Scholarly article on topic 'Thermoelectric properties of GeSe'

Thermoelectric properties of GeSe Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Journal of Materiomics
OECD Field of science
Keywords
{Thermoelectric / GeSe}

Abstract of research paper on Materials engineering, author of scientific article — Xinyue Zhang, Jiawen Shen, Siqi Lin, Juan Li, Zhiwei Chen, et al.

Abstract The recently reported superior thermoelectric performance of SnSe, motivates the current work on the thermoelectric properties of polycrystalline GeSe, an analog compound with the same crystal structure. Due to the extremely low carrier concentration in intrinsic GeSe, various dopants are utilized to substitute either Ge or Se for increasing the carrier concentration and therefore for optimizing the thermoelectric power factor. It is shown that Ag-substitution on Ge site is the most effective, which enables a hole concentration up to ∼1018 cm−3. A further isovalent substitution by Pb and Sn leads to an effective reduction in the lattice thermal conductivity. A peak figure of merit, zT of ∼0.2 at 700 K can be achieved in Ag0.01Ge0.79Sn0.2Se, a composition with the highest carrier concentration. The transport properties can be well described by a single parabolic band model with a dominant carrier scattering by acoustic phonons at high temperatures (>500 K). This further enables a prediction on the maximal zT of ∼0.6 at 700 K and the corresponding carrier concentration of ∼5 × 1019 cm−3.

Academic research paper on topic "Thermoelectric properties of GeSe"

Accepted Manuscript

Thermoelectric properties of GeSe

Xinyue Zhang, Jiawen Shen, Siqi Lin, Juan Li, Zhiwei Chen, Wen Li, Yanzhong Pei

PII: S2352-8478(16)30067-3

DOI: 10.1016/j.jmat.2016.09.001

Reference: JMAT 71

Materiomics

To appear in: Journal of Materiomics

Received Date: 27 July 2016 Accepted Date: 5 September 2016

Please cite this article as: Zhang X, Shen J, Lin S, Li J, Chen Z, Li W, Pei Y, Thermoelectric properties of GeSe, Journal of Materiomics (2016), doi: 10.1016/j.jmat.2016.09.001.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A peak figure of merit, zT of ~0.2 at 700 K is achieved in Ago o1Geo.79Sno.2Se, a composition with the highest carrier concentration, through a combination method of doping and alloying. The transport properties can be well described by a single parabolic band model which further enables a prediction on the maximal zT of ~0.6 at

700 K and the corresponding carrier concentration of -5*10 cm- , indicating that GeSe shows a potentially high thermoelectric figure of merit but requires a much higher carrier concentration.

Xinyue Zhang, Jiawen Shen, Siqi Lin, Juan Li, Zhiwei Chen, Wen Li, Yanzhong Pei Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji Univ.,

4800 Caoan Rd„ Shanghai 201804, China.

Email: yanzhongiaitongji.edu.cn

Abstract

The recently reported superior thermoelectric performance of SnSe, motivates the current work on the thermoelectric properties of polycrystalline GeSe, an analogue compound with the same crystal structure. Due to the extremely low carrier concentration in intrinsic GeSe, various dopants are utilized to substitute either Ge or Se for increasing the carrier concentration and therefore for optimizing the thermoelectric power factor. It is shown that Ag-substitution on Ge site is the most effective, which enables a hole concentration up to ~1018cm"3. A further isovalent substitution by Pb and Sn leads to an effective reduction in the lattice thermal conductivity. A peak figure of merit, zT of -0.2 at 700 K can be achieved in Agoo1Geo.79Sno.2Se, a composition with the highest carrier concentration. The transport properties can be well described by a single parabolic band model with a dominant carrier scattering by acoustic phonons at high temperatures (>500 K). This further enables a prediction on the maximal zT of ~0.6 at 700 K and the corresponding carrier concentration of ~5><1019cm"3.

