Enhanced Thermoelectric Properties of Cu₃SbSe₄ Compounds via Gallium Doping

Abstract

In this study, the p-type Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds were fabricated by melting, annealing, grinding, and spark plasma sintering (SPS). The transport properties of Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds were investigated. As Ga content increased, the hole concentration of Cu₃Sb_1−xGa_xSe₄ compounds increased, which led to an increase in electrical conductivity. Meanwhile, the Seebeck coefficient of the Cu₃Sb_1−xGa_xSe₄ compounds decreased as Ga content increased. The extra phonon scattering originating from Ga-doping effectively depressed the lattice thermal conductivity of the Cu₃Sb_1−xGa_xSe₄ compounds. The ZT value of Cu₃SbSe₄ markedly improved, which is primarily ascribed to the depressed lattice thermal conductivity and the increased electrical conductivity. The highest ZT value for the Cu₃Sb_0.985Ga_0.015Se₄ compound was 0.54 at 650 K, which is two times higher than that of a pure Cu₃SbSe₄ compound.

1. Introduction

With the global environmental issue and energy crisis becoming more and more serious, developing renewable and eco-friendly technologies for the sustainable development has gained more attention. Moreover, substantial amounts of waste heat from industrial, private, and transport sectors in modern society should be effectively recovered. Thermoelectric material provides a possibility to solve the issues mentioned above. Thermoelectric material is a kind of energy conversion material, which can realize the conversion between heat energy and electric energy. Thermoelectric material is expected to play a significant role in the field of electronic cooling, power generation, and waste heat recovery. The efficiency of thermoelectric material is usually characterized by the dimensionless figure of merit ZT. The ZT value can be calculated using the equation ZT = σα²T/κ, where κ, T, α, and σ are the total thermal conductivity, absolute temperature, Seebeck coefficient, and electrical conductivity, respectively [1,2,3,4,5]. The total thermal conductivity consists of a carrier part (κ_c) and a phonon part (κ_l). Therefore, a large ZT requires the thermoelectric material to have a low κ, a high α, and a high σ. Nowadays, developing high ZT material has been a research focus in the field of thermoelectric materials. As the α, σ, and κ_e of a material are associated closely with carrier concentration, how to optimize the carrier concentration to realize the maximum ZT is a key issue in this field. To improve ZT, many feasible methods have been developed and applied. Band engineering including electric band structure and valley degeneracy has been regarded as an efficient approach to improve the power factor (PF = α²σ), thereby enhancing the ZT. Doping or nanostructuring are also effective ways of enhancing the ZT by introducing extra phonon scattering centers [6,7,8,9,10].

Recently, copper-based chalcogenide semiconductors have attracted much attention because of their relatively high carrier mobility (μ_H) and low κ, such as CuGa(In)Te₂, Cu₂CdSnX₄ (X = Se, S), Cu₂SnSe₃, and Cu₃SbSe₄ [11,12,13,14]. Among these compounds, ternary Cu₃SbSe₄ semiconductor has emerged as a promising thermoelectric material because of its narrow band gap and large carrier effective mass. Cu₃SbSe₄ has a superlattice of a zinc-blended structure and is of the type Cu₂FeSnS₄ with space group I-42m. The Cu/Se atoms form an electrically conductive framework and the remaining Sb atoms form the one- dimensional [SbSe₄] tetrahedra. This special tetrahedra in the Cu₃SbSe₄ crystal structure can enhance phonon scattering, similar to the “rattling atom” in skutterudite, resulting in a decrease in lattice thermal conductivity. Therefore, Cu₃SbSe₄ has a relatively low thermal conductivity. However, the electrical properties of intrinsic Cu₃SbSe₄ is poor due to its low hole concentration (p), which decreases the thermoelectric performance and leads to a low ZT value in the middle temperature range [15,16,17,18]. Theoretically, partial substitution on the Sb site of the Cu₃SbSe₄ can tune its electrical conductivity so as to enhance the thermoelectric performance. Previous studies about doping on the Sb site have been carried out, and some valuable work has been achieved [19,20]. Qin et al. synthesized the Al-doped Cu₃Sb_1−xAl_xSe₄ compounds and the maximum ZT reached 0.58 @ 600 K [21]. Similarly, Ge-doping or In-doping on the Sn site of Cu₃SbSe₄ was carried out, and the ZT value was enhanced to some extent [22,23]. Gallium has been shown to be a promising dopant in copper-based chalcogenide systems [24], but very little literature on Cu₃SbSe₄ has been reported. In the present work, the Ga substation on the Sb site is investigated in synthesized Cu₃Sb_1−xGa_xSe₄ compounds, and our experimental results demonstrate that Ga-doping can effectively optimize carrier concentration (p) and decrease κ simultaneously. The paper investigated the phase composition, microstructure, and transport properties of Cu₃Sb_1−xGa_xSe₄ compounds. The highest ZT of 0.54 was obtained for the Cu₃Sb_0.985Ga_0.015Se₄ compound.

