Raising thermoelectric performance of n-type SnSe via Br doping and Pb alloying

Abstract

High ZT value of ∼1.2 at 773 K was achieved in n-type polycrystalline SnSe. The high thermoelectric performance derives from the low thermal conductivity of SnSe and enhanced electrical conductivity induced by Br doping and Pb alloying.

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Thermoelectric technology, capable of directly converting heat into electricity, provides a promising route to power generation. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit, ZT = (S²σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively.^1–6 Over decades, various efforts have been made to enhance ZT.^7–13 Recently, IV–VI compound SnSe received extensive attention for its remarkable high thermoelectric performance in undoped SnSe crystals and high ZT_ave of ∼1.34 at 300–773 K in hole-doped SnSe crystals.^14–17 SnSe crystallizes in a layered orthorhombic crystal structure with the space group Pnma at room temperature and lattice parameters: a = 11.49 Å, b = 4.44 Å, c = 4.135 Å, which can be considered as layered distorted rock-salt structure. Fig. S1† shows the view of SnSe along three axes respectively. Due to the highly layered structure and phase transition from the orthorhombic structure (Pnma) to orthorhombic structure (Cmcm) at ∼800 K, SnSe single crystals cleave easily along the (001) plane, resulting in poor mechanical properties. Compared with SnSe single crystals, polycrystalline SnSe is more suitable for application. Therefore, various researches are focused on polycrystalline SnSe. However, most of the reported polycrystalline SnSe samples were focused on p-type conductors but few reports on n-type polycrystalline SnSe.^18,19

Iodine dopant was chose to optimize the carrier concentration for n-type polycrystalline SnSe, resulting in ZT value of ∼0.8 at 773 K.¹⁸ the low ZT values are limited due to the lower power factor. In this work, to enhance electrical transport properties (power factor), we chose Br as the dopant considering the similar ionic radii between Se²⁻ (1.98 Å) and Br⁻ (1.96 Å). We found that the power factor can be enhanced from ∼2 μW cm⁻¹ K⁻² for undoped SnSe to ∼5 μW cm⁻¹ K⁻² for 3% Br doped SnSe. After Pb alloying, the power factor of polycrystalline SnSe was further enhanced to ∼7 μW cm⁻¹ K⁻². As a result, the peak ZT value of ∼1.2 was achieved at ∼773 K. These results indicate that n-type polycrystalline SnSe is a promising candidate for thermoelectric applications.

Fig. 1(a) shows the powder XRD patterns of SnSe_1−xBr_x (x = 0–0.04). All peaks are indexed as orthorhombic SnSe phase, no other phases are observed within the detection limits of the measurements. The powder XRD patterns of Sn_1−xPb_xSe_0.97Br_0.03 (x = 0–0.3) are shown in Fig. 1(b). The XRD peaks shift towards lower angles with the increasing Pb fractions, indicating Pb goes into SnSe lattice. When Pb fraction reaches as high as 30%, a PbSe character peak can be observed. The variation of calculated lattice parameters with Br and Pb concentration are shown in Fig. 2. Due to the similar ionic radii between Se²⁻ (1.98 Å) and Br⁻ (1.96 Å), there is little variation in lattice parameters with increasing amount of Br, Fig. 2(a). However, the lattice parameters increase with increasing Pb fraction from 0.25% to 20%, consistent with the fact that the ionic radius of Pb²⁺ (1.49 Å) is larger than that of Sn²⁺ (1.22 Å). XRD results indicate that the solid solubility limit of Pb in SnSe is around 20%. The result is consistent with the SnSe–PbSe phase diagram in previous work.²⁰

Fig. 1 Powder XRD patterns for (a) SnSe_1−xBr_x (x = 0–0.04); (b) Sn_1−xPb_xSe_0.97Br_0.03 (x = 0–0.3).

Fig. 2 Lattice parameters for (a) SnSe_1−xBr_x (x = 0, 0.01, 0.02, 0.03, and 0.04) and (b) Sn_1−xPb_xSe_0.97Br_0.03 (x = 0, 0.025, 0.05, 0.1, 0.2 and 0.3).

SnSe crystallizes in a layered orthorhombic crystal structure, which can be considered as layered distorted rock-salt structure, resulting in strong anisotropic thermoelectric performance in single SnSe crystals along three axes.^14,15 To identify the direction possessing superior thermoelectric performance in polycrystalline SnSe, the thermal and electrical transport properties are measured along two directions that perpendicular to and parallel to the SPS pressing direction, respectively. It can be seen in Fig. S2† that the electrical transport properties (power factor) measured perpendicular to the pressing direction is superior to that measured parallel to the pressing direction. However, the thermal conductivity is higher along the direction perpendicular to the pressing direction, Fig. S2(d) and (e).† Finally, a higher ZT value is obtained parallel to the pressing direction, Fig. S2(f).† This is consistent with previous reports on polycrystalline SnSe.^18,21 In the next work, all the thermoelectric transport properties are measure along the direction parallel to the SPS pressing direction.