Keywords: Thermoelectric; GeSe

1. Introduction

Thermoelectric materials, which can directly convert heat into electricity based on either Seebeck or Peltier effect, has drawn increasing attention in the research community concerning the energy crisis[l]. The thermoelectric performance of a material depends on the dimensionless figure of merit, zT=S2T/p(KE+KLJ, where S, p, T, ke, and kl represent the Seebeck coefficient, electrical resistivity, absolute temperature, electronic thermal conductivity and lattice thermal conductivity, respectively. Materials possessing a high zT require a high power factor (S2/p) and a low thermal conductivity {ke+kl). However, the strongly coupled S, p, and lead to the difficulty for obtaining high zT value [2].

Since S, p, and ke mentioned above are all closely related to the carrier concentration (;?), an optimization of n is fundamentally required for realizing the maximal available zT in a given material[3-5]. To further enhance the maximal zT, proven strategies are typified either by enhancing the power factor {S2lp) through band engineering [6] including band convergence[7-ll], band nestification[12], band effective mass[13], or by minimizing the lattice thermal conductivity (kl), the only one independent material property. Several approaches have been taken to reduce the lattice thermal conductivity, through alloying[9, 14], nanostructuring[15-18], lattice anharmonicity[19, 20], liquid-like transport behaviors compounds [21] and more recently a low sound velocity [22]. Providing the carrier concentration is optimized, these strategies have been demonstrated to be effective for advancing the thermoelectric performance in many materials.

As a new thermoelectric material, single-crystal SnSe has recently been reported to show an extraordinarily high thermoelectric figure of merit at high temperatures (zT of 2.6 at 973 K along the b axis), due to the ultralow thermal conductivity [20]. With a further optimization in the carrier concentration by sodium doping on Sn site, single-crystal SnSe was reported to show a large increase in zT at moderate temperatures and therefore a high average zT (zTave) of -1.3, as compared with the zTave of only -0.2 for pristine single-crystal SnSe [23, 24]. Afterwards, polycrystalline SnSe has also been reported to have a low lattice thermal conductivity and a high zT (-0.8) [25-28].

These literatures send a message that high zT value can be probably realized in the same orthorhombic bulk materials. It is therefore important to explore new thermoelectric

materials having the same or similar crystal structure with SnSe, since its low lattice thermal conductivity and high zT are probably inherent to materials with this type of crystal structure or similar, particularly for those having constituent elements with similar chemical properties to SnSe.

Actually, all the layer-like IV-VI compounds, such as GeS, GeSe, SnS, and SnSe, crystallize in the GeS-type structure at room temperature. This structure belongs to the orthorhombic family with a space group of D^fi (Pnma)[29, 30]. Among these IV-VI compounds, SnS and GeSe show not only the same crystal structure but also very similar chemical composition with SnSe. The thermoelectric properties of SnS have recently reported to show a decent zT of 0.6[31]. However, thermoelectric properties of GeSe have so far been rarely reported, and most of the available work focused on the theoretical calculations[32, 33]. A minimum lattice thermal conductivity of 0.39 W-m^K"1 is predicted for GeSe, which is lower than that of SnS and GeS[33], Meanwhile, a large value of Seebeck coefficient and power factor can be achieved by proper p-type or n-type doping[33]. In another work [32], the predicted peak zT of 2.5 at 800 K for GeSe along b axis due to a multiband effect, is even higher than that of SnSe. All these finding indicates GeSe should potentially be a superior thermoelectric material.

This motivates the current work focusing on the measured transport properties of GeSe. The chemical doping for tuning the carrier concentration, the transport properties and the relevant physics of polycrystalline of GeSe are discussed. The resistivity of undoped GeSe is very large due to the extremely low carrier concentration, as compared with conventional thermoelectric materials[14, 34]. This leaves a primary purpose to improving its electrical properties by increasing its carrier concentration by doping. Therefore, various elements (Cu, Ag and Na for p-type, and Bi, Sb, La, As and I for n-type) are used to achieve a high enough carrier concentration. It turns out that, among the above mentioned elements, only Ag enables a carrier concentration up to -1018 cm"3. Additionally, isovalent alloying with Sn and Pb on Ge site, is found to achieve an effectively reduction in the lattice thermal conductivity approaching the theoretical minimal estimated by the Cahill model[35]. As a result of both doping and alloying, the measured maximal zT is about 0.2 at 700 K. The transport properties can be well described by a single parabolic band model with a dominant carrier scattering

of acoustic scattering at high temperatures. This predicts a maximal zT of 0.6 at 700 K for polycrystalline GeSe with an optimize carrier concentration of ~5x1019 cm-3.

