Optimizing the thermoelectric performance of low-temperature SnSe compounds by electronic structure design

Abstract

Recently SnSe compound was reported to have a peak thermoelectric figure-of-merit (ZT) of ~2.62 at 923 K, but the ZT values at temperatures below 750 K are relatively low. In this work, the electronic structures of SnSe are calculated using the density functional theory, and the electro-and thermo-transport properties upon carrier density are evaluated by the semi-classic Boltzmann transport theory, revealing that the calculated ZT values along the a-and c-axes below 675 K are in 10 agreement with reported values, but that along the b-axis can be as high as 2.57 by optimizing the carrier concentration to n ~ 3.6×10 19 cm -3 . It is suggested that a mixed ionic-covalent bonding and heavy-light band overlapping near the valence band are the reasons for the higher thermoelectric performance.

First-Principles Calculations of Thermoelectric Properties of IV–VI Chalcogenides 2D Materials

“…The lattice thermal conductivity values obtained in the present work are in good agreement with recently reported results (Qin et al, 2016;Shafique and Shin, 2017). If we consider the lattice thermal conductivity as a good approach to obtain high efficiency (ZT) in TE materials, we can expect, from our results (Figure 6), that the monolayer SnSe should be one of the best candidate for TE applications as we can see in previously reports for this material (Carrete et al, 2014;Zhao et al, 2014Zhao et al, , 2016Guan et al, 2015;Guo et al, 2015;Hong et al, 2015;Kutorasinski et al, 2015;Sassi et al, 2015;Wang et al, 2015;Chere et al, 2016;Leng et al, 2016;Morales Ferreiro et al, 2016;Popuri et al, 2016;Li et al, 2017).…”

Band Structure of the IV-VI Black Phosphorus Analog and Thermoelectric SnSe

“…With the predicted band gap of around 800 meV, and the valence band maximum at 200 meV below the chemical potential, our SnSe samples appear naturally p-doped, and thus well suited for voltage generation. We note, however, that part of the asymmetry at low temperatures is likely coming from unequal onsets of density of states at two ends of the gap and may change as the chemical potential shifts at higher T [21]. The higher voltage, in general, will be generated by a temperature gradient the farther in kT (at 600K, kT =50 meV) the chemical potential is from the extremum of the conducting band (that is the valence band in p-doped, or the conduction band in n-doped crystals).…”

“…Density functional studies of bulk SnSe [21][22][23][24][25][26][27][28][29][30][31][32][33] generally agree on the indirect nature of the band gap that forms between the valence band pockets along Γ-Z and several electron pockets in the conduction band showing along Γ-Y . The electron pocket minima themselves, however, happen to be only a few tens of meV apart.…”

Investigation of the bipolar effect in the thermoelectric material CaMg2Bi2 using a first-principles study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The TE conversion efficiency and performance-price ratios of TE devices developed so far and those under development are insufficient to compete with fossil fuel based energy resources. Some intrinsic issues need to be solved for TE materials design and synthesis, and some of them have been lasting for decades.…”

“…9,12,13,16,[26][27][28][29][30][31][32] To date, it is claimed that all the TE properties can be quantitatively calculated for the given lattice composition/structure and carrier concentration (n). 9,30 This framework takes into account the thermally induced carrier excitation and in this sense the bipolar effect is at least partially considered. Nevertheless, in this framework, the electronic and phononic structures are calculated using the ideal stoichiometric compound instead of the real doped compound, which does not consider the influence of any carrier doping, lattice imperfection, and impurity on the electronic and phononic structures.…”