Bi and Zn co-doped SnTe thermoelectrics: interplay of resonance levels and heavy hole band dominance leading to enhanced performance and a record high room temperature ZT

Abstract

Interplay of resonance levels in Bi–Zn co-doped SnTe thermoelectrics showcasing a record high room temperature and average ZT.

Improving the ZT of SnTe using electronic structure engineering: unusual behavior of Bi dopant in the presence of Pb as a co-dopant

“…The electronic structure of SnTe reveals a direct band gap of 0.081 eV at Γ point due to the typical underestimation in DFT based calculations (Figure 1a). 4,22 It is well known that PbTe and SnTe have inverted bands and the addition of Pb into SnTe gradually decreases the band gap to zero when Sn = 0.4 and Pb = 0.6. [25][26][27] When we simulate the electronic structure for a composition of Sn0.7Pb0.3Te, we observe that the system appears metallic as DFT fails to capture the extremely low band gap (Figure 1b).…”

“…26,27 We know that Bi is a n-type resonant dopant in SnTe known to introduce resonance levels just beneath the conduction band and improve the room temperature Seebeck co-efficient. 21,22,28 Hence, we co-doped Bi to beneficially tune the electronic structure. In the electronic structure of Pb-Bi co-doped SnTe, we observe an increase of 0.147 eV in the band gap at Γ point, effectively solving the problem of bipolar effect (Figure 1c).…”

“…21 In addition to this, it also introduces resonance level which appears as a distinct peak near the Fermi level in the density of states (DOS) plot (Figure 1d). 21,22 These resonance levels are formed due to the hybridization of Bi 'p' states with Sn and Pb 'p' states near the Fermi level just below the conduction band (Figure 2). In SnTe, we observe that the resonance states introduced by 'p' valence electron impurities are rather broad as seen in this case than that of 's' valence electron impurities as seen in the case of Zn resonance levels.…”

“…20 But recent studies have revealed Bi to be a successful n type resonant dopant in SnTe. 21,22 Hence, we thought it is worthwhile to study the 3 effect of Bi doping in the presence of Pb in SnTe. The combined electronic structure and transport property studies using first principles calculations reveal the unusual synergistic nature of Bi and Pb in improving the thermoelectric properties of SnTe making it a suitable material for further experimental study.…”

Control of the Thermoelectric Properties of Mg2Sn Single Crystals via Point-Defect Engineering

“…Second, thermal conductivity can be reduced by introducing point defects because they can act as phonon scattering centres. Although enhanced zT values have been realised for a variety of TE materials via point-defect engineering [3][4][5][6][7][8][9][10][11][12][13][14][15] , the fraction of point defects has rarely been evaluated quantitatively. Thus, point-defect engineering cannot yet be considered an established strategy to enhance TE performance.…”