Manipulating Band Convergence and Resonant State in

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New. Energy Materials, Department of Physics, Jinan University, Guangzh...
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Manipulating Band Convergence and Resonant State in Thermoelectric Material SnTe by Mn−In Codoping Ling Wang,†,‡ Xiaojian Tan,*,‡ Guoqiang Liu,‡ Jingtao Xu,‡ Hezhu Shao,‡ Bo Yu,‡ Haochuan Jiang,‡ Song Yue,*,† and Jun Jiang*,‡ †

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou 510632, China ‡ Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo 315201, China S Supporting Information *

ABSTRACT: We report an enhanced thermoelectric performance by manipulating band engineering in Mn−In codoped SnTe. It has been revealed that SnTe is a unique example achieving the synergy of band convergence and resonant state. According to band theory, band convergence favors heavy doping, while resonant state favors light doping. Following this idea, a series of Mn−In codoped SnTe samples are prepared by hot pressing. A significantly enhanced Seebeck c o e ffic i e n t o f 1 1 6 μ V K − 1 a t 3 0 0 K i s o b s e r v e d i n Sn0.915Mn0.11In0.005Te. By carefully tuning the band structure of the solid solution, we achieve a high ZTmax of 1.15 at 823 K and an overall enhanced ZTave of 0.62. The improved thermoelectric performance in a large temperature range leads to a competitive conversion efficiency of 10.1% with Tc = 300 K and Th = 850 K, suggesting Mn−In codoped SnTe is a promising candidate for medium-temperature thermoelectric applications.

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and the valley number is thus increased, which enhances the S without the penalty of degrading σ. Lead-free SnTe was expected to be a promising substitute for PbTe, due to the similar crystal and band structures.10−12 In recent years, valence band convergence13−23 and resonant state at the edge of valence bands24−26 have been achieved in SnTe, and the peak ZTs are significantly improved from 0.6 to 1.2− 1.4 around 900 K. Nevertheless, such results are still far from the expectation, and TE applications not only require a high peak ZT but also require high ZTs in a wide temperature region. Thus, it is urgent to further improve the overall TE performance of SnTe. Very recently, both experimental and theoretical investigations indicated that the synergy of band convergence and resonant state can be realized in SnTe, leading to further improved TE performance.26−28 Different from other TE materials, SnTe is a unique example achieving such a synergistic effect. Because the location of resonant state of In doping mutually matches the energy separation between light and heavy valence bands,26 hole concentration tuning may shift the

hermoelectric (TE) materials have attracted much attention because they can realize directly convert between the waste heat to renewable electricity.1−3 The conversion efficiency of TE material is determined by the dimensionless figure of merit ZT = S2σT/κ, where S, σ, T, and κ denote the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively.4 The pure electrical properties S2σ should be maximized, not only for better conversion efficiency but also for higher output power in generation. However, it remains a challenge because S and σ are coupled to each other and always show opposite trends. Since the electrical properties are strongly dependent on the geometry of the band structure, the concept of “band engineering” has been shown to enable high S2σ by somewhat decoupling S and σ.5−7 Resonate level, which has been applied in PbTe, β-Zn4Sb3, etc.,6,8 is a method of band engineering by creating a “hump” in the density of state (DOS). It somewhat lowers carrier mobility (μ) but greatly enhances S, especially at low temperatures, leading to improved S2σ. Another method is band convergence, which has been widely used to enhance S2σ in many TE materials such as PbTe, Mg2Si1−xSnx, etc.5,7,9 The band edges of two valence or conduction bands are brought close together, © XXXX American Chemical Society

Received: April 1, 2017 Accepted: May 1, 2017

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Figure 1. (a) Schematic band structure for pristine and Mn−In codoped SnTe describing the synergistic band engineering. (b) Calculated DOS for pristine, Mn-doped, In-doped, and Mn−In codoped SnTe with spin−orbit coupling. The top of the light valence band is at 0 eV.

