Research Update: Prediction of high figure of merit plateau in SnS and solid solution of (Pb,Sn)S

Abstract

Direct conversion between thermal and electrical energy can be achieved by thermoelectric materials, which provide a viable route for power generation and solid state refrigeration. Here, we use a combination of energetic, electronic, and vibrational first-principles based results to predict the figure of merit performance in hole doped single crystals of SnS and (Pb,Sn)S. We find high ZT values for both materials, specifically for (Pb,Sn)S along the b-axis. Both SnS and (Pb,Sn)S have excellent power factors when doped, due to a combination of increased electrical conductivity (due to doping) and a significantly enhanced Seebeck coefficient obtained by a doping-induced multiband effect. Anharmonic phonon calculations combined with a Debye-Calloway model show that the lattice thermal conductivity of both compounds is low, due to intrinsic anharmonicity, and is lowered further by the random, solid solution nature of the cation sublattice of (Pb,Sn)S. (Pb,Sn)S exhibits a high ZT plateau ranging from 1.3 at 300 K to 1.9 at 800 K. The overall ZT of the hole doped (Pb,Sn)S crystals is predicted to outperform most of the current state-of-the-art thermoelectric sulfide materials.

Sodium‐Doped Tin Sulfide Single Crystal: A Nontoxic Earth‐Abundant Material with High Thermoelectric Performance

“…As shown in Figure 2a, the calculated bandgap of SnS is ≈0.89 eV, which agrees with the results of previous calculations [35] and is smaller than that which we obtained experimentally from optical measurements (≈1.1 eV; see Figure S6 in the Supporting Information), because of the intrinsic underestimation of bandgap sizes in the density functional theory (DFT) calculations. As shown in Figure 2a, the calculated bandgap of SnS is ≈0.89 eV, which agrees with the results of previous calculations [35] and is smaller than that which we obtained experimentally from optical measurements (≈1.1 eV; see Figure S6 in the Supporting Information), because of the intrinsic underestimation of bandgap sizes in the density functional theory (DFT) calculations.…”

“…The turn over shifted to higher temperatures as the carrier concentration of holes increased. [35] We also observed comparably low lattice thermal conductivity values and large Grüneisen parameters for polycrystalline SnS. We achieved the maximum power factor of 2.0 mW m −1 K −2 at room temperature in a SnS crystal doped with 2 at% Na.…”

“…The data in Figure 5a show that the highest zT value of ≈1.1 at ≈870 K was achieved for the SnS crystal with the Na content of x = 0.02. [40,41] According to theoretical calculations, [35] a higher zT value (≈1.9 at 800 K) for SnS single crystal samples along the b-axis direction could be realized by combining improved doping efficiencies for the higher PF with the reduced total thermal conductivity via forming solid solutions. [39] As shown in Figure S3 (Supporting Information) and Figure 5b, we compared the highest zT values and the average zT values obtained with our SnS single crystals to corresponding values of several representative metal sulfides except superionic conductors as reported in the previous literature.…”

Coupling effects of strain on structural transformation and bandgap engineering in SnS monolayer

“…[36][37][38] The PBE methods have been employed to investigate the structural stability and electronic properties, and the HSE hybrid functional methods have been used to check the band gaps of all the cases employed here to overcome the problem of band gap underestimation in PBE functional. 39,40 The projector augmented wave (PAW) method is used to describe the electron-ion interaction, and the valence electrons of 4s 2 4p 2 for Sn and 3s 2 3p 4 for S atoms have been adopted in the calculations. 33,41 A energy cutoff of 450 eV and appropriate Monkhorst-Pack K meshes grid under structure relax and the more precise K mesh with 15 Â 15 Â 1 are adopted in electrons calculations.…”

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…Some theoretical studies predict that applied pressure or stress can greatly enhance the figure of merit of thermoelectric materials, giving values in the range ZT ∼ 2-3 and above. 21,22,24,25 In general, properties of materials can be changed dramatically under high pressure: for example, not only can insulators become metals under appropriate compression, but, vice versa, certain metals can transform into insulators if strongly pressurized, as in the cases of sodium (Na) 26 and lithium (Li). 27 Thus, high-pressure methods open a portal to a novel "alternative" world of materials in which it is possible to use the application of variable pressure to tune material properties to meet certain technological requirements.…”