Zhengliang Du

Promising defect thermoelectric semiconductors Cu1−xGaSbxTe2 (x = 0–0.1) with the chalcopyrite structure

Through calculating the band structures of the defect chalcopyrite semiconductor Cu1−xGaSbxTe2 with the proper addition of Sb to CuGaTe2, we have demonstrated that Sb actually mostly occupies the Te rather than Cu lattice sites. Such a dominant occupation increases the density of states (DOS) at the Fermi level and the effective mass of the valence band, and thereby results in an increase in the Seebeck coefficient. The electrical conductivity (σ) is hardly related to Sb content when x < 0.1, due to the subtle change in the concentration n and mobility μ at the degenerate state of holes. The attempted addition of Sb causes a decrease in the lattice thermal conductivity (κL), but as the Sb content increases there is a limited enhancement of κL. We have thus determined the mechanism, proposing that the dual effect on the κL resulted from the extra lattice mismatch and crystal structure distortion. By considering all the effects mentioned above on the transport properties, we have attained the highest thermoelectric ZT value (1.07 ± 0.1) of the sample Cu1−xGaSbxTe2 (x = 0.02) at 721 K, which shows promise for application in the intermediate temperatures.

Manipulation of the crystal structure defects: An alternative route to the reduction in lattice thermal conductivity and improvement in thermoelectric performance of CuGaTe2

Here, we present the manipulation of the crystal structure defects: an alternative route to reduce the lattice thermal conductivity (κL) on an atomic scale and improve the thermoelectric performance of CuGaTe2. This semiconductor with defects, represented by anion position displacement (u) and tetragonal deformation (η), generally gives low κL values when u and η distinctly deviate from 0.25 and 1 in the ideal zinc-blende structure, respectively. However, this semiconductor will show high Seebeck coefficients and low electrical conductivities when u and η are close to 0.25 and 1, respectively, due to the electrical inactivity caused by an attractive interaction between donor-acceptor defect pairs (GaCu2+ + 2VCu−).

High thermoelectric performance of a defect in α-In2Se3-based solid solution upon substitution of Zn for In

In this project, we have successfully manipulated the lattice defects in α-In2Se3-based solid solutions (In2−xZnxSe3) by appropriate substitution of Zn for In, via a non-equilibrium fabrication technology (NEFT) of materials. The manipulation of the defect centers involves reduction of the number of interstitial In atoms (Ini) and Se vacancies (VSe), and creation of a new antisite defect ZnIn as a donor. Through this technique, the lattice structure tends to be ordered, and also more stabilized than that of pure α-In2Se3. In the meantime, the carrier concentration (n) and mobility (μ) have increased by 1–2 orders of magnitude. As a consequence, the solid solution at x = 0.01 gives the highest TE figure of merit (ZT) of 1.23(±0.22) in the pressing direction at 916 K, which is about 4.7 times that of pure α-In2Se3 (ZT = 0.26). This achieved TE performance is mainly due to the remarkable improvement in the electrical conductivity from 0.53 × 103 (Ω−1 m−1) at x = 0 to 4.88 × 103 (Ω−1 m−1) at x = 0.01 at 916 K, in spite of the enhancement in the lattice thermal conductivity (κL) from 0.26 (W m−1 K−1) to 0.32 (W m−1 K−1).

Site occupations of Zn in AgInSe2-based chalcopyrites responsible for modified structures and significantly improved thermoelectric performance

The band structures of AgInSe2-based semiconductors have been calculated and the lifting of the Fermi level toward the conduction band in AgInSe2 when Ag is replaced by Zn has been observed. This is mainly caused by the site occupation of Zn on the cation Ag site, which leads to the formation of the defect ZnAg1+ as an active donor. While the Fermi level lowers toward the valence band when In is replaced by Zn, due to the primary formation of an acceptor ZnIn1−. The ZT value reaches 0.95 ± 0.10 at ∼815 K through substituting Zn for Ag and In simultaneously. However, a higher ZT value of 1.05 ± 0.12 has been achieved by substituting an appropriate amount of Zn for Ag through largely enhancing the carrier concentration n and reducing the lattice thermal conductivity via modifying the crystal structure. Hence, we propose that when Ag is replaced by Zn in AgInSe2 there are at least two factors i.e. the carrier concentration n and bandgap Eg that govern the electrical property, and that the enhancement in carrier concentration n seems to have a more prominent effect than the widening of bandgap Eg does.

