Realizing High Thermoelectric Performance in Polycrystalline

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Realizing High Thermoelectric Performance in Polycrystalline SnSe via Silver Doping and Germanium Alloying Qian Zhao,† Bingchao Qin,† Dongyang Wang,† Yuting Qiu,*,‡ and Li-Dong Zhao*,† †

School of Materials Science and Engineering, Beihang University, Beijing 100191, China Beihang School, Beihang University, Beijing 100191, China

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S Supporting Information *

ABSTRACT: It has been reported that SnSe crystals possess outstanding thermoelectric property, while polycrystals are inferior on account of the poor electrical transport properties. Therefore, we try to improve the disadvantage of polycrystalline SnSe via synergistic Ag doping and Ge alloying. First, the carrier concentration of SnSe is enhanced by Ag doping, resulting in a maximum carrier concentration ∼1.0 × 1019 cm−3. Second, the Seebeck coefficient is increased by Ge alloying through enlarging the band effective mass and narrowing the band gap, resulting in a highest power factor of ∼10.0 μW cm−1 K−2 at 793 K. In addition, Ge alloying contributes greatly to reducing the lattice thermal conductivity through scattering phonons induced by the point defects. Above all, a maximum ZT value of ∼1.5 at 793 K is obtained for the Sn0.975Ag0.01Ge0.015Se sample with the simultaneously optimized thermoelectric transport parameters. KEYWORDS: thermoelectric, p-type polycrystalline SnSe, carrier concentration, band effective mass, band gap

1. INTRODUCTION Thermoelectric materials, which can directly and reversibly convert heat to electricity, has arouse worldwide interest due to energy issues.1−4 The thermoelectric conversion efficiency is determined with the dimensionless figure of merit ZT, which is defined as ZT = (S2σT/(κl + κe)), where S, σ, T, κl, and κe are the Seebeck coefficient, electrical conductivity, absolute temperature in kelvin, lattice thermal conductivity, and electronic thermal conductivity, respectively.5−8 However, the interrelationships between these parameters make it pretty hard to improve the thermoelectric performance. To date, numerous strategies have been taken to optimize the figure of merit ZT. Typical cases include fixing the carrier concentration,8−10 manipulating the band structures,11−14 reducing the thermal conductivity,15−17 and searching after thermoelectric systems with intrinsically low lattice thermal conductivity,18,19 etc. SnSe crystals have attracted the interest of the thermoelectric community owing to their extraordinary thermoelectric properties.18,20,21 The remarkably high ZT values have been obtained both in p-type and n-type SnSe crystals.22,23 To date, the polycrystalline SnSe system has also achieved significant development with a simpler preparation process.24,25 However, SnSe polycrystals exhibit lower ZTs owing to the poor carrier mobility and electrical conductivity. Recently, SnSe polycrystal upon proper hole-doping with PbSe-alloying shows the recordhigh ZT value of ∼2.5 through removing most of the tin oxide films covering the SnSe grains,26 which encourages us to additionally explore the potential thermoelectric performance in SnSe polycrystals. It has been well elucidated that the approach of optimizing carrier concentration is effective to enhance the electrical transport properties.22,27 Compared with © XXXX American Chemical Society

the p-type dopants of Na and K, Ag is easily processed and also could optimize the carrier concentration of p-type SnSe.9,12,28,29 Moreover, it is demonstrated that forming a solid solution is a pretty efficient strategy to decrease the lattice thermal conductivity of SnSe.30 The typical example, GeSe, possesses an orthorhombic layered structure that is the same as that of SnSe, and it is predicted that alloying with SnSe shows promising thermoelectric performance.31 Experimentally, it is reported that GeSe alloying in SnSe can obtain a giant Seebeck coefficient owing to the increased band gap energy and a modified band structure.32 Meanwhile, GeSe alloying in SnSe induces point defects; it can greatly suppress the lattice thermal conductivity.33 Motivated by these reports, we have investigated SnSe polycrystals, aiming to enhance their thermoelectric performance through stepwise Ag doping and Ge alloying. First, the Sn1−xAgxSe samples with x = 0−0.03 are produced and the thermoelectric properties are discussed. With Ag doping, the carrier concentration is optimized due to introducing more holes, resulting in a maximum carrier concentration ∼ 1.0 × 1019 cm−3 in Sn0.99Ag0.01Se. Then the Seebeck coefficients are enhanced through Ge alloying in Sn0.99‑yAg0.01GeySe, so that the power factor is improved to ∼10 μWbcm−1 K−2 at 793 K. The point defects produced by Ag doping and Ge alloying yield a lower lattice thermal conductivity via significantly scattering phonons, which could be theoretically confirmed Special Issue: Thermoelectrics Received: July 29, 2019 Accepted: August 12, 2019


DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials with the Callaway model. Such a system maintained a higher power factor with a lower thermal conductivity; a maximum ZT value of ∼1.5 at 793 K is achieved in Sn0.975Ag0.01Ge0.015Se, which is almost twice as that of pristine SnSe, showing that polycrystalline SnSe is one of the most promising thermoelectric materials.

