Optimization of thermoelectric efficiency in SnTe: the case for the light band

Abstract

bd p-Type PbTe is an outstanding high temperature thermoelectric material with zT of 2 at high temperatures due to its complex band structure which leads to high valley degeneracy. Lead-free SnTe has a similar electronic band structure, which suggests that it may also be a good thermoelectric material. However, stoichiometric SnTe is a strongly p-type semiconductor with a carrier concentration of about 1 Â 10 20 cm À3 , which corresponds to a minimum Seebeck coefficient and zT. While in the case of p-PbTe (and n-type La 3 Te 4 ) one would normally achieve higher zT by using high carrier density in order to populate the secondary band with higher valley degeneracy, SnTe behaves differently. It has a very light, upper valence band which is shown in this work to provide higher zT than doping towards the heavier second band. Therefore, decreasing the hole concentration to maximize the performance of the light band results in higher zT than doping into the high degeneracy heavy band. Here we tune the electrical transport properties of SnTe by decreasing the carrier concentration with iodine doping, and increasing the carrier concentration with Gd doping or by making the samples Te deficient. A peak zT value of 0.6 at 700 K was obtained for SnTe 0.985 I 0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for Gd z Sn 1ÀzT e and SnTe 1+y which utilize the high valley degeneracy secondary valence band.

Thermoelectric performance of PbSnTeSe high-entropy alloys

“…[2] Ideal thermoelectric materials should possess a high dimensionless figure of merit, ZT, defined as ZT = S 2 T/ ρ(κ e + κ L ), where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electronic resistivity, and κ e and κ L are the carrier and lattice thermal conductivity, respectively. [1,3] Majority of IV-VI compounds tend to be dominant thermoelectric materials in the medium-temperature (500-900 K) range; these include most of lead chalcogenides (PbTe, [4][5][6][7][8] PbSe, [9,10] and PbS [11,12]), and tin chalcogenides (SnTe, [13,14] SnSe, [15][16][17] and SnS [18]). In addition, many mixtures composed of these compounds, such as PbTe-PbSe alloys, [19][20][21] PbTe-PbS alloys, [22,23] PbSe-PbS alloys, [24,25] SnSe-SnS alloys, [26] and PbTerich quaternary alloys of PbTe-PbSe-PbS, [27,28] have been extensively studied for further improving their performance.…”

Limit ofzTenhancement in rocksalt structured chalcogenides by band convergence

“…[28]). The feasibility of our calculations was verified via comparing our calculated S, μ H , S 2 σ , and zT as a function of n H with the reported experimental values of Na-doped PbSe [10], Na-doped PbTe [29], and I-doped SnTe [14]. All the comparisons are shown in Figs.…”

“…They share similar band structures, in which two extrema at the L (E V L ) and (E V ) points of the Brillouin zone are separated by an energy bias ( E = E V L − E V ), which is comparable to the band gap (E g = E C − E V L , with E C denoting the extreme of the conduction band) [14]. Since the valance band (VB ) locates further away from the Fermi level (E f ) compared with the L valance band (VB L ), the S tensor of VB is larger than that of V B L [15][16][17].…”

“…(S7) and (S15), respectively, whereas κ l for SnTe was obtained from Ref. [14]. Figure 2(a) shows the calculated κ e as a function of η, in which κ e increases with increasing T and η.…”

“…In addition, we investigated n H dependent thermoelectric properties and determined the temperature-dependent n opt H values for S 2 σ and zT in PbTe, PbSe, and SnTe, respectively. Using the reported κ l values for these rocksalt structured chalcogenides from the literature [10,14,29], we predicted the maximum zT to be 2.2, 1.8, and 1.6 for PbTe, PbSe, and SnTe, respectively. If the κ l reaches the amorphous limit, zT can be further enhanced to 3.1, 2.4, and 2.2 for PbTe, PbSe, and SnTe, respectively.…”