μWcm^-1K^-2 and a peak thermoelectric figure of merit zT of 2.5 at 673 K. This exceptional performance is attributed to an increased hole concentration resulting from the formation of the impurity band and a lower formation energy of the defect complexes ( $V_{Ag}^{1-}$ + ${Sn}_{Sb}^{1-}$ ). Besides, the doped materials exhibit a significantly improved Seebeck coefficient by inhibiting bipolar conductivity and preventing the formation of n-type Ag₂Te. Additionally, the optimized AgSbTe₂ is used to fabricate a unicouple thermoelectric device that achieves energy conversion efficiencies of up to 12.1% and a high power density of 1.13 Wcm^-2. This study provides critical insights and guidance for optimizing the performance of p-type AgSbTe₂ in thermoelectric applications.

Fig. 1. TE performance and characterizations on Sn doped AgSbTe₂ samples.

a Schematic illustration of improved TE performance in Sn doped AgSbTe₂. b XRD patterns of as-synthesized AgSb_1-xSn_xTe₂ pellets. c DSC curves of AgSbTe₂ and Sn-doped AgSbTe₂. d High-resolution XPS spectra of AgSb_0.94Sn_0.06Te₂ pellet.

Fig. 2. Theoretical simulations on band structure, defect formation energy, and density of states.

DFT-calculated band structure of (a) Ag₁₆Sb₁₆Te₃₂ and (b) Ag₁₆Sb₁₅SnTe₃₂. Defect formation energies of individual defects and complex defects with respect to the Fermi level: (c) Ag₁₆Sb₁₆Te₃₂ and (d) Ag₁₆Sb₁₅SnTe₃₂. DOS of (e) Ag₁₆Sb₁₆Te₃₂ and (f) Ag₁₆Sb₁₅SnTe₃₂ for the Ag, Sb, and Te atoms. Zero energy corresponds to the Fermi level.

Fig. 3. Microstructure characterization on AgSb_0.94Sn_0.06Te₂ sample.

a Atomic resolution HAADF-STEM image obtained from AgSb_0.94Sn_0.06Te₂ sample and its corresponding FFT, the presence of double-diffraction spots are circled in red and green. b Inverse composite and individual FFT images obtained from the red and green circled diffraction spots from panel a. c Schematic illustration of Sn doping induced short-range Ag/Sb ordering in AgSbTe₂. d HRTEM micrograph of a AgSb_0.94Sn_0.06Te₂ grain visualized along its [211] zone axis and (e) corresponding selected area diffraction pattern. HRTEM images and corresponding FFTs of the (f) red and (g) green squared regions in (d).

Fig. 4. Transport properties of AgSb_0.94Sn_0.06Te₂ sample.

Temperature-dependent TE properties of AgSb_1-xSn_xTe₂. a Electric conductivity, σ. b Carrier concentration, n_H. c Mobility, µ_H. d Seebeck coefficient, S. e Power factors, PF. f Maximum PF comparison with state-of-art AgSbTe₂ materials^,,,,–. g Thermal conductivity, κ. h Combination of bipolar and lattice contribution to the thermal conductivity. i TE figure of merit, zT, the uncertainty of zT measurement is ~20% as indicated by error bar.

Fig. 5. Device performance.

a Current dependent output power of an AgSb_0.94Sn_0.06Te_{2 /} Yb_0.25Co_3.75Fe_0.25Sb₁₂ unicouple module. b Maximum power density as a function of ΔT. c Current-dependent conversion efficiency (η) of the unicouple module. d Comparison of the maximum conversion efficiency (η_max) as a function of ΔT of the AgSb_0.94Sn_0.06Te₂ unicouple device with that of other state-of-art AgSbTe₂ devices^,,,,,.