2. EXPERIMENTAL SECTION All of the samples were synthesized using melting method and subsequently densified with spark plasma sintering (SPS-211Lx). The sample phases were identified using powder X-ray diffraction (PXRD), and the distributions of elements were measured by scanning electron microscope (SEM; JSM7500) using the attached EDS equipment. The thermal parameters consisting of the electrical resistivities, Seebeck coefficients and thermal diffusivities were estimated using related measurement systems (Cryoall CTA and Netzsh LFA 457 system). The UV−vis−near-IR spectrophotometer measurement was established to obtain the band gap energies of all samples. Hall parameters were estimated using the Lake Shore 8400 series. The detailed experimental sections are shown in the Supporting Information.

3. RESULTS AND DISCUSSION Powder X-ray diffraction data for Sn1−xAgxSe samples with x = 0−0.03 are presented in Figure 1. Those character peaks are

Figure 2. Temperature dependent thermoelectric transport properties for Sn1−xAgxSe samples with x = 0−0.03: (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) specific heat, (e) total thermal conductivity, and (f) ZT.

Figure 2b demonstrates that the Seebeck coefficients of Sn1−xAgxSe (x = 0−0.03) samples possess much lower Seebeck coefficients when compared with undoped SnSe, which is rooted in the optimized nH, as Table 1 demonstrates. The Seebeck coefficient is defined via

Figure 1. PXRD patterns for Sn1−xAgxSe samples with x = 0−0.03.

iπ y m*T jjj zzz 2 (1) 3eh k 3n { where kB is the Boltzmann constant, h is the Planck constant, and m* is the band effective mass.34,35 Those decreased Seebeck coefficients could be explained by those increased carrier concentrations. Owing to the remarkably increased carrier concentration and slightly reduced Seebeck coefficients, the power factors for Ag-doping specimens lie much higher than the undoped SnSe as shown in Figure 2c, indicating that Ag can be a pretty efficient dopant for the SnSe system. Resultantly, an extremely high power factor exceeding 7 μW cm−1 K−2 at 793 K is achieved in Sn0.99Ag0.01Se, which is almost twice as that of undoped SnSe. The specific heat (Cp) and total thermal conductivities (κtot) for Sn1−xAgxSe samples with x = 0−0.03 are presented in Figure 2d,e, respectively. The κtots of Sn1−xAgxSe samples are much higher compared to those of undoped SnSe, which arises from the optimized carrier concentration and yields largely enhanced electronic thermal conductivity. On the basis of the optimized carrier concentration, ZT value ∼ 1.0 around 793 K is achieved for the sample Sn0.99Ag0.01Se, as Figure 2f depicts. Ag-doping optimizes the carrier concentration and successfully increases the power factor, suggesting that hole doping is valid for enhancing the electrical transport performance in the p-type SnSe system. Furthermore, forming a solid solution can

identified as a low temperature Pnma phase (PDF No. 01-0890234), without any impurity phases. These XRD data are in relation to our Hall measurement data, as depicted in Table 1, suggesting that Ag atoms are introduced into the SnSe matrix, which yields the increased carrier concentration.


Table 1. Hall Carrier Concentrations and Mobilities for Sn1−xAgxSe Samples with x = 0−0.03 at 300 K composition

nH (cm−3)

μH (cm2 V−1 s−1)

SnSe Sn0.99Ag0.01Se Sn0.98Ag0.02Se Sn0.97Ag0.03Se

× × × ×

49.64 35.83 34.84 28.27

2.80 1.00 6.90 8.16


10 1019 1018 1018

The thermoelectric transport properties of Sn1−xAgxSe specimens are shown in Figure 2. As apparently shown, Agdoping samples have much higher electrical conductivities compared to those of undoped SnSe. Because Ag has one fewer electron than Sn, Sn1−xAgxSe samples should have higher carrier concentrations (nH), bringing about higher electrical conductivities. This is further confirmed by Hall carrier concentrations as demonstrated in Table 1. As a matter of fact, the Sn0.99Ag0.01Se sample achieves the highest carrier concentration of ∼1019 cm−3, much higher than that of SnSe. B

8π 2kB 2


DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials also be a significant strategy to improve the thermoelectric properties. As suggested by the previous reports,31,32,36 GeSe, SnS, and SnSe, both IV−VI semiconductors, can form a complete solid solution. The XRD data for Sn0.99‑yAg0.01GeySe (y = 0−0.02) in Figure 3a indicates that all samples exhibit a

Figure 3. (a) PXRD patterns for Sn0.99‑yAg0.01GeySe samples (y = 0− 0.02). (b) Lattice parameters for Sn0.99‑yAg0.01GeySe changing with the Ge alloying fraction.

