Engineering the energy gap near the valence band edge in Mn-incorporated Cu3Ga5Te9 for an enhanced thermoelectric performance

Cu3Ga5Te9-based compounds Cu3-xGa5MnxTe9 (x=0-0.2) with Mn substitution for Cu have been synthesized. The engineered energy gap (∆EA) between impurity and valence band is reduced from 44.4 meV at x=0 to 25.7 meV at x=0.1, which is directly responsible for the reduction of potential barrier for thermal excitation of carriers and enhancement in carrier concentration. However, the Seebeck coefficient shows an increasing tendency with the increasing of determined Hall carrier concentration (n). This anomalous behavior suggests that the Pisarenko plots under assumed effective masses do not fit the current relationship between the Seebeck coefficient and carrier density. With the combination of enhanced electrical conductivities and reduced thermal conductivities at high temperatures, the maximum thermoelectric (TE) figure of merit (ZT) of 0.81 has been achieved at 804 K with x=0.1, which is about 1.65 and 2.9 times the value of current and reported intrinsic Cu3Ga5Te9. The remarkable improvement in TE performance proves that we have succeeded in engineering the energy gap near the valence band edge upon Mn incorporation in Cu3Ga5Te9

The role of excess Sn in Cu4Sn7S16 for modification of the band structure and a reduction in lattice thermal conductivity

In this work, we have investigated the band structures of ternary Cu4Sn7+xS16 (x = 0–1.0) compounds with an excess of Sn, and examined their thermoelectric (TE) properties. First principles calculations reveal that the excess Sn, which exists as Sn2+ and is preferentially located at the intrinsic Cu vacancies, unpins the Fermi level (Fr) and allows Fr to enter the conduction band (CB) at x = 0.5. Accordingly, the Hall carrier concentration (nH) is enhanced by about two orders of magnitude when the x value increases from x = 0 to x = 0.5. Meanwhile, the lattice thermal conductivity (κL) is reduced significantly to 0.39 W K−1 m−1 at 893 K, which is in reasonably good agreement with the estimation using the Callaway model. As a consequence, the dimensionless TE figure of merit (ZT) of the compound Cu4Sn7+xS16 with x = 0.5 reaches 0.41 at 863 K. This value is double that of the stoichiometric Cu4Sn7S16, proving that excess Sn in Cu4Sn7S16 is beneficial for improving the TE performance

Engineering band structure via the site preference of Pb2+ in the In+ site for enhanced thermoelectric performance of In6Se7

Although binary In-Se based alloys as thermoelectric (TE) candidates are of interests in recent years, little attention has been paid into In6Se7 based compounds. With substituting Pb in In6Se7, the preference of Pb2+ in the In+ site has been observed, allowing the Fermi level (Fr) shift towards the conduction band and the localized state conduction becomes dominated. Consequently, the Hall carrier concentration (nH) has been enhanced significantly with the highest nH value being about 2~3 orders of magnitude higher than that of Pb-free sample. Meanwhile, the lattice thermal conductivity (κL) tends to be reduced as nH value increases, owing to an increased phonon scattering on carriers. As a result, a significantly enhanced TE performance has been achieved with the highest TE figure of merit (ZT) of 0.4 at ~850 K. This ZT value is 27 times that of intrinsic In6Se7 (ZT=0.015 at 640 K), which proves a successful band structure engineering through site preference of Pb in In6Se7

Significantly Enhanced Thermoelectric Performance of γ-In2Se3 through Lithiation via Chemical Diffusion

γ-In2Se3 is selected as a thermoelectric candidate because it has a unique crystal structure and thermal stability at relatively high temperatures. In this work we have prepared lithiated γ-In2Se3 through chemical diffusion and investigated its band structures and thermoelectric performance. After lithiation of γ-In2Se3 in lithium acetate (CH3COOLi) solution at 50oC, we have observed a high Hall carrier concentration (nH) up to ≤1.71×1018 cm-3 at room temperature (RT), which is about ∼4 orders of magnitude compared to that of pristine γ-In2Se3. The enhancement in nH is directly responsible for the remarkable improvement in electrical conductivity, and can be elucidated as the Fermi level (Fr) unpinning and moving towards the conduction band (CB) through the dominant interstitial occupation of Li+ in the γ-In2Se3 lattice. Combined with the minimum lattice thermal conductivity (κL=0.30-0.34 WK-1m-1) at ~923 K, the highest ZT value of 0.62-0.67 is attained, which is about 9-10 times that of pristine γ-In2Se3, proving that the lithiation in γ-In2Se3 is an effective approach on the improvement of the thermoelectric performance

