Jiaolin Cui

High thermoelectric performance of solid solutions CuGa1−xInxTe2 (x = 0–1.0)

We synthesized the solid solutions CuGa1−xInxTe2 (x = 0–1.0) by isoelectronic substitution of element In (Ga) for Ga(In) in the CuMTe2 (M = Ga, In) lattices and examined their thermoelectric properties. The structure upon substitution provides much high Seebeck coefficient (α), relatively low thermal (κ), and electrical conductivity (σ). AT 701 K, the α, σ, and κ are 283.15 µV K−1, 1.15 × 104 Ω−1 m−1, and 0.71 W m−1 K−1, respectively, for CuGa0.36In0.64Te2, which give the figure of merit (ZT) of 0.91, about two times those of the mother compounds CuGaTe2 and CuInTe2. This material holds great application perspectives at intermediate temperatures.

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.

Bandgap reduction responsible for the improved thermoelectric performance of bulk polycrystalline In2–xCuxSe3 (x = 0−0.2)

α-In2Se3 is of large bandgap (∼1.4 eV) semiconductor and its structure is based on two-layer hexagonally packed arrays of selenium atoms with 1/3 of the sites of indium atoms being empty. Here we report a bandgap Eg reduction due mainly to the formation of a Cu2Se slab in the host In2Se3, which is responsible for the remarkable improvement of thermoelectric performance of bulk polycrystalline In2−xCuxSe3 (x = 0.1–0.2). When x = 0.2 the dimensionless figure of merit ZT and power factor were increased by a factor of 2 and 3, respectively, at 846 K if compared to those of Cu-free In2Se3. Interestingly, an incorporation of Cu into the lattice of In2Se3 results in a change in morphology from amorphouslike structure represented by In2Se3 to a visible polycrystalline form attributed to partial crystallization of the structure. This change enhances lattice thermal conductivities κL over the very low values of In2Se3. However, the enhancement is only moderate because of the effective scattering of phonons in the p...

Thermoelectric properties in nanostructured homologous series alloys GamSbnTe1.5(m+n)

In this paper we reported the thermoelectric (TE) properties in nanostructured homologous series alloys GamSbnTe1.5(m+n) over the temperature range of 318–482 K and observed the maximum TE figure of merit (ZT) value of 0.98 for the alloy with m:n=1:10 at 482 K, which is approximately 0.24 higher than that of undoped Sb2Te3 at the corresponding temperature. This improvement is mainly attributed to the substantial reduction in lattice thermal conductivity due to the phonon scattering caused partly by the nanograins (<30 nm) and amorphous structure conceived in the matrix and partly by the lattice distortion resulted from an occupation of some Ga atoms in the Sb sites and a certain amount of Ga2Te3 precipitation. If in comparison with the TE properties for Ga directly doped Bi–Sb–Te solid solutions, we conclude that these Bi-free nanostructured homologous series alloys GamSbnTe1.5(m+n) with proper compositions are of great potentiality for the improvement of TE performance.

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.