Zhang et al. suggested that iodine is an effective dopant to convert SnSe from p-type to n-type semiconductor.¹⁸ It is intriguing to explore the effect of Br on SnSe, considering the similar ionic radii between Se²⁻ and Br⁻. Fig. 3(a) shows the temperature dependence of electrical conductivity of SnSe_1−xBr_x, the increasing electrical conductivity with rising temperature indicates the nondegenerate characteristic. As the content of Br rises from 1% to 4%, the electrical conductivity at room temperature increases significantly from ∼10⁻³ S cm⁻¹ to ∼10⁻¹ S cm⁻¹. There is a sharp rising of electrical conductivity at ∼523 K due to the thermal excitation of carriers. However, the values are still much lower than those of IV–VI thermoelectric materials, such as PbTe,²² PbSe,²³ PbS,^24–26 SnTe.²⁷ For the 3% Br doped sample, the maximum electrical conductivity reaches ∼30 S cm⁻¹ at 773 K, nearly twice as much as that of 3% iodine doped samples,¹⁸ indicating Br is an effective dopant. The enhanced electrical conductivity can be understood with insighting into carrier concentration and carrier mobility. The electrical conductivity is determined by the following equation:

σ = neμ (1)

where n, μ are carrier concentration and carrier mobility respectively. As shown in Table S1,† the carrier concentrations increase with rising amount of Br and reach 1.83 × 10¹⁸ cm⁻³ and 1.02 × 10¹⁹ cm⁻³ for SnSe_0.97Br_0.03 and SnSe_0.96Br_0.04, respectively. These values are much higher than that (2.25 × 10¹⁷ cm⁻³) for undoped SnSe, which reconfirms that Br is an effective dopant. The carrier mobility decreases with increasing carrier concentrations and Br doping fractions, namely, carrier mobility significantly decreases from 12.5 cm² V⁻¹ s⁻¹ for undoped SnSe to 0.32 cm² V⁻¹ s⁻¹ for SnSe_0.96Br_0.04. The results indicate that the carrier mobility values obtained in polycrystalline SnSe are much lower than 250 cm² V⁻¹ s⁻¹ along b axis in SnSe crystals,¹⁴ which is the critical factor that causes the low power factor and ZT in polycrystalline SnSe samples.^18,28–32

Fig. 3 Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) total thermal conductivity, (e) lattice thermal conductivity, and (f) ZT for SnSe_1−xBr_x (x = 0, 0.01, 0.02, 0.03, and 0.04) measured along the pressing direction. The thermoelectric properties for I-doped SnSe²⁸ and single SnSe crystals²¹ are also plotted for comparisons. Specific heat, Lorenz number, thermal diffusivity and electrical conductivity are shown in Fig. S4.†

As shown in Fig. 3(b), the Seebeck coefficient shows increasing trend with rising temperature and reaches ∼−400 μV K⁻¹ at ∼773 K. The negative values demonstrate all samples are n-type semiconductors. According to the Pisarenko relations:³³

where k_B is the Boltzmann constant, e the electron charge, h the Plank constant, m* the density of states effective mass of carriers. Seebeck coefficient decreases with increasing carrier concentration.

For Br-doped SnSe samples, the Seebeck coefficients decrease with rising Br dopants at room temperature, ranging from ∼−250 μV K⁻¹ to −190 μV K⁻¹, which is consistent with the increasing carrier concentrations induced by Br⁻. Fig. 3(c) shows the temperature dependence of power factor, the power factor reaches ∼5 μW cm⁻¹ K⁻² at 773 K for 3% Br doped SnSe, which is higher than those of thermoelectric materials with low thermal conductivity such as Yb₁₄MnSb₁₁,³⁴ Ag₉TlTe₅,³⁵ AgSbTe₂.³⁶

Fig. 3(d) and (e) show the temperature dependence of total thermal conductivity and lattice thermal conductivity respectively. The lattice thermal conductivity (κ_lat) is determined by subtracting electrical thermal conductivity (κ_ele) from total thermal conductivity (κ_tot). κ_ele is calculated via the equation:

κ_ele = LσT (3)

where L, σ, T represent the Lorenz number, electrical conductivity and absolute temperature respectively. Calculation details are given in ESI.†

In consideration of the low electrical conductivity of Br doped SnSe, κ_lat is close to κ_tot from room temperature to ∼573 K. It can be seen from Fig. 3(d) that the total thermal conductivity rises with increasing amount of Br, ranging from 0.5 W m⁻¹ K⁻¹ to 0.7 W m⁻¹ K⁻¹ at room temperature. Furthermore, the value reaches ∼0.3 W m⁻¹ K⁻¹ for total thermal conductivity and even as low as ∼0.27 W m⁻¹ K⁻¹ for lattice thermal conductivity at 773 K, Fig. 3(e). The low thermal conductivity obtained in n-type polycrystalline SnSe is close to the minimum lattice thermal conductivity ∼0.26 W m⁻¹ K⁻¹ at ∼773 K.¹⁸ Compared to SnSe crystals,¹⁴ κ_lat values of n-type polycrystalline SnSe fall between the thermal conductivity values along a-axis and b-axis direction, where the lowest and highest thermal conductivity values in SnSe crystals are observed, respectively.

Fig. 3(f) shows the ZT values as a function of temperature. Due to the low electrical conductivity at low temperature, SnSe shows low ZT values below 523 K. But the ZT values increase significantly with rising temperature from 523 K to 773 K. Combined with moderate electrical transport properties and exceedingly low thermal conductivities, the SnSe doped with 3% Br reaches high ZT value of ∼1.1 at 773 K, superior to most of the previous reported p- and n-type polycrystalline SnSe.^{18,19,21,28,29,31,32,37} To further confirm the anisotropic transport, and the ZT along parallel direction is higher than that along perpendicular direction in polycrystalline SnSe. Fig. S5† shows the thermoelectric properties along two directions in the sample with 3% Br, because of the lower power factor and much higher thermal conductivity, the thermoelectric performance perpendicular to the pressing direction is much lower than that parallel to the pressing direction.

In consideration of the low thermal conductivity of SnSe, a slight increase of power factor may result in a significant enhancement of ZT. This inspired us to study the effect of Pb alloying on the electrical transport property of SnSe.

Fig. 4(a) shows the electrical conductivity of Sn_1−xPb_xSe_0.97Br_0.03 as a function of temperature. The room temperature electrical conductivity exhibits unexpected behavior with increasing amount of Pb. First, the room temperature electrical conductivity decreases when Pb fraction is lower than 10%, then increases with further increasing fraction of Pb > 10%. The room transport behaviour is consistent with the variations of carrier concentration and carrier mobility. As shown in Table S1,† the carrier concentration shows a decreasing trend with the amount of Pb < 10%. Low carrier concentration and reduced carrier mobility lead to inferior electrical conductivity compared to SnSe_0.97Br_0.03. For samples alloyed with Pb > 10%, due to the existence of PbSe second phase,²⁰ the carrier concentration and mobility are much enhanced, leading to higher electrical conductivity. The electrical conductivities show the same trend with increasing temperature as SnSe_1−xBr_x: first, electrical conductivity shows very low value and temperature-independent behavior below 523 K, then a change to a thermally activated semiconductor behavior from 523 K to 773 K. As shown in Fig. 4(b), the Seebeck coefficients decrease with rising amount of Pb at room temperature, ranging from ∼−230 μV K⁻¹ to −110 μV K⁻¹. The Seebeck coefficients show increasing trend with rising temperature. And it is notable that the Seebeck coefficients reduce slightly when Pb is within the solid solubility and reduced significantly when Pb is beyond solid solution limit. Finally high power factor value reaching 7.8 μW cm⁻¹ K⁻² is obtained in samples alloyed with 30% Pb at 773 K, which is about four times higher than that of undoped SnSe.

Fig. 4 Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) total thermal conductivity, (e) lattice thermal conductivity, and (f) ZT for Sn_1−xPb_xSe_0.97Br_0.03 (x = 0–0.3) measured along the direction parallel to the pressing direction. Specific heat, Lorenz number, thermal diffusivity and electrical conductivity are shown in Fig. S6.†

Thermal conductivity as a function of temperature are shown in Fig. 4(d) and (e). κ_tot decrease with rising temperature and then shows an upturn, which is attributed to the bipolar effect induced by Pb. However, when alloyed with <10% Pb, the bipolar effect is not obvious and low total thermal conductivity <0.4 W m⁻¹ K⁻¹ still can be obtained. As a result, SnSe alloyed with 10% Pb achieves high ZT value of ∼1.2 at 773 K, which is attribute to the high power factor, Fig. 4(f).

In summary, we demonstrated that n-type polycrystalline SnSe, an earth-abundant material, can achieve high ZT value of ∼1.2 at 773 K. The high performance of SnSe is attributed to the enhanced power factor and low thermal conductivity. We identified that Br is an effective dopant to optimize the carrier concentration in n-type polycrystalline SnSe. The effect of Pb was also studied to further enhance the electrical transport properties of SnSe. All these results indicate that n-type polycrystalline SnSe is a robust candidate for thermoelectric applications.

Acknowledgements

This work was supported by the “Zhuoyue” program of Beihang University, the Recruitment Program for Young Professionals, NSFC under Grant No. 51571007, and the Fundamental Research Funds for the Central Universities. This work was also supported by the Science, Technology and Innovation Commission of Shenzhen Municipality (Grant No. ZDSYS20141118160434515 & JCYJ20140612140151884).

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Footnote

† Electronic supplementary information (ESI) available: Information about additional experimental data including Lorenz number calculation, sample density, carrier concentration, carrier mobility, specific heat, thermal diffusivity, Lorenz number and electron thermal conductivity. See DOI: 10.1039/c6ra21884a

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