The synergistic effect of band engineering in Mn−In codoped SnTe is well-confirmed by the following experimental measurements. In Figure 2, we compare the present room-

Fermi level to an ideal position, where both resonant state and valence band convergence can affect S. The synergy in PbTe is impossible because its resonant state by Tl doping is energetically separated from the level of band convergence.29 To realize the synergistic effect in SnTe, In doping is irreplaceable in introducing the element-selective resonant state near the Fermi level,26 and another dopant could be Cd, Mg, Mn, Ag, Hg, etc.13−21 The codoping of In−Cd and In−Ag has shown that the synergistic effect strongly improves the TE performance.27,28 However, the doping concentrations of Cd/Ag and In were treated equally, which may not lead to the best performance of the two different effects. Band convergence is a tuning of the periodical band structure, and it requires a relative high doping level to influence the k-space band dispersion. Resonant state is a distortion of the DOS, and it may largely decrease σ owing to the reduction of μ at high doping levels. Therefore, a lower resonant state doping and a higher band convergence doping are preferred. As is well-known, the solubility of Mn is greater than that of Cd or Ag, which enables a significant valence band convergence in SnTe.19,20 In this work, we take Mn−In codoping as an example to realize the optimized synergistic effect of band convergence and resonant state in SnTe. Figure 1 presents the calculated DOS and schematic band structure for pristine, Mndoped, In-doped, and Mn−In codoped SnTe. As can be seen, the DOS near the top of the valence band for Sn1−xMnxTe is significantly higher than that of pristine SnTe, which originates from the band convergence of the light and heavy valence bands.22 For In doping, an obvious DOS peak could be found about 0.2 eV lower than the edge of the band gap, which indicates a very local resonant level resulting from the In−Te antibonding state.26 Both the band convergence of Mn doping and resonant level of In doping have been demonstrated by previous experimental works to be effective in enhancing the S and S2σ for SnTe.17−20,24,25 In the Mn−In codoped system, we find that the obvious DOS peak by In doping is still present when incorporating Mn atoms in the lattice. Interestingly, the increased DOS near the top of valence band is markedly higher than that of In-doped SnTe. It can therefore be concluded that the resonant state still exists and is even encouraged by Mn doping in Mn−In codoped SnTe. The synergy of band convergence and resonant level in SnTe is naturally expected to further enhance the S.

Figure 2. Seebeck coefficients as a function of hole concentration for SnTe with different dopants at room temperature.

temperature S versus n data of Mn-doped and Mn−In codoped SnTe samples with previously reported experimental results on undoped,10,11 In-doped,24,25 and Mn-doped SnTe.17−20 The theoretical Pisarenko curve at room temperature is calculated based on a two valence band model24 and is presented for comparison. The results of undoped SnTe fall exactly on the Pisarenko line and demonstrate the validity of the reference line. As Figure 2 shows, in the hole concentration range of 1−5 × 1020 cm−3, the S of Mn-doped SnTe is enhanced to 20−50 μV K−1 by the valence band convergence, compared with the prediction of the Pisarenko curve (10−20 μV K−1). The Indoped SnTe samples also have S values that are about 30−40 μV K−1 higher than the predicted values, which was reported to arise from the In resonant level localized in the valence band.24,25,27 The S enhancement by Mn doping is relatively mild compared to the In doping. Such a trend is due to resonant levels in SnTe working at relatively lower temperature while band convergence gradually affects the TE transport with increasing temperature.24,27 Additionally, Mn doping in SnTe can reduce the energy separation between the two valence bands and leads to band convergence, and this effect is proportional to the Mn content. Thus, better band convergence 1204