SnSe-type phase, without any impurity phases. As clearly presented in Figure 3b, the lattice parameters change with Ge amount, indicating that Ge substitutes the Sn site. To further demonstrate that a solid solution has completely formed between GeSe and SnSe, we carry out a microscopic investigation on the sample Sn0.975Ag0.01Ge0.015Se. The EDS maps for this sample significantly certify that Ag atoms and Ge atoms are homogeneously doped into the SnSe matrix, as Figure 4 depicts.

Figure 5. Temperature dependent thermoelectric transport properties for Sn0.99‑yAg0.01GeySe samples (y = 0−0.02): (a) electrical conductivity and (b) Seebeck coefficient. (c) Room temperature Pisarenko plot vs the hall carrier concentration (nH). (d) Optical absorption spectra. (e) Power factor. (f) Power factor comparisons for Sn0.975Ag0.01Ge0.015Se and reported Sn0.99Ag0.01Se,39 Sn0.96Ge0.04Se,40 Sn0.985Na0.015Se,29 and Sn0.97 Na0.03Se0.7S0.3.38

Table 2. Hall Carrier Concentrations and Mobilities of Sn0.99‑yAg0.01GeySe samples (y = 0-0.02) at 300 K composition Sn0.99Ag0.01Se Sn0.98Ag0.01Ge0.01Se Sn0.975Ag0.01Ge0.015Se Sn0.97Ag0.01Ge0.02Se

nH (cm−3)

μH (cm2 V−1 s−1)

× × × ×

35.83 24.69 19.38 27.77

1.00 7.31 9.55 6.11

1019 1018 1018 1018

high Seebeck coefficients are observed for all of the Ge-alloying samples, both higher than 280 μV/K at 300 K, which corresponds to the decreased carrier concentration. To investigate those giant Seebeck coefficients, the Pisarenko plot among 300 K is calculated using the SPB model. As shown in Figure 5c, the obtained Seeebck coefficients of Sn0.99‑yAg0.01GeySe samples slightly deviate from the Pisarenko line when the effective mass was fixed as 1.0 m0, indicating a large band effective mass after Ge alloying, which is fully corresponding with the Hall mobility changing trend, as presented in Table 2. Essentially, those are the main reasons for the increase of Seebeck coefficients after forming SnSe−GeSe solid solution. Figure 5d exhibits that the experimental band gaps for Sn0.99‑yAg0.01GeySe (y = 0−0.02) decrease with rising Ge amount. Because Ge atom is more electrically negative, it could obtain the impurity dopant level slightly above the valence band formed with Sn atom, which is significant to decrease the band gap.31 Compared with the Gefree samples, the Ge-alloying samples have much higher power

Figure 4. EDS Sn maps (a), Se maps (b), Ag maps (c), and Ge maps (d) for Sn0.975Ag0.01Ge0.015Se sample.

As shown in Figure 5a, all Ge-alloyed samples possess lower electrical conductivity than that of the Ge-free sample. This could be demonstrated by a lower carrier concentration and mobility, as Table 2 shows. The introduction of Ge may affect the defect formation energy in SnSe, which leads to a higher formation energy of Ag substituting Sn, resulting in the reduced carrier concentration.37 Additionally, compared with Sn atom, the reduced size of the Ge atom could introduce extra point defects into the matrix, resulting in a lower carrier mobility.32,38 Temperature dependent Seebeck coefficients of Sn0.99‑yAg0.01GeySe samples are shown in Figure 5b. Apparently, C

DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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The final ZT values vs temperature are presented in Figure 7a, in which the Sn0.975Ag0.01Ge0.015Se sample achieves the

factors, which are ascribed to the larger Seebeck coefficients as shown in Figure 5e. It is apparent that Sn0.975Ag0.01Ge0.015Se can achieve a higher power factor ∼ 10.0 μW cm−1 K−2. Comparably, former research demonstrated much lower values of only ∼5.0 μW cm−1 K−2 for the polycrystalline SnSe at 793 K.29,38−40 κtot and Cp are presented in Figure 6a,b. κtots for Ge-alloying samples are lower than those of Ge-free samples, which arises

