The Role of Ionized Impurity Scattering on the Thermoelectric Performances of Rock Salt AgPb_mSnSe_2+m

Abstract

5149wileyonlinelibrary.com the Seebeck coeffi cient is related to the electrical conductivity according to the Boltzmann transport equations, [ 3 ] hence, it will be hard to optimize one without degrading the other. Many approaches have been developed in the recent years to enhance the thermoelectric power factor ( S 2 / ρ ) including modifying the band structure by electronic resonant states, [ 4 ] quantum confi nement effects, [ 5 ] band convergence, [ 6 ] energy barrier fi ltering, [ 7,8 ] and intensifying impurity scattering of the carriers [ 9 ] to enhance the Seebeck coefficient. Among these, the carrier impurity scattering effect on the Seebeck coeffi cient has been recently investigated in Skutterudite [ 9 ] after it had nearly been ignored during the past half century. The carrier scattering probability by ionized impurity is inversely proportional to ε 3/2 and ν 3 , where ε is the energy of the carriers and ν is the drift velocity. [ 10 ] Therefore the low energy carriers are more scattered than the high energy ones leading to a positive energy dependence of the scattering time, which can in turn enhance the Seebeck coeffi cient. On the other hand, it is worthy to point out that strong ionized impurity scattering will signifi cantly reduce the carrier mobility which thus leads to a deterioration of the electrical conductivity. [ 11 ] Hence, the key point to use carrier ionized impurity scattering for improving the thermoelectric performances is to fi nd an effective strong ionized impurity scattering center, which could simultaneously enable to tune the concentration of carriers in order to reduce the degradation of the electrical conductivity due to much lower mobility. Typically, the lattice of the ionized impurity scattering center should match well with the matrix, which could minimize the detrimental effects on electrical transport properties.Recently, AgSnSe 2 with rock salt crystal structure, which is identical to PbSe, has attracted much attention as a natural valence-skipping (the Sn ions separated into 1:1 mixture of Sn 2+ and Sn 4+ ) 3D superconductor with quantum phase fl uctuations. [ 12,13 ] In the normal state, the electrical resistivity of AgSnSe 2 is around 0.23 mΩ⋅cm at room temperature (RT) which originate from high carrier concentration of 2.0 × 10 22 cm −3 . Besides, the mean-free path of the carriers in AgSnSe 2 with strong disorder is around 0.9 nm, which is very short, implying the existence of intrinsically strong electron scattering possibly due to the valence fl uctuations of Sn. [ 13 ] On the other hand, rocksalt-structured PbSe was considered as an alternative for its analogue PbTe due to the lower cost of This study reports on the successful synthesis and on the properties of polycrystalline AgPb m SnSe 2+ m ( m = ∞ , 100, 50, 25) samples with a rock salt structure. Between ≈160 and ≈400 K, the dominant scattering process of the carriers in this system changes from acoustic phonon scattering in PbSe to ionized impurity scattering in AgPb m SnSe 2+ m , which synergistica...

High Thermoelectric Performance in PbSe–NaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity

“…To investigate if the unusual electrical behavior is from a phase change or temperature-dependent increase in dopant solubility, we performed in situ PXRD over 300-650 K. The variable temperature PXRD patterns shown in Figure S7 in the Supporting Information are unchanged with heating, and the corresponding lattice parameters increase linearly, indicating a different origin for the strange charge transport. Similar results have been reported in the analogous systems AgPb m SbSe m+2 [58] and AgPb m SnSe m+2 , [59] and also in SnSe- [60] and Mg 3 Sb 2 -based thermoelectrics, [31,32] all of which show thermally activated charge carrier mobilities well above room temperature. In AgPb m SnSe m+2 and Mg 3 Sb 2 , the atypical charge transport mechanism was first suggested to be ionized impurity scattering, which generally gives carrier mobilities with positive T 1.5 temperature dependence and was shown to be in good agreement with the experimental data for these respective compounds.…”

“…Clearly, however, the experimental electrical conductivities shown in Figure 3a deviate dramatically from this expectation, with semiconducting-like thermally activated conduction up to ≈500 K. The Hall effect data adds to the puzzle, showing that the carrier concentrations are nearly constant over the temperatures of interest, implying the charge carrier mobilities increase with temperature under 500 K as shown in the inset of Figure 3d. Similar results have been reported in the analogous systems AgPb m SbSe m+2 [58] and AgPb m SnSe m+2 , [59] and also in SnSe- [60] and Mg 3 Sb 2 -based thermoelectrics, [31,32] all of which show thermally activated charge carrier mobilities well above room temperature. To investigate if the unusual electrical behavior is from a phase change or temperature-dependent increase in dopant solubility, we performed in situ PXRD over 300-650 K. The variable temperature PXRD patterns shown in Figure S7 in the Supporting Information are unchanged with heating, and the corresponding lattice parameters increase linearly, indicating a different origin for the strange charge transport.…”

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“…The same behavior was observed for the cubic binary In‐S thiospinel . In relaxation time approximation ( τ = τ 0 × E r × T − s ) [assuming effective electron mass m *( E , T )=constant], the carrier mobility μ is proportional to T r − s , in which r is the scattering parameter [i.e., r =−0.5 or 1.5 for acoustic phonon scattering (APS) or ionized impurity scattering (IIS), respectively], and s is the number of phonons taking part in the scattering (typically s =0 and 1 for IIS and APS, respectively) . Consequently, ( r − s )=−0.5 and 1.5 are expected for APS and IIS, respectively.…”

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“…This result indicates that they behave like heavily doped degenerate semiconductors and the minority‐carrier contribution to the Seebeck coefficient starts to play a role in the high‐temperature range . The electrical conductivity ( σ ) decreased with increasing temperature and the slopes of σ – T varied in different samples, implying slightly different mechanisms of charge carrier scattering (Figure a, solid symbols) . The σ of the PbBi 12.666 Te 20 sample (Figure a, blue solid triangles) started to ascend with respect to increasing T at greater than 500 K, possibly because of the bipolar electronic conduction.…”