Thermoelectric transport properties of pristine and Na-doped SnSe_1−xTe_x polycrystals

Abstract

SnSe, a "simple" and "old" binary compound composed of earth-abundant elements, has been reported to exhibit a high thermoelectric performance in single crystals, which stimulated recent interest in its polycrystalline counterparts. This work investigated the electrical and thermal transport properties of pristine and Na-doped SnSe1-xTex polycrystals prepared by mechanical alloying and spark plasma sintering. It is revealed that SnSe1-xTex solid solutions are formed when x ranges from 0 to 0.2. An energy barrier scattering mechanism is suitable for understanding the electrical conducting behaviour observed in the present SnSe polycrystalline materials, which may be associated with abundant defects at grain boundaries. The thermal conductivity was greatly reduced upon Te substitution due to alloy scattering of phonons as well explained by the Debye model. Due to the increased carrier concentration by Na-doping, thermoelectric figure of merit (ZT) was enhanced in the whole temperature range with a maximum value of 0.72 obtained at a relatively low temperature (773 K) for Sn0.99Na0.01Se0.84Te0.16.

Facile Surfactant‐Free Synthesis of p‐Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors

“…The indirect (direct) optical band gap from diffuse reflectance (DR) UV/Vis spectra10c narrows slightly from ≈0.89 (≈1.1) eV to ≈0.85 (≈1.0) eV (Figure S10) when the nanoplates are consolidated into dense pellets, which could be related to sintering effects. The values are very similar to the indirect band gaps reported for both single crystalline and polycrystalline SnSe 4, 5c, 5d3 b, S11a).…”

“…It is also notable that both σ and S increase with temperature for 1 . This phenomenon has been observed in both un‐doped and iodine‐doped polycrystalline SnSe 5c,5d, 16. Although the origins of the behavior for 1 require further investigation, the combination of superior σ values coupled with high values of S leads to exceptional power factors (≈0.05 mW m −1  K −2 at 300 K to ≈0.40 mW m −1  K −2 at 550 K; Figure 4 c).…”

“…The huge differences in performance between 1 and 2 further emphasize the importance of the surfactant‐free synthesis route, not just in the context of a simpler, more sustainable synthesis method, but also in delivering significantly improved electrical properties consistently (Figure S15). Notably, the power factors for 1 far exceed those for un‐doped polycrystalline SnSe across a similar temperature range (0.028–0.04 mW m −1  K −2 ),5c–5e and are comparable to those for hole‐doped materials with high carrier concentrations 5d, 17. Recent Na‐ and Ag‐doping studies have elegantly demonstrated how the electrical performance and Z   T values of SnSe single crystals can be dramatically improved 18.…”

Phonon anharmonicity and negative thermal expansion in SnSe

“…This phase transition has been the subject of a number of early crystallographic and thermomechanical experimental investigations [22][23][24][25][26][27][28][29][30][31]. More recently, the ultra-low lattice thermal conductivity of SnSe has attracted great interest for thermoelectric applications, and has been studied both experimentally and through ab-initio simulations [10,[18][19][20][32][33][34][35][36][37][38][39][40][41][42]. The coupling between lattice distortion and electronic structure was recently shown to be responsible for the large anharmonicity, which persists in a broad range of temperatures, and leads to the low thermal conductivity [10,17].…”

Texturing degree boosts thermoelectric performance of silver-doped polycrystalline SnSe

“…[2][3][4] Therefore, chemical doping has been used to tune the carrier concentration and improve the thermoelectric performance of SnSe. Ag, 21,22 Na, [23][24][25] and K 26 have been used to increase hole concentration, and I 27 and BiCl 3 28 have been used as n-type dopants. Significant enhancement in ZT values below the phase transition was observed in doped SCs.…”