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ature. Compared with the pristine SnTe,19 the S2σ of Mndoped SnTe is significantly enhanced in the whole temperature range, which benefits from the valence band degeneracy. Among these Mn-doped SnTe samples, Sn0.92Mn0.11Te and Sn0.88Mn0.15Te exhibit S2σ that are much higher than those of Sn0.96Mn0.07Te near room temperature and smaller values than the latter at higher temperature. Owing to the moderate S2σ and relatively lower κtotal, Sn0.92Mn0.11Te exhibits ZTs that are relatively higher than those of the others in a broad temperature region. Thus, Mn content of 0.11 can be fixed for sufficient band convergence in SnTe. In is an irreplaceable dopant for resonant state in SnTe,26 and previous experimental results found that In could generate resonant states even at very low doping concentration (0.001− 0.01).24 As is well-known, resonant state sharply increases the DOS effective mass, which is beneficial to the S but risks the reduction of mobility (μ). Therefore, a small In content should be carefully chosen to realize the synergy of resonant state and band convergence for optimizing the S2σ. Table 1 summarizes the electronic transport coefficients: hole concentration (n), mobility (μ), electrical conductivity (σ), Seebeck coefficient (S), and power factor (S2σ) for Sn0.92−yMn0.11InyTe (y = 0, 0.005, 0.01) samples. As may be seen, In doping with y = 0.005 decreases μ and significantly increases S as expected, which demonstrates that Mn−In codoping is synergic in SnTe. When the In content increases to 0.01, the n and σ slightly increase but the S obviously decreases, leading to a much lower S2σ. Therefore, the appropriate In content in Mn−In codoped SnTe is 0.005, which is much lower than the Mn content. In contrast to the equal contents of Cd/Ag and In reported previously,27,28 such different contents of Mn and In need to be carefully manipulated according to the different mechanisms of band convergence and resonant state. As shown in Figure 3c, the S2σ of Mn−In codoped SnTe exhibits obvious improvement compared to those of Mn doping. In particular, the S2σ of Sn0.915Mn0.11In0.005Te at room temperature is 18% higher than that of Sn0.92Mn0.11Te. For further enhancement of the TE performance for Mn−In codoped SnTe, we fix the Mn content x = 0.11 and In content y = 0.005 and change the total cation ratio to tune the carrier concentration. Besides changing Sn content, aliovalent doping (such as I) can also efficiently tune the carrier concentration of SnTe.30 From the S−n relationship in Figure 2, such Sn content tuning slightly increases the n from 1.2 to 1.5 × 1020 cm−3 and mildly decreases the corresponding S from 116 to 108 μV K−1; more detailed transport coefficients are summarized in Figure S3. Benefiting from the synergistic effects of Mn−In codoping, the power factor S2σ of Sn0.905Mn0.11In0.005Te is much higher than that of Sn0.92Mn0.11Te from 300 to 600 K. For example, the S2σ of the former is 28% higher than that of the latter at room temperature (see Figure 3c). At higher temperature above 600 K, the Seebeck coefficient of Mn−In codoped system is no longer improved, but the increased electrical

is obtained in higher Mn content samples, resulting in higher S. In our Mn-doped SnTe samples, Mn content x = 7−13% is actually high and close to the solid solubility limit (Figure S1 in the Supporting Information); thus, the corresponding S value (blue solid dots) is about 80 μV K−1 higher than those reported previously. As expected, Mn−In codoped SnTe has S at room temperature that is significantly higher than the Pisarenko curve. Specifically, the S of Mn−In codoped samples at n = 1.2 × 1020 cm−3 reaches 116 μV K−1, which is much higher than those of Mn- or In- doped samples at similar hole concentration. These results indicate that band convergence caused by Mn doping and resonant state caused by In doping may harmoniously coexist and synergistically work in Mn−In codoped SnTe. Considering the fact that SnTe has a very high hole concentration owing to the intrinsic Sn vacancy, we choose self-compensated Sn1.03Te16 as the base material in this work. The following experimental investigation consists of three steps: (1) introducing Mn dopant to Sn1.03−xMnxTe for sufficient band convergence; (2) based on Sn1.03−xMnxTe, introducing tiny In dopant to realize resonant levels; (3) with the contents of Mn and In optimized, tuning the hole concentration via Sn content adjustment. Figure 3a,b shows the temperature-dependent power factor S2σ and ZTs for Sn1.03−xMnxTe (x = 0.07, 0.11, 0.15) samples,

Figure 3. Temperature-dependent (a) power factor and (b) ZT values for Mn-doped SnTe. Panels c and d are S2σ and ZT values for Mn−In codoped SnTe.

and the corresponding Seebeck coefficient (S), electronic conductivity (σ), total thermal conductivity (κtotal), as well as the lattice portion (κl) are presented in Figure S2. With temperature increases, the S2σ increases initially, then mildly reaches a maximum, and finally decreases at higher temper-

Table 1. Hole Concentration (n), Mobility (μ), Electrical Conductivity (σ), Seebeck Coefficient (S), and Power Factor (S2σ) of Sn0.92−yMn0.11InyTe (y = 0, 0.005, 0.01) at Room Temperature sample

n (×1020 cm−3)

μ (cm2 V−1 s−1)

σ (104 Sm−1)

S (μV K−1)