2. Experimental Procedures

Cu₃Sb_1−xGa_xSe₄ (x = 0, 0.005, 0.010, 0.015) compounds were conventionally synthesized via melting, annealing, grinding, and spark plasma sintering (SPS). The stoichiometric mixtures of pure elements Cu (powder, 99.98%), Sb (powder, 99.998%), Ga (granule, 99.998%), and Se (granule, 99.998%) were loaded in a graphite crucible. Then, the graphite crucible was sealed in a quartz tube, heated to 1173 K, and left for 720 min. The quartz tube was slowly cooled to 773 K at the rate of 0.5 K/min and subsequently quenched in salt water. Then, the sample was annealed at 573 K and the holding time is 48 h to ensure homogeneity. Lastly, the resultant alloys were ground in ethyl alcohol in an agate mortar. The obtained powder was sintered via SPS at 683 K in a vacuum of 0.1 Pa. The axial pressure and holding time were 50 MPa and 5 min, respectively. The Archimedes method was adopted to measure the density (d) of samples.

X-ray diffractometer equipment with Cu K_α radiation (Rigaku Rint 2000) was used to analyze the phase composition of the Cu₃Sb_1−xGa_xSe₄ samples. Scanning electron microscopy (SEM, JXA-8200, JEOL, Tokyo, Japan) was employed to characterize the microstructure of Cu₃Sb_1−xGa_xSe₄ samples. ZEM-3 apparatus (ULVAC-RIKO, Yokohama, Japan) was used to measure the σ and α in the temperature range of 300–650 K in an argon atmosphere. The measurement of thermal diffusivity (λ) of Cu₃Sb_1−xGa_xSe₄ compounds was carried out using a laser flash equipment (Netzsch, LFA427) in an argon atmosphere under a vacuum of 0.001 Pa. A differential scanning calorimetry (Netzsch, DSC404, Munich, Germany) was used to measure the specific heat capacity (C_p) of Cu₃Sb_1−xGa_xSe₄ compounds. The thermal conductivity was then obtained by the equation κ = dλC_p. Van der Pauw’s method was adopted to measure the Hall coefficient (R_H). Hall measurement was carried out in a vacuum of 0.1 Pa with a constant magnetic strength of 0.5 T. The p can be calculated using the equation p_H = 1/(R_He), where e is the electron charge. The μ_H was obtained using the equations of μ_H = R_Hσ.

3. Results and Discussion

3.1. XRD Analysis and Microstructure

The X-ray diffraction patterns of Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds is present in Figure 1. All major XRD peaks coincide well with the stand JCPDS card of Cu₃SbSe₄ (No. 01-085-0003). Therefore, the Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds are single phase and have the same crystallographic structure with a pure Cu₃SbSe₄ phase. In addition, no impurity phase was detected in the XRD results, suggesting the amount of Ga-doping in this study is within the doping limit. However, as the Ga content in Cu₃Sb_1−xGa_xSe₄ increases, no obvious peak shift is found. On the one hand, the Ga content is very low; on the other hand, it is possibly related with the similar atomic radius of Ga and Sb. Chen et al. synthesized the Cu₃Sb_1−xGe_xSe₄ compounds and the small atomic radius of Ge resulted in a decrease in the lattice constant of the Cu₃Sb_1−xGe_xSe₄ compounds [22]. The SEM image and elemental distribution maps, including Cu, Sb, and Se elements for the Cu₃Sb_0.985Ga_0.015Se₄ compound is displayed in Figure 2. It can be seen that each element (Cu, Sb and Se) was uniform with no notable brighter regions, indicating that all elements distributed homogeneously in the matrix. Meanwhile, no visible other phase can be found in the SEM, which is also in agreement with the XRD result above.

