Thermoelectrics (TEs) are an important class of technologies for harvesting electric power directly from heat sources. To design high performace TE materials, valleytronics has great theoretical potential to maximize a dimensionless figure of merit ZT but has not yet been demonstrated experimentally. Pseudocubic structure approach based on valleytronics paves a new path to manipulate valley degeneracy and anisotoropy with low thermal conductivity caused by short-range lattice distortion. Here, we report a record high ZT = 1.6 around 800 K, realized in totally enviromentally benign Na-doped Cu2ZnSnS4 (CZTS) single crystal. The exceptional performance comes from a high power factor while maintaining intrinsically low thermal conductivity. The results demonstrate that valleytronics is a new strategy and direction in the TE field, which takes advantage of simple material nature tuning without complex techniques.
Article
High thermoelectric performance of environmentally friendly sodium-doped Cu2ZnSnS4 single crystal: Evidence of valleytronics based strategy
https://doi.org/10.21203/rs.3.rs-50170/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted 11 Aug, 2020
You are reading this latest preprint version
There is NO Competing Interest.
- SupplementaryinformationTECZTS.pdf
Supplementary information (TE CZTS).pdf
- FigureS1.jpg
Structural properties for samples 1 (stoichiometric), 2 (Cu-poor), and 3 (Na: 0.1 mol%) for confirming of kesterite phase. a, the powder XRD patterns; (b) Raman spectroscopy; (c) XRD of the cutting plane along a- and c-axes. Inset images: how crystals were cut for directional measurements.
- FigureS2.jpg
The reproducibility of TE characterization for CZTS single crystals. Temperature dependence of a, electrical conductivity σ; b, Seebeck coefficient S; c, power factor PF by multiple measurements. The uncertainty of σ is 4-8 % and S is 2-6 %, and the measurement uncertainty of PF is 8-20 %.
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The reproducibility of thermal transport properties for CZTS single crystals. Temperature dependence of a, thermal conductivity κ = λCpD; b, thermal diffusivity λ; c, specific heat capacity Cp by multiple measurements. The density D between 4.4-4.5 g/cm3 were measured using the Archimedes method at room temperature. The uncertainty of κ is 8-12 % comprising those of 3-5 % for λ, 3-5 % for Cp, and 2 % for D.
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The reproducibility of the dimensionless figure of merit ZT for CZTS single crystals. The ZT measurement uncertainty is about 30 %.
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Temperature dependence of the measured hole mobility in CZTS single crystals. a, Fitting by Tk relation; b-d, Fitting by the combination of hopping conduction μI and acoustic (AC) and non-polar (NPO) phonon scattering μAC, NPO. The slope parameter k at low temperature T < 100 K are higher than 1.5, which indicates hopping conduction in the impurity band. The absolute k values at high temperature T > 300 K are less than 1.5 expected for typical lattice scattering. Total hole mobility μ composed of μI and μAC, NPO is excellent agreement with experimental data.
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The thermogravimetric analysis (TGA) measurements of samples 1-4. All samples are stable at the temperature up to 800 K in N2 atmosphere.
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Electronic band structures along the two symmetry directions (100) and (001) by DFT calculation. a, Non-doped; b, 6.25% Na-doped CZTS by DFT calculation. The top of the valence band is split into the topmost (v1) and second (v2) bands with Γ7+8 symmetry and the third band (v3) with Γ5+6 symmetry. Dashed line denote the Fermi energy. The color scale represents the number of band crossing.
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Calculated DOS of each atoms. a, Non-doped; b, 6.25% Na-doped CZTS. Dashed line denote the Fermi energy.
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The detailed composition of each sample determined by ICP-AES.
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Unintentional impurity levels detected by ICP-AES in the CZTS single crystals.
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Results and parameters of the CZTS single crystals determined as described in the main text.
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Scattring parameters of the CZTS single crystals determined from temperature dependence of hole mobility.
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High temperature TE properties of CZTS single crystal compared to some relating quaternary materials.
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Anisotropy of the effective hole masses (mn for n = v1, v2, and v3 in Fig. 1) in CZTS. me is electron mass. The transverse ⊥ masses are determined from the energy dispersions in (100) direction, and the longitudinal ∥ masses are determined from the dispersions in (001) direction.