Enhanced thermoelectric performance of a chalcopyrite compound CuIn3Se5-xTex (x=0~0.5) through crystal structure engineering

In this work the chalcopyrite CuIn3Se5−xTex (x = 0~0.5) with space group through isoelectronic substitution of Te for Se have been prepared, and the crystal structure dilation has been observed with increasing Te content. This substitution allows the anion position displacement ∆u = 0.25-u to be zero at x ≈ 0.15. However, the material at x = 0.1 (∆u = 0.15 × 10−3), which is the critical Te content, presents the best thermoelectric (TE) performance with dimensionless figure of merit ZT = 0.4 at 930 K. As x value increases from 0.1, the quality factor B, which informs about how large a ZT can be expected for any given material, decreases, and the TE performance degrades gradually due to the reduction in nH and enhancement in κL. Combining with the ZTs from several chalcopyrite compounds, it is believable that the best thermoelectric performance can be achieved at a certain ∆u value (∆u ≠ 0) for a specific space group if their crystal structures can be engineered.

Improvement of the thermoelectric performance of InSe-based alloys doped with Sn

Here we present InSe-based alloys InSeSnx (x = 0–0.02) with improved thermoelectric performance upon Sns preferential occupation on In lattice sites. This improvement is attributed to the enhancement in carrier concentration (n) and reduction in lattice thermal conductivity (κL). However, the enhancement in n is limited due to the presence of the intermediate band in the middle of the bandgap, which acts as an annihilation center for electrons and holes. The reduction in κL is caused by increased phonon scattering on the newly-created defect SnIn+. As a result, we attain the highest ZT value of 0.23 at x = 0.01@830 K, which is about 2.9 times that of virgin InSe.

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 (ΔEA) between impurity and valence bands 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 the potential barrier for thermal excitation of carriers and the increase in carrier concentration. However, the Seebeck coefficient shows an increasing tendency with the increase 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 the 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 into 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.

Enhancing the 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.

Significant improvement in the thermoelectric performance of Sb-incorporated chalcopyrite compounds Cu18Ga25SbxTe50−x (x = 0–3.125) through the coordination of energy band and crystal structures

A newly developed chalcopyrite semiconductor Cu18Ga25Te50 (Cu/Ga = 0.72) has an inherent deficiency in Cu. Therefore, it is taken as a thermoelectric candidate due to a high vacancy rate of copper. In this work, we have observed that after Sb substitution for Te in Cu18Ga25SbxTe50−x, the active Sb-5p orbital hybridizes with those of Cu-4s and Te-5p in the valence band, which makes the Fermi level (Ef) unpin and move toward the inner side of the valence band as Sb content increases. The alteration in the band structure, which is the determining factor, causes the Hall carrier concentration (nH) to rise by more than one order of magnitude compared with those in pristine Cu18Ga25Te50, thereby significantly increasing the power factor (PF). Combined with the relatively low thermal conductivity, caused by the increased lattice disorder and general diminution of the crystal structure distortion as the x value increases, we have attained the best TE performance, where ZT reaches the highest value of 1.2 for the Sb-incorporated Cu18Ga25SbxTe50−x (x = 2.5) at 854 K. This result suggests that the coordination of energy band and crystal structures is a good approach to achieving high TE performance via the appropriate distortion of the crystal structure of ternary chalcopyrite semiconductors.