Enhancing thermoelectric performance of Cu3SnS4-based solid solutions through coordination of the Seebeck coefficient and carrier concentration

Improving the thermoelectric (TE) performance of Cu3SnS4 is challenging because it exhibits a metallic behavior, therefore, a strategy should be envisaged to coordinate the carrier concentration (nH) and Seebeck coefficient (α). The coordination in this work has been realized through the Fermi level (Ef) unpinning and shifting towards the conduction band (CB) via addition of excess Sn in Cu3SnS4. As a result, the solid solution Cu3Sn1+xS4 (x = 0.2) has a moderate α (178.0 μV K−1) at 790 K and a high nH (1.54 × 1021 cm−3) value. Along with the lowest lattice thermal conductivity κL (0.39 W K−1 m−1) caused by the increased phonon scattering by carriers, the highest ZT value of 0.75 is attained at ∼790 K. This value is 2.8 times that of the stoichiometric Cu3SnS4, and stands among the highest for ternary Cu–Sn–S sulfide thermoelectrics at the corresponding temperatures. More importantly, this approach used in the case of ternary Cu3SnS4 provides a guidance or reference to improve the TE performance of other materials

Thin-film solar thermoelectric generator with enhanced power output: Integrated optimization design to obtain directional heat flow

Thin-film STEGs (solar thermoelectric generators) show promise in effective use of solar energy as a power supply for wireless sensors and microscale devices. This paper reports a simulation procedure that aims to identify desirable heat flow and temperature distribution to improve the performance of thin-film STEGs. The temperature distribution, heat flux, and voltage of a thin-film STEG are simulated using the finite element method, resulting in an optimal design of the substrate, heat conductive layer, and thermoelectric legs of thin-film STEGs. The effect of air convection on the STEG's performance is also studied. Based on the simulation, a thin-film STEG was designed and fabricated, which exhibits an open-circuit voltage of 22 mV. In addition, the experimental results demonstrate that the measured temperature distribution is in good agreement with the simulated result. To minimize the heat loss from the passive region of the device, an improved design was created in an attempt to confine the heat flow within the thermoelectric legs. This improved design resulted in a 21.4% increase of the output voltage

Engineering Band Structure via the Site Preference of Pb²⁺ in the In⁺ Site for Enhanced Thermoelectric Performance of In₆Se₇

Although binary In–Se based alloys have in recent years gained interest as thermoelectric (TE) candidates, little attention has been paid to In₆Se₇-based compounds. Substituting Pb in In₆Se₇, preference for Pb²⁺ in the In⁺ site has been observed, allowing Fermi level (<i>F</i>_r) shift toward the conduction band, where the localized state conduction becomes dominant. Consequently, the Hall carrier concentration (<i>n</i>_H) has been significantly enhanced with the highest <i>n</i>_H value being about 2–3 orders of magnitude higher than that of the Pb-free sample. Meanwhile, the lattice thermal conductivity (κ_L) tends to be reduced as the <i>n</i>_H value increases, owing to an increased phonon scattering on carriers. As a result, a significantly enhanced TE performance has been achieved with the highest TE figure of merit (ZT) of 0.4 at ∼850 K. This ZT value is 27 times that of intrinsic In₆Se₇ (ZT = 0.015 at 640 K), which proves a successful band structure engineering through site preference of Pb in In₆Se₇

Significantly Enhanced Thermoelectric Performance of γ‑In₂Se₃ through Lithiation via Chemical Diffusion

γ-In₂Se₃ is selected as a thermoelectric candidate because it has a unique crystal structure and thermal stability at relatively high temperatures. In this work, we have prepared lithiated γ-In₂Se₃ through chemical diffusion and investigated its band structures and thermoelectric performance. After lithiation of γ-In₂Se₃ in a lithium acetate (CH₃COOLi) solution at 50 °C for 30 h, we have observed a high Hall carrier concentration (<i>n</i>_H) of ≤1.71 × 10¹⁸ cm^–3 at room temperature, which is ∼4 orders of magnitude higher than that of pristine γ-In₂Se₃. The enhancement of <i>n</i>_H is directly responsible for the remarkable improvement in electrical conductivity and can be elucidated as the Fermi level (<i>F</i>_r) unpinning and moving toward the conduction band through the dominant interstitial occupation of Li⁺ in the γ-In₂Se₃ lattice. Combined with the minimum lattice thermal conductivity (κ_L = 0.30–0.34 W K^–1 m^–1) at ∼923 K, the highest ZT value of 0.62–0.67 is attained, which is approximately 9–10 times that of pristine γ-In₂Se₃, proving that the lithiation in γ-In₂Se₃ is an effective approach to the improvement in thermoelectric performance