2. Experiment

Various dopants, including sodium (Na), iodine (I), antimony (Sb), lanthanum (La), arsenic (As), copper (Cu) and silver (Ag), were used to dope GeSe to tune the carrier concentration. Polycrystalline GeSe samples and alloys with PbSe and SnSe, with and without doping, were synthesized by melting the stoichiometric quantities of high purity elements (>99.99%) sealed in silica ampoule at 1123 K for 7 hours, quenching in cold water and then annealing at 853 K for 3 days. The obtained ingots were hand ground into fine powders for X-ray diffraction (XRD) to identify the phase composition, and for hot press by induction heating at 800 K for 30 min under a uniaxial pressure of ~ 70 MPa. The resulting pellets with a density higher than 96% of the theoretical density value, were ~12.0 mm in diameter and ~1 mm in thickness.

The electrical transport properties including resistivity, Seebeck coefficient and Hall coefficient of the pellet samples were simultaneously measured, under vacuum in the temperature range of 300-700 K. The resistivity and Hall coefficient were measured using the van der Pauw technique under a reversible magnetic field of 1.5 T. The Seebeck coefficient was obtained from the slope of the thermopower versus temperature gradients[36]. Thermal diffusivity (l) measured by a laser flash technique (the Netzsch LFA457 system) was used to calculate the thermal conductivity, k=dkCp, where d is the density measured by a mass/volume method and Cp is assumed to be a Dulong-Petit limit of heat capacity and to be temperature independent as well. The measurement uncertainty for S, p and ris 5% approximately. /

Sound velocities were measured on the pellet samples at room temperature, using pulse-receiver (Olympus-NDT) equipped with an oscilloscope (Keysight). Water and Shear gel (Olympus) were used as couplant during the measurements of longitudinal and transverse sound velocity (vL and vS) respectively.

3. Results and Discussion

The room temperature transport properties for doped polycrystalline GeSe with various dopants, excluding Ag, are listed in Table 1. These elements are considered as ineffective dopants since the resistivity and Seebeck coefficient are too large to enable a high thermoelectric performance. It is found that Ag is the most effective dopant enabling a carrier concentration up to ~1018 cm-3, probably due to the small electronegative and atomic size differences between Ag and Ge. Therefore Ag-doped samples are focused on for the following discussion.

Table 1. Room temperature transport properties of GeSe doped with Cu, Na, La, and As.

p-type doping p (mQ-cm) S (uV-K-1) r (W-m-1K-1)

Ge1-xCuxSe x=2% 1.51x108 591.25 1.54

x=2% 6.81x105 626.93 1.48

Ge1-xNaxSe x=4% 2.99x107 631.24 1.40

x=6% 3.39x107 500.80 1.43

n-type doping p (mQ- cm) S (uV-K-1) r (W-m-1K-1)

Ge1-xLaxSe x=2% x=6% 2.03x108 4.24x108 668.85 441.10 1.52 1.03

GeSe1-xAsx x=2% x=4% 3.25x108 3.99x107 670.66 614.29 1.63 1.55

The crystal structure of orthorhombic GeSe has been well investigated previously[30]. The crystal structure of GeSe in the room-temperature phase (Pnma) is shown in Fig. 1a, which is a distorted NaCl-type structure. The primitive cell contains eight atoms and two adjacent double layers. Each atom is connected with three nearest neighbors through covalent bond in a single layer, creating zigzag chains along the b axis. It is known that GeSe undergoes a first-order phase transition from orthorhombic structure to NaCl-type structure at ~920-930K and remains cubic up to the melting point[37].

The XRD patterns for Ge1-xAgxSe (x = 0%, 0.2%, 0.5%, 1%, 1.5%, 3%) samples and Ge1-xAgxSe alloyed with PbSe and SnSe (x=1%) are shown in Fig. 1b. All the peaks can be well indexed to orthorhombic GeSe structure without any observable peaks of impurities. The diffraction peaks of PbSe-and SnSe-alloyed samples shift toward the lower angle, indicating the increase of lattice constant.

Fig. 2 shows the temperature dependent Hall carrier concentration (a) and Hall mobility (b) for Gei_xAgxSe. It can be seen that Ag-doping can effectively increase the Hall carrier concentration (nH) up to ~1018 cm-3. It is also shown that the Hall carrier concentration of Ge1-xAgxSe quickly saturates at x ~0.002, meaning that nH does not increase with a further increase in high nominal doping levels. It is seen that alloying with Sn on Ge site can still enable a hall carrier concentration of ~1018 cm-3 while alloying with Pb significantly reduces nH.

The Hall mobility (uh) for all Ag-doped samples show a very similar decrease with increasing temperature via ¡uHx T"15 when temperature is above 500 K, indicating a dominant mechanism of charge carriers scattering by acoustic phonons at these temperatures. However, at lower temperatures (300~500 K), the Hall mobility rises with increasing temperature via uHxT2 5 approximately, revealing additional scattering mechanism which is believed to be due to grain-boundary potential barrier scattering. This observed increase in the Hall mobility with increasing temperature has been seen in SnSe, SnS and PbTe based materials[31, 38, 39].

According to the recent band structure calculation for GeSe [32], the energy difference between the first and second valence band maximum is small. Therefore it is in principle easy to achieve a two-band conduction behavior for enhancing the thermoelectric performance. It should keep in mind that the Fermi level needs to be deep in the valence band, in order to involve the beneficial contribution from the second valence band. This usually means that a heavily doping is required, which is unfortunately not achieved in this work due to low doping effectiveness of the dopants considered. In fact, the obtainable carrier concentration by Ag-doping is not high enough to position the Fermi level deep enough into the valence band to enable a two-band conduction.

The band gap of 1.1 eV in GeSe [40, 41] is much larger than that of conventional thermoelectric materials (0.2~0.5 eV for Bi2Te3, PbTe and PbSe)[42-44], leading to a much weaker interaction between the valence and conduction bands. Therefore, it is reasonable to approximate the band as parabolic[45].

Therefore, the Seebeck coefficient at 700 K can be very well described by a single parabolic band (SPB) model in the Hall carrier concentration range obtained in this work (1017~1018 cm-3) as shown in Fig. 3a. It should be noted that the SPB model here assumes a dominant acoustic scattering, as evident from Fig. 2b. In other words, polycrystalline GeSe in this work can be effectively approximated as a single band

conduction due to its low carrier concentration (only up to ~1018 cm"3), although the band calculation indeed shows a multiband structure.

The approximation of a single parabolic band conduction can be further supported by the carrier concentration dependent Hall mobility as shown in Fig. 3b. It is seen that the Hall mobility can be nicely predicted by the same SPB model as well, for all the Ag-doped GeSe samples obtained in this work. When GeSe is further alloyed with PbSe (blue symbols, concentrations of 8%, 10% and 14%) or SnSe (olive symbols, concentrations of 10% and 20%), the mobility decreases due to the additional scattering by substitutional defects.

The measured temperature Seebeck coefficient and electrical resistivity for Gei_.rAgrSe are shown in Fig. 4a and 4b, respectively. Seebeck coefficient for all the samples is positive, indicating a p-type conduction. Seebeck coefficient decreases with the increasing Ag content. Due to the low carrier concentration, resistivity of the samples here is high, as compared with traditional thermoelectrics. With increasing Ag-doping level, the resistivity decreases by orders of magnitude. When further alloying with PbSe, both resistivity and Seebeck coefficient increase, because of the reduced Hall carrier concentration as shown in Fig. 2a. The decrease in Seebeck coefficient and resistivity at 7>600 K is believed to be due to the bipolar conduction.

The Dulong-Petit limit of heat capacity ((',,) and density (cl) of all samples are listed in Table 2.

Table 2. Dulong-Petit limit of heat capacity (Cp) and density (d) of Gei.jAgjSe and Ge0S9Ag0 0iSe-based alloys with PbSe and SnSe.

Samples CVJ-g^K-1) 4g-cm-3)

x=0 0.3291 5.52

x=0.2% 0.3290 5.47

x=0.5% 0.3287 5.6

x=l% 0.3284 5.58

x=l.5% 0.3280 5.56

x=3% 0.3268 5.62

Ge0.89Ag0.01Pb0.1Se 0.3016 5.77

Geo.79Ago.01Sno.2Se 0.3096 5.69

Temperature dependent total thermal conductivity and its lattice component for Gei_.rAgrSe and its alloys are shown in Fig. 4c and 4d. The lattice thermal conductivity (kl) is obtained by subtracting the electronic thermal conductivity (ke) from the total thermal conductivity. The electronic thermal conductivity is estimated by KE=LT/p according to the Wiedemann-Franz law, where L is the Lorenz factor determined by the single parabolic band (SPB) model discussed above. Both k and kl of all the samples decline as temperature rises. It should be noted that the electronic contribution to the total thermal conductivity is less than 1% for all the samples in this work, even at temperatures that the bipolar conduction happens. This is again due to the low carrier concentration.

The lattice thermal conductivity decreases as MT approximately, indicating a dominant phonon scattering by Umklapp processes in intrinsic GeSe and Ag-doped compounds. As compared with polycrystalline SnSe[27], GeSe obtained here shows a very similar lattice thermal conductivity in the important temperature range of 500-700 K for thermoelectric applications, which can be understood by the same crystal structure between these two compounds.

The lattice thermal conductivity decreases with increasing concentration of Ag-doping in the whole temperature range. Through a further alloying with PbSe or

SnSe, the lattice thermal conductivity is significantly reduced down to -0.4 W/m-K at 700 K. With the measured average longitudinal (г'/=3210 m/s) and transvers (vs=1952 m/s) sound velocities for polycrystalline samples obtained in this work, the theoretical minimal lattice thermal conductivity (к"пп) is estimated to be -0.4 W/m-K at high temperatures, based on the Cahill model [35]:

= -2/3/-о- Л/ T Л2 c0D/t

'fcBn2/3(3yD)(^)2J0£

(ex—l)2

Where kB is Boltzmann constant, n is the number density of atoms and ©D is the Debye temperature, 0D= vD(h/kB)(6n2n)y\ And vD is average sound velocity determined by

3 "L3 "S3

It should be noted that the sound velocity measurements here show well agreement with previous calculations for single crystal GeSe (Ref. 33).

This estimation on minimal lattice thermal conductivity shows an excellent agreement with the calculations[33]. Most importantly, the samples obtained here show a very comparable lattice thermal conductivity with the theoretical minimal value.

Temperature-dependent figure of merit, zT is shown in Fig. 5a. A maximal zT of 0.2 is achieved in Ge0.79Ag0.01 Sn^Se. With the measured average lattice thermal conductivity for these materials, the SPB model enables a prediction on the carrier concentration dependent zT at a given temperature. Shown in Fig. 5b is a case for 700 K. First of all, the model prediction agrees well with the experimental data. It is shown that a maximal zT of -0.6 is predicted, if the carrier concentration can be further increased to ~5><1019cm"3.

zT achieved so far in this compound is low, however, the band calculation[32] indicated a multiband structure, which should in principle enable a significantly enhanced zT if the material can be doped to have a carrier concentration of -102" cm"3. It should be, once again, noted that such a multiband effect has not been taken into account in the prediction here, because it is unnecessary to include this complexity in the carrier concentration range studied here. However, It is still believed that GeSe has a potential as a promising thermoelectric material, through a further carrier concentration increase and/or band engineering.

4. Summary

In summary, Ag acts as an effective dopant on Ge site for increasing the hole concentration, which results in an enhancement on power factor and therefore figure of merit, zT. A further alloying with Pb and Sn on Ge site can reduce the thermal conductivity effectively in the entire temperature range. The obtained lattice thermal conductivity is actually approaching the theoretical minimum as estimated by the Cahill model. The combined approach of both doping and alloying leads to a maximal zT of 0.2 at 700 K for GeSe. The high temperature carrier concentration dependent transport properties here can be well understood by a single parabolic band conduction with acoustic scattering, which further enables a prediction of a peak zT of 0.6 at 700 K. According to the model prediction, the carrier concentration obtained in this work stays not fully optimized, suggesting available room for further improvements, especially by doping and band engineering approaches.

5. Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grant No. 51422208, 11474219 and 51401147) and the national Recruitment Program of Globa Youth Experts (1000 Plan).

Reference

[1] Bell LE. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science. 2008;321:1457-1461.

[2] Snyder GJ, Toberer ES. Complex thermoelectric materials. Nat Mater. 2008;7:105-114.

[3] Pei Y, Gibbs ZM, Balke B, Zeier WG, Snyder GJ. Optimum Carrier Concentration in n-type PbTe Thermoelectrics. Advanced Energy Materials. 2014;4:1400486.

[4] Pei Y, May AF, Snyder GJ. Self-tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance. Advanced Energy Materials. 2011;1:291-296.

[5] Androulakis J, Todorov I, Chung DY, Ballikaya S, Wang GY, Uher C, et al. Thermoelectric enhancement in PbTe with K or Na codoping from tuning the interaction of the light- and heavy-hole valence bands. Phys Rev B. 2010;82:115209.

[6] Pei Y, Wang H, Snyder GJ. Band Engineering of Thermoelectric Materials. Adv Mater. 2012;24:6125-6135.

[7] Pei Y, Shi X, LaLonde A, Wang H, Chen L, Snyder GJ. Convergence of electronic bands for high performance bulk thermoelectrics. Nature. 2011;473:66-69.

[8] Liu W, Tan X, Yin K, Liu H, Tang X, Shi J, et al. Convergence of Conduction Bands as a Means of Enhancing Thermoelectric Performance of n-Type Mg2Si1-xSnx Solid Solutions. Phys Rev Lett. 2012;108:166601.

[9] Li W, Chen Z, Lin S, Chang Y, Ge B, Chen Y, et al. Band and scattering tuning for high performance thermoelectric Sn1-xMnxTe alloys. Journal of Materiomics. 2015;1:307-315.

[10] Jian Z, Chen Z, Li W, Yang J, Zhang W, Pei Y. Significant band engineering effect of YbTe for high performance thermoelectric PbTe. J. Mater. Chem. C. 2015;3:12410-12417.

[11] Pei Y, Wang H, Gibbs ZM, LaLonde AD, Snyder GJ. Thermopower Enhancement in Pb1-xMnxTe alloys and its Effect on Thermoelectric Efficiency. NPG Asia Materials 2012;4:e28.

[12] Chattopadhyay D, Queisser HJ. Electron scattering by ionized impurities in semiconductors. Rev. Mod. Phys. 1981;53:745-768.

[13] Peng H, Wang CL, Li JC. Enhanced thermoelectric properties of AgGaTe2 utilizing carrier concentration adjusting. Physica B: Condensed Matter. 2014;441:68-71.

14] Long D, Myers J. Ionized-Impurity Scattering Mobility •ons in Silicon. Phys Rev. 1959;115:1107-1118.

[15] Pei Y, Lensch-Falk J, Toberer ES, Medlin DL, Snyder GJ. High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping. Adv Funct Mater. 2011;21:241-249.

[16] Biswas K, He J, Blum ID, Wu C-I, Hogan TP, Seidman DN, et al. High-performance bulk thermoelectrics with all-scale hierarchical architectures. Nature. 2012;489:414-418.

[17] Poudel B, Hao Q, Ma Y, Lan YC, Minnich A, Yu B, et al. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science. 2008;320:634-638.

[18] Hsu KF, Loo S, Guo F, Chen W, Dyck JS, Uher C, et al. Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit. Science. 2004;303:818-821.

[19] Morelli DT, Jovovic V, Heremans JP. Intrinsically Minimal Thermal Conductivity in Cubic I-V-VI_{2} Semiconductors. Phys Rev Lett. 2008;101:035901.

[20] Zhao L-D, Lo S-H, Zhang Y, Sun H, Tan G, Uher C, et al. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature. 2014;508:373-377.

[21] Liu H, Shi X, Xu F, Zhang L, Zhang W. Copper ion liquid-like thermoelectrics. Nat Mater. 2012;11:422-425.

[22] Li W, Lin S, Ge B, Yang J, Zhang W, Pei Y. Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6. Advanced Science. 2016:1600196.

[23] Zhao L-D, Tan G, Hao S, He J, Pei Y, Chi H, et al. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science. 2016;351:141-144.

[24] Peng K, Lu X, Zhan H, Hui S, Tang X, Wang G, et al. Broad temperature plateau for high ZTs in heavily doped p-type SnSe single crystals. Energy Environ. Sci. 2016;9:454-460.

[25] Li Y, Shi X, Ren D, Chen J, Chen L. Investigation of the Anisotropic Thermoelectric Properties of Oriented Polycrystalline SnSe. Energies. 2015;8:6275-6285.

[26] Sassi S, Candolfi C, Vaney JB, Ohorodniichuk V, Masschelein P, Dauscher A, et al. Assessment of the thermoelectric performance of polycrystalline p-type SnSe. Appl Phys Lett. 2014;104:212105.

[27] Chen C-L, Wang H, Chen Y-Y, Day T, Snyder GJ. Thermoelectric properties of p-type polycrystalline SnSe doped with Ag. Journal of Materials Chemistry A. 2014;2:11171.

[28] Wei TR, Tan G, Zhang X, Wu CF, Li JF, Dravid VP, et al. Distinct Impact of Alkali-Ion Doping on Electrical Transport Properties of Thermoelectric p-Type Polycrystalline SnSe. J Am Chem Soc. 2016.

[29] Taniguchi M, Johnson RL, Ghijsen J, Cardona M. Core

excitons and conduction-band structures in orthorhombic GeS, GeSe, SnS, and SnSe single crystals. Phys'ERev B. M 1990;42:3634-3643.

[30] Okazaki A. The crystal structure of Germanium Selenide GeSe. J Phys Soc Jpn. 1958;13:1151-1155.

[31] Tan Q, Zhao L-D, Li J-F, Wu C-F, Wei T-R, Xing Z-B, et al. Thermoelectrics with earth abundant elements: low thermal conductivity and high thermopower in doped SnS. J. Mater. Chem. A. 2014;2:17302-17306.

[32] Hao S, Shi F, Dravid VP, Kanatzidis MG, Wolverton C. Computational Prediction of High Thermoelectric Performance in Hole Doped Layered GeSe. Chem Mater. 2016;28:3218-3226.

[33] Ding G, Gao G, Yao K. High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds. Scientific reports. 2015;5:9567.

[34] Pei Y, LaLonde A, Iwanaga S, Snyder GJ. High Thermoelectric Figure of Merit in Heavy-hole Dominated PbTe. Energ Environ Sci. 2011;4:2085-2089.

[35] Cahill DG, Watson SK, Pohl RO. Lower limit to the thermal conductivity of disordered crystals. Phys Rev B. 1992;46:6131.

[36] Zhou ZH, Uher C. Apparatus for Seebeck coefficient and electrical resistivity measurements of bulk thermoelectric materials at high temperature. Rev Sci Instrum. 2005;76:023901.

37] Wiedemeier H, Siemers P. The thermal expansion and

rature transformation of GeSe. Z Anorg Allg Chem. 1975;411:90-96.

[38] Li Y, Li F, Dong J, Ge Z, Kang F, He J, et al. Enhanced mid-temperature thermoelectric performance of textured SnSe polycrystals made of solvothermally synthesized powders. J. Mater. Chem. C. 2016;4:2047-2055.

[39] J. Martin LW, Lidong Chen, and G. S. Nolas. Enhanced Seebeck coefficient through energy-barrier scattering in PbTe nanocomposites. Phys Rev B. 2009;79:115311.

[40] Dimitri D. Vaughn II RJP, Michael A. Hickner and Raymond E. Schaak. Single-Crystal Colloidal Nanosheets of GeS and GeSe. Journal of American Chemical Society. 2010;132:15170-15172.

[41] Elkorashy AM. Photoconductivity in Germanium Selenide Single Crystals. Phys. Status Solidi B 1989;152:249-259.

[42] Goldsmid HJ, Introduction to Thermoelectricity. Springer: Heidelberg, 2009.

