This is a preprint, a preliminary version of a manuscript that has not completed peer review at a journal. Research Square does not conduct peer review prior to posting preprints. The posting of a preprint on this server should not be interpreted as an endorsement of its validity or suitability for dissemination as established information or for guiding clinical practice.
Article
Akira Nagaoka
University of Miyazaki
Corresponding Author
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This work is licensed under a CC BY 4.0 License. Read Full License
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.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
This preprint is available for download as a PDF.
There is NO Competing Interest.
This is a list of supplementary files associated with this preprint. Click to download.
- 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 %.
- FigureS3.jpg
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.
- FigureS4.jpg
The reproducibility of the dimensionless figure of merit ZT for CZTS single crystals. The ZT measurement uncertainty is about 30 %.
- FigureS5.jpg
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.
- FigureS6.jpg
The thermogravimetric analysis (TGA) measurements of samples 1-4. All samples are stable at the temperature up to 800 K in N2 atmosphere.
- FigureS7.jpg
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.
- FigureS8.jpg
Calculated DOS of each atoms. a, Non-doped; b, 6.25% Na-doped CZTS. Dashed line denote the Fermi energy.
- TableS1.png
The detailed composition of each sample determined by ICP-AES.
- TableS2.png
Unintentional impurity levels detected by ICP-AES in the CZTS single crystals.
- TableS3.png
Results and parameters of the CZTS single crystals determined as described in the main text.
- TableS4.png
Scattring parameters of the CZTS single crystals determined from temperature dependence of hole mobility.
- TableS5.png
High temperature TE properties of CZTS single crystal compared to some relating quaternary materials.
- TableS6.png
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.
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This is a preprint, a preliminary version of a manuscript that has not completed peer review at a journal. Research Square does not conduct peer review prior to posting preprints. The posting of a preprint on this server should not be interpreted as an endorsement of its validity or suitability for dissemination as established information or for guiding clinical practice.
Article
Akira Nagaoka
University of Miyazaki
Corresponding Author
Badges
Prescreen
This manuscript passed our Prescreen assessment
Complete author information
Appropriate declaration statements
Potential risks to human health
Prescreen
History
CURRENT STATUS: Posted
Version 1
Posted 11 Aug, 2020
No community comments so far
Metrics
Comments: 0
PDF Downloads: ...
HTML Views: ...
License:
This work is licensed under a CC BY 4.0 License. Read Full License
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.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
This preprint is available for download as a PDF.
There is NO Competing Interest.
This is a list of supplementary files associated with this preprint. Click to download.
- 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 %.
- FigureS3.jpg
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.
- FigureS4.jpg
The reproducibility of the dimensionless figure of merit ZT for CZTS single crystals. The ZT measurement uncertainty is about 30 %.
- FigureS5.jpg
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.
- FigureS6.jpg
The thermogravimetric analysis (TGA) measurements of samples 1-4. All samples are stable at the temperature up to 800 K in N2 atmosphere.
- FigureS7.jpg
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.
- FigureS8.jpg
Calculated DOS of each atoms. a, Non-doped; b, 6.25% Na-doped CZTS. Dashed line denote the Fermi energy.
- TableS1.png
The detailed composition of each sample determined by ICP-AES.
- TableS2.png
Unintentional impurity levels detected by ICP-AES in the CZTS single crystals.
- TableS3.png
Results and parameters of the CZTS single crystals determined as described in the main text.
- TableS4.png
Scattring parameters of the CZTS single crystals determined from temperature dependence of hole mobility.
- TableS5.png
High temperature TE properties of CZTS single crystal compared to some relating quaternary materials.
- TableS6.png
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.
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