3.2. Electrical Performance

The temperature dependence of σ for the Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds is present in Figure 3. As the temperature increases, the σ of the Cu₃Sb_1−xGa_xSe₄ samples increases, indicating typical heavily doped semiconducting behavior. Moreover, the σ of these samples increases as the Ga content increases. The improvement in σ for Cu₃Sb_1−xGa_xSe₄ should be ascribed to an increase in carrier concentration (p) resulting from the Ga-doping. The calculated carrier concentration of pure Cu₃SbSe₄ was about 1.90 × 10¹⁸ cm⁻³. The thermoelectric properties and structural parameters of Cu₃Sb_1−xGa_xSe₄ compounds at room temperature are listed in

Table 1. Thermoelectric properties and structural parameter of Ga-doped Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds at room temperature.

x	κL (W m−1 K−1)	Relative Density	α (μV/K)	σ (Ω−1 cm−1)	p (1018 cm−3)	μH (cm2/Vs)	m* (m0)
0	3.19	98.5%	405	23.2	1.90	76.2	1.2
0.005	2.71	98.7%	244	50.8	8.01	39.2	1.4
0.010	2.54	98.3%	222	55.9	9.84	35.5	1.6
0.015	2.29	98.8%	208	62.7	12.7	30.8	1.5

. The hole concentration of Ga-doped Cu₃Sb_1−xGa_xSe₄ is higher than that of pure Cu₃SbSe₄. The hole concentration increases rapidly from 1.90 × 10¹⁸ to 12.7 × 10¹⁸ cm⁻³ when the Ga content increases from 0 to 0.015. Meanwhile, the corresponding μ_H decreases from 76.2 cm²/Vs for pure Cu₃SbSe₄ to 30.8 cm²/Vs for the Cu₃Sb_0.985Ga_0.015Se₄ sample. The extra ionized impurity scattering and alloy scattering should result in a decrease in μ_H. The μ_H of the Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds is present in Figure 4. It can be seen that the μ_H of these compounds shows a gradual downward trend with the increase in temperature. In addition, the relationship of μ_H ∞ T^−3/2 can be found at high temperature, which indicates that the dominant scattering mechanism of Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds is phonon scattering. As the Ga content increases, the relationship of μ_H ∞ T^−3/2 of these compounds becomes weak, indicating that the dominant mechanism is mixed scattering for the Cu₃Sb_1−xGa_xSe₄ compounds at high temperature [8]. Moreover, the μ_H of Ga-doped Cu₃Sb_1−xGa_xSe₄ samples in this study is between 30 and 40 cm² V⁻¹ s⁻¹ at room temperature. Shi et al. calculated the μ_H of Cu₂SnSe₃ materials, and the results showed that the μ_H was about 52 cm² V⁻¹ s⁻¹ at room temperature. The similar Hall mobility is possibly related to the similar density of states effective mass [25,26].

Figure 5 demonstrates the Seebeck coefficients for Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds. All of the Cu₃Sb_1−xGa_xSe₄ samples exhibit a p-type character, and the major charge carriers are holes. As the temperature increases, the α of the pure Cu₃SbSe₄ samples decreases, from 405 μV/K at 300 K to 291 μV/K at 650 K. Nevertheless, the α of Ga-doped samples firstly increases approximately linearly to a maximum value, and then decreases, suggesting a heavily degenerate semiconductor behavior. For example, the peak value of α for the Cu₃Sb_0.985Ga_0.015Se₄ compound is 295 μV/K at 500 K. The α decreases linearly to 260 μV/K at 650 K. Similar behaviors have been reported in In-doped Cu₃SbSe₄ samples [26]. In addition, the α of Ga-doped samples decreases as the Ga-doped content increases because of the increase in hole concentration. Generally, the Seebeck coefficient can be written as:

$$\alpha =\pm \frac{k_B}{e}\left[2+\ln \frac{2{\left(2\pi m^{\ast }{k}_{B}T\right)}^{\frac{3}{2}}}{h^{3}p}\right]$$

(1)

where m* is the density of states effective mass, h is Planck’s constant, and k_B is Boltzmann constant [4,5,6]. As the increase in hole concentration has a more significant effect than the increase in the density of states effective mass (m*, Table 1), the α of the Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds decreases as Ga-doped content increases.