[43] Ravich YI, Efimova BA, Smirnov IA, Semiconducting Lead Chalcogenides. Plenum Press: New York, 1970; p xv, 377 p.

[44] Wang H, Pei Y, LaLonde AD, Snyder GJ. Heavily Doped p-Type PbSe with High Thermoelectric Performance: An Alternative for PbTe. Adv Mater. 2011;23:1366 -1370

[45] Wolfe CM. Electron Mobility in High-Purity GaAs. J Appl Phys. 1970;41:3088.

Table Captions:

Table 1. Room temperature transport properties of GeSe doped with Cu, Na, La, and As.

Table 2. Dulong-Petit limit of heat capacity (Cp) and density (d) of Ge^Ag^Se and Ge0.99Ag0.0iSe-based alloys with PbSe and SnSe.

Figure Captions:

Fig. 1. (a) Crystal structure of orthorhombic GeSe, in which the blue and red atoms represent Ge and Se respectively. (b) XRD patterns for Ge1-tAgtSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe.

Fig. 2. (a) Temperature dependent Hall carrier concentration for Ge1-tAgtSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe. (b) Temperature dependent Hall mobility for Ge1-tAgtSe.

Fig. 3. Hall carrier concentration dependent Seebeck coefficient (a) and Hall mobility (b) for Ge1-tAgtSe and its alloys at 700 K. The curves represent the prediction based on the SPB model with a constant density-of-states effective mass (m*) of 1.04 me and a deformation potential coefficient (Edef) of ~17 eV.

Fig. 4. Temperature dependent Seebeck coefficient (a), resistivity (b), total thermal conductivity (c), thermal diffusivity (inset) and lattice thermal conductivity (d) for Ge1-tAgtSe and its alloys. The thermal conductivity of polycrystalline SnSe[27] is also included for comparison.

Fig. 5. Temperature dependent figure of merit, zT (a) for Ge1-tAgtSe and its alloys, and the predicted Hall carrier concentration dependent zT at 700K (b).

RV&B3iS33SllX!

NUSCRIPT

20(deg.)

Fig. 1. (a) Crystal structure of orthorhombic GeSe, in which the blue and red atoms represent Ge and Se respectively. (b) XRD patterns for Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe.

500 T(K)

400 500 600 700 / (K)

Fig. 2. (a) Temperature dependent Hall carrier concentration for Ge^AgrSe and Ge0.99Ag0.0iSe-based alloys with PbSe and SnSe. (b) Temperature dependent Hall mobility for Ge1.xAgxSe.

D -SPB model

Ge1.xAgxSe

700K, m*=1.04mQ

SPB model Gel-xAgxSe

,AgxSe alloyed with PbSe Ge1-xAgxSe alloyed with SnSe

njcm-3)

и 2» "E ■H. 10

Ge1-xAgxSe alloyed with PbSe Ge1-xAgxSe alloyed with SnSe

700K, m*=1.04me

(cm-3)

Fig. 3. Hall carrier concentration dependent Seebeck coefficient (a) and Hall mobility (b) for Ge1-xAg*Se and its alloys at 700 K. The curves represent the prediction based on the SPB model with a constant density-of-states effective mass (m*) of 1.04 me and a deformation potential coefficient (Ef of ~17 eV.

Fig. 4. Temperature dependent Seebeck coefficient (a), resistivity (b), total thermal conductivity (c), thermal diffusivity (inset) and lattice thermal conductivity (d) for Ge^AgSe and its alloys. The thermal conductivity of polycrystalline SnSe[27] is also included for comparison.

Fig. 5. Temperature dependent figure of merit, zT (a) for Ge1.xAgxSe and its alloys, and the predicted Hall carrier concentration dependent zT at 700K (b).

Professor Yanzhong Pei, Tongji University Email: yanzhong@tongji.edu.cn

Yanzhong Pei is a professor at Tongji University, China. He has been working on advanced thermoelectric semiconductors for about a decade, from synthesizing the materials to understanding the underlying physics and chemistry. He holds a B.E. from Central South University in China, a Ph. D from Shanghai Institute of Ceramics, CAS and postdoctoral research experience from Michigan State University and the California Institute of Technology.