S2σ (μW cm−1 K−2)

y=0 y = 0.005 y = 0.01

0.94 1.22 1.58

81.3 32.6 31.3

12.2 6.34 7.86

78.1 117 91.2

7.44 8.68 6.54

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MnTe,19 and CdS,13,27 can significantly reduce κl to about 0.6 W m−1 K−1 near 823 K. Combining the synergistic band engineering and phonon scattering design, the TE performance of Mn−In codoped SnTe would be more competitive. In summary, our first-principles calculations predict that Mn−In codoping may effectively combine the effects of valence band convergence and resonant state to enhance the TE properties of p-type SnTe. We then manipulate the Mn−In content to realize the synergistic effect of band engineering in SnTe by melting and hot pressing method, and an appropriate scheme is found to be heavy Mn doping of 0.11 and relatively light In doping of 0.005. The measured S−n relationship demonstrates that Mn−In codoping has a cumulative beneficial effect on the TE performance. Moreover, when the hole concentration is tuned, the power factor of Mn−In codoped sample is increased within a broad temperature range. As a consequence, the ZTmax at 823 K and ZTave between 300−823 K are improved to 1.12 and 0.62, respectively, and the predicted conversion efficiency is over 10% for the temperature difference of 525 K.

conductivity can still lead to the improvement of power factor (see Figure S4). Figure 3d compares the ZT values of our Mn-doped and Mn−In codoped SnTe sample with the results of previously reported Mn-doped,19 In-doped, and Cd−In codoped SnTe samples.24 Consistent with the prediction from band engineering, band convergence by Mn doping and resonant state by In doping synergistically improve the ZTs of SnTe. The maximum ZT value of our Mn−In codoped SnTe is 1.15 at 823 K, which is significantly higher than those of Mn-doped or In-doped SnTe sample.19,24 Such ZT value is also higher than that of previously reported codoped SnTe with equal content of Cd and In.27 This observation implies that the doping concentration in codoped SnTe should be well-designed, i.e., lower In content and higher Mn (Cd, Mg, Ag, Hg, etc.) content is basically demanded for the synergy of resonant state and band convergence. In practice, average ZT value in a wide temperature is even more important than the peak value. Figure 4a schematically

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00285. Brief statement of experimental method for sample synthesis, physical characterization, and band structure calculations; XRD diffraction patterns, lattice parameters, and TE transport coefficients (Seebeck coefficient, electrical conductivity, and thermal conductivity) for doped SnTe samples (PDF)

Figure 4. (a) Average ZT (ZTave) between 300−823 K and the maximum ZT (ZTmax) at 823 K, and (b) the conversion efficiency η as Tc = 300 K for pristine SnTe,19 Mn-doped,19 In-doped,24 and Mn−In codoped SnTe. The results of Cd−In codoping27 are also shown for comparison.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

presents the average ZT (ZTave) between 300−823 K and the maximum ZT (ZTmax) at 823 K for SnTe samples. As shown, utilizing band convergence (Mn doping)19 or resonant levels (In doping)24 to modify the top of valence band for SnTe individually, one can readily improve the ZTave relative to pristine SnTe. Combining both effects by Mn−In codoping could yield even higher ZTave of 0.62 between 300 and 823 K, which is actually higher than those of pristine SnTe (0.16),19 Mn-doped (0.31),19 and In-doped SnTe (0.33).24 A developed cumulative temperature dependence model has been proved to reliably predict the TE conversion efficiency with the Thomson effect and Joule heat considered.31 Following this scheme, we evaluate the conversion efficiency for these SnTe samples based on the experimentally measured temperature dependence of S, σ, and κ. Figure 4b displays the predicted efficiency of pristine, Mn-doped, In-doped, Mn−In codoped, and Cd−In codoped SnTe samples. It can be found that the conversion efficiency of the Sn0.905Mn0.11In0.005Te is 10.1% with Tc = 300 K and Th = 850 K, which is much higher than those of other SnTe samples. It should be emphasized that there is still room to further improve the TE performance of the SnTe-based materials. Multiscale phonon scattering is a well-demonstrated independent strategy by reducing κl. As shown in Figure S3d, the κl of Sn0.905Mn0.11In0.005Te is 0.91 W m−1 K−1 at 823 K. In particular, second-phase nanostructures, such as AgBiTe2,32 AgSbSe2,33

ORCID

Xiaojian Tan: 0000-0002-6949-9255 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11404350, 11404348, and 11234012), Zhejiang Provincial Science Foundation for Distinguished Young Scholars (LR16E020001), and Ningbo Science and Technology Innovation Team (2014B82004).

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