3.3. Thermal Conductivity

Figure 6a,b show the temperature dependences of the total thermal conductivity κ and the phonon part κ_l for Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds, respectively. The κ of all Cu₃Sb_1−xGa_xSe₄ compounds decreases as the temperature increases. In addition, the κ of Ga-doped Cu₃Sb_1−xGa_xSe₄ samples is markedly lower that of pure Cu₃SbSe₄, which should be attributed to the decrease in κ_l, resulting from the increase of point defect scattering. The κ of the material consists of a carrier part (κ_c) and a lattice part (κ_l). The electron part (κ_c) can be obtained using the Wiedemann–Franz equation, κ_c = L₀σT, where L₀ is the Lorenz number. As the Lorenz number varies with the temperature and the composition of materials, the precise Lorenz number is adopted according to the method in [27]. Therefore, the κ_l can be obtained by subtracting the κ_c from the κ. The κ_l of Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds drastically decreases with increasing Ga content, as shown in Figure 6b. In addition, the κ_l shows a temperature dependence of T⁻¹, as illustrated by the blue dotted line, indicating that phonon–phonon scattering is the dominant scattering for the pure Cu₃SbSe₄ sample and the Ga-doped Cu₃Sb_1−xGa_xSe₄ samples. For the Cu₃Sb_0.985Ga_0.015Se₄ sample, the κ_l is 2.27 W/mK at room temperature, which is reduced by 30% than that of pure Cu₃SbSe₄. The minimum κ_l of the Cu₃Sb_0.985Ga_0.015Se₄ sample in this study is 0.62 W/mK at 650K. As far as is known, the theoretical minimal value of lattice thermal conductivity, κ_lmin, can be evaluated according to the equation κ_lmin = 1/3lν_mC_v, where l, C_v, and ν_m are the mean free path of the phonon, the isochoric specific heat, and the mean sound velocity, respectively. The red dashed line in Figure 6b presents the theoretical minimal value of lattice thermal conductivity for pure Cu₃SbSe₄ and the obtained κ_lmin is 0.47 W m⁻¹ K⁻¹, as shown in the red dashed line. It also can be concluded from Figure 6b that there is still a potential possibility to further decrease the κ_l of the Cu₃SbSe₄ compound.

3.4. Figure of Merit

The ZT value for Cu₃Sb_1−xGa_xSe₄ (0 ≤ x ≤ 0.015) compounds is present in Figure 7. The ZT value of Cu₃SbSe₄ sample increases from 0.02 to 0.18 in the temperature ranged from room temperature to 650 K. Compared with the ZT value of pure Cu₃SbSe₄, the ZT of Ga-doped Cu₃Sb_1−xGa_xSe₄ sample is obviously improved. For example, the ZT of the Cu₃Sb_0.995Ga_0.005Se₄ compound is 0.36 at 650 K, which is one higher than the ZT of pure Cu₃SbSe₄. The maximal ZT value of the Cu₃Sb_0.985Ga_0.015Se₄ compound can reach 0.54 at 650 K, which is about 3 times as large as that of the pure Cu₃SbSe₄ compound.

4. Conclusions

In this study, p-type Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds were fabricated by melting, annealing, and SPS. Compared with a pure Cu₃SbSe₄ compound, Ga-doped Cu₃Sb_1−xGa_xSe₄ compounds showed a large increase in electrical conductivity resulting from the substantial increase in carrier concentration. However, the Seebeck coefficient of the Cu₃Sb_1−xGa_xSe₄ compounds decreased as the Ga content increased. The Seebeck coefficient of Ga-doped samples firstly increased approximately linearly to a maximum value and then decreased. Meanwhile, the thermal conductivity of the Cu₃Sb_1−xGa_xSe₄ compounds markedly decreased because of the extra phonon scattering originating from the Ga-doping on the Sb site. Therefore, the increased electrical conductivity and the depressed lattice thermal conductivity effectively enhanced the ZT value of Cu₃SbSe₄. The maximum ZT value for the Cu₃Sb_0.985Ga_0.015Se₄ compounds was 0.54 at 650 K, which is around two times larger than that of pure Cu₃SbSe₄ compounds.