Figure 7. (a) Temperature dependent ZT values of Sn0.99‑yAg0.01GeySe samples (y = 0−0.02). (b, c) ZT values and the maxima in Sn0.975Ag0.01Ge0.015Se and reported Sn0.99Ag0.01Se,39 Sn0.96Ge0.04Se,40 Sn0.985Na0.015Se,29 and Na0.03Sn0.97Se0.7S0.338 as comparisons. (d) ZT values changing with nH at 300, 473, 673, and 793 K for the Sn0.975Ag0.01Ge0.015Se sample.

maximum ZT value ∼ 1.5 at 793 K. Those reported ZT values for Sn0.99Ag0.01Se,39 Sn0.96Ge0.04Se,40 Sn0.985Na0.015Se,29 and Sn0.97 Na0.03Se0.7S0.338 are also plotted in Figure 7b,c for comparisons, indicating that the approach of Ag doping and Ge alloying is apparently effective to improve thermoelectric performance of SnSe polycrystals. Using the single band (SPB) model with adopted thermoelectric parameters in Supporting Information,35,43 the predicted curves for ZT changing with nH can be estimated, as Figure 7d demonstrates. Obviously, a much higher ZT approaching ∼1.8 can be obtained at 793 K when the nH is optimized to 5.7 × 1019 cm−3. Therefore, a higher thermoelectric performance in polycrystalline SnSe can be expected.

Figure 6. Temperature dependent thermoelectric transport properties for Sn0.99‑yAg0.01GeySe samples (y = 0−0.02): (a) total thermal conductivity, (b) specific heat, (c) Lorenz number, (d) lattice thermal conductivity, and (e) electronic thermal conductivity. (f) Comparisons of Callaway model and experimental data.

from the decreased carrier concentration and strong point defects due to Ge alloying. The electronic thermal conductivities (κele) which can be calculated with κele = LσT are shown in Figure 6d. The Lorenz number is calculated using the reduced chemical potential method,24,41 which is presented in Figure 6c. The details for the calculation can be seen in the Supporting Information. It is apparent that the κeles for Sn0.99‑yAg0.01GeySe samples are lower than those of the Ge-free sample, which arises from the inferior carrier concentrations and yields decreased electrical conductivities. The lattice thermal conductivities present a similar reduction trend with the increase of Ge content as shown in Figure 6e. This can be explained by the smaller size of Ge atoms causing strong point defect scattering when Ge occupies the Sn site. The theoretical values of κlat at 300 K are calculated according to the Callaway model for further estimating the reduction on κlat.42 Details for this calculation are demonstrated in our Supporting Information. The experimental lattice thermal conductivities are consistent with the theoretical plot, as seen in Figure 6f. This can be a good confirmation for the enhanced phonon scattering and decreased lattice thermal conductivities due to Ge alloying.

4. CONCLUSION In this work, two sets of samples are synthesized by melting and sintered by SPS method, Sn1−xAgxSe with x changing from 0 to 0.03 and Sn0.99‑yAg0.01GeySe with y changing from 0.01 to 0.02. The carrier concentration of undoped SnSe can be increased to 1.0 × 1019 cm−3 through doping with 1% Ag. The Seebeck coefficients are properly improved through enhancing the band effective mass through Ge alloying, which further leads to the greatly enhanced power factor ∼ 10.0 μW cm−1 K−2 at 793 K. Meanwhile, Ge-alloying induce strong point defects to decrease the lattice thermal conductivity. Consequently, remarkable improvement upon ZT ∼ 1.5 is obtained for Sn0.975Ag0.01Ge0.015Se because of the synergistically optimized power factor with significantly lower thermal conductivity. Present results elucidate that polycrystalline SnSe could achieve more excellent thermoelectric property by further optimizing the carrier concentration. D

DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b01475. Experimental details, sample densities, Callaway model details, theoretical ZT prediction, and Lorenz number calculations, (PDF)


Corresponding Authors

*(Y.Q.) E-mail: [email protected] *(L.-D.Z.) E-mail: [email protected] ORCID

Li-Dong Zhao: 0000-0003-1247-4345 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51671015, 51571007, and 51772012), the National Key Research and Development Program of China (Grant Nos. 2018YFA0702100 and 2018YFB0703600), the Beijing Natural Science Foundation (Grant No. JQ18004), Shenzhen Peacock Plan team (Grant No. KQTD2016022619565991), and the 111 Project (Grant No. B17002).


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DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsaem.9b01475 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX