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
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.
ACCEPTED MANUSCRIPT b 700 K
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Thermoelectric GeSe 700K
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A peak figure of merit, zT of ~0.2 at 700 K is achieved in Ag0.01Ge0.79Sn0.2Se, 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 ~5×1019 cm-3, indicating that GeSe shows a potentially high thermoelectric figure of merit but requires a much higher carrier concentration.
Thermoelectric properties of GeSe
ACCEPTED MANUSCRIPT 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:
[email protected] Abstract
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 (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-1K-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,
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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[1]. The thermoelectric performance of a material depends on the dimensionless figure of merit, zT=S2T/ρ(κE+κL), where S, ρ, T, κE, and κL 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/ρ) and a low thermal conductivity (κE+κL). However, the strongly coupled S, ρ, and κE, lead to the difficulty for obtaining high zT value[2]. Since S, ρ, and κE mentioned above are all closely related to the carrier concentration (n), 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 (S2/ρ) through band engineering [6] including band convergence[7-11], band nestification[12], band effective mass[13], or by minimizing the lattice thermal conductivity (κL), 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
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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 ~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.
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
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of acoustic scattering at high temperatures. This predicts a The crystal structure of orthorhombic GeSe has been maximal zT of 0.6 at 700 K for polycrystalline GeSe with an MANUSCRIPT well investigated previously[30]. The crystal structure of ACCEPTED -3 optimize carrier concentration of ~5×1019 cm . GeSe in the room-temperature phase (Pnma) is shown in Fig. 2. Experiment 1a, which is a distorted NaCl-type structure. The primitive cell Various dopants, including sodium (Na), iodine (I), contains eight atoms and two adjacent double layers. Each antimony (Sb), lanthanum (La), arsenic (As), copper (Cu) and atom is connected with three nearest neighbors through silver (Ag), were used to dope GeSe to tune the carrier covalent bond in a single layer, creating zigzag chains along concentration. Polycrystalline GeSe samples and alloys with the b axis. It is known that GeSe undergoes a first-order phase PbSe and SnSe, with and without doping, were synthesized by transition from orthorhombic structure to NaCl-type structure melting the stoichiometric quantities of high purity elements at ~920-930K and remains cubic up to the melting point[37]. (>99.99%) sealed in silica ampoule at 1123 K for 7 hours, The XRD patterns for Ge1-xAgxSe (x = 0%, 0.2%, 0.5%, quenching in cold water and then annealing at 853 K for 3 1%, 1.5%, 3%) samples and Ge1-xAgxSe alloyed with PbSe days. The obtained ingots were hand ground into fine powders and SnSe (x=1%) are shown in Fig. 1b. All the peaks can be for X-ray diffraction (XRD) to identify the phase composition, well indexed to orthorhombic GeSe structure without any and for hot press by induction heating at 800 K for 30 min observable peaks of impurities. The diffraction peaks of PbSeunder a uniaxial pressure of ~ 70 MPa. The resulting pellets and SnSe-alloyed samples shift toward the lower angle, with a density higher than 96% of the theoretical density value, indicating the increase of lattice constant. were ~12.0 mm in diameter and ~1 mm in thickness. Fig. 2 shows the temperature dependent Hall carrier The electrical transport properties including resistivity, concentration (a) and Hall mobility (b) for Ge1-xAgxSe. It can Seebeck coefficient and Hall coefficient of the pellet samples be seen that Ag-doping can effectively increase the Hall were simultaneously measured, under vacuum in the carrier concentration (nH) up to ~1018 cm-3. It is also shown temperature range of 300–700 K. The resistivity and Hall that the Hall carrier concentration of Ge1-xAgxSe quickly coefficient were measured using the van der Pauw technique saturates at x ~0.002, meaning that nH does not increase with a under a reversible magnetic field of 1.5 T. The Seebeck further increase in high nominal doping levels. It is seen that coefficient was obtained from the slope of the thermopower alloying with Sn on Ge site can still enable a hall carrier versus temperature gradients[36]. Thermal diffusivity (λ) concentration of ~1018 cm-3 while alloying with Pb measured by a laser flash technique (the Netzsch LFA457 significantly reduces nH. system) was used to calculate the thermal conductivity, The Hall mobility (µH) for all Ag-doped samples show a κ=dλCp, where d is the density measured by a mass/volume very similar decrease with increasing temperature via µH∝ method and Cp is assumed to be a Dulong-Petit limit of heat T-1.5 when temperature is above 500 K, indicating a dominant capacity and to be temperature independent as well. The mechanism of charge carriers scattering by acoustic phonons measurement uncertainty for S, ρ and κ is 5% approximately. at these temperatures. However, at lower temperatures Sound velocities were measured on the pellet samples at (300~500 K), the Hall mobility rises with increasing room temperature, using pulse-receiver (Olympus-NDT) temperature via µH∝T2.5 approximately, revealing additional equipped with an oscilloscope (Keysight). Water and Shear scattering mechanism which is believed to be due to gel (Olympus) were used as couplant during the grain-boundary potential barrier scattering. This observed measurements of longitudinal and transverse sound velocity increase in the Hall mobility with increasing temperature has (vL and vS) respectively. been seen in SnSe, SnS and PbTe based materials[31, 38, 39]. According to the recent band structure calculation for 3. Results and Discussion GeSe [32], the energy difference between the first and second The room temperature transport properties for doped valence band maximum is small. Therefore it is in principle polycrystalline GeSe with various dopants, excluding Ag, easy to achieve a two-band conduction behavior for enhancing are listed in Table 1. These elements are considered as the thermoelectric performance. It should keep in mind that ineffective dopants since the resistivity and Seebeck the Fermi level needs to be deep in the valence band, in order coefficient are too large to enable a high thermoelectric to involve the beneficial contribution from the second valence performance. It is found that Ag is the most effective band. This usually means that a heavily doping is required, dopant enabling a carrier concentration up to ~1018 cm-3, which is unfortunately not achieved in this work due to low probably due to the small electronegative and atomic size doping effectiveness of the dopants considered. In fact, the differences between Ag and Ge. Therefore Ag-doped obtainable carrier concentration by Ag-doping is not high samples are focused on for the following discussion. enough to position the Fermi level deep enough into the Table 1. Room temperature transport properties of GeSe doped valence band to enable a two-band conduction. with Cu, Na, La, and As. 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 p-type doping ρ (mΩ·cm) S (µV·K-1) κ (W·m-1K-1) for Bi2Te3, PbTe and PbSe)[42-44], leading to a much weaker Ge1-xCuxSe x=2% 1.51×108 591.25 1.54 interaction between the valence and conduction bands. x=2% 6.81×105 626.93 1.48 Therefore, it is reasonable to approximate the band as Ge1-xNaxSe x=4% 2.99×107 631.24 1.40 7 parabolic[45]. x=6% 3.39×10 500.80 1.43 -1 -1 -1 Therefore, the Seebeck coefficient at 700 K can be very n-type doping ρ (mΩ·cm) S (µV·K ) κ (W·m K ) 8 well described by a single parabolic band (SPB) model in the x=2% 2.03×10 668.85 1.52 Ge1-xLaxSe 8 Hall carrier concentration range obtained in this work x=6% 4.24×10 441.10 1.03 17 18 ~10 cm-3) as shown in Fig. 3a. It should be noted that (10 8 x=2% 3.25×10 670.66 1.63 GeSe1-xAsx the SPB model here assumes a dominant acoustic scattering, x=4% 3.99×107 614.29 1.55 as evident from Fig. 2b. In other words, polycrystalline GeSe in this work can be effectively approximated as a single band
The Dulong-Petit limit of heat capacity (Cp) and density (d) of all samples are listed in Table 2. Table 2. Dulong-Petit limit of heat capacity (Cp) and density (d) of Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe.
Cp(J·g-1K-1) 0.3291 0.3290 0.3287 0.3284 0.3280 0.3268 0.3016 0.3096
d(g·cm-3) 5.52 5.47 5.6 5.58 5.56 5.62 5.77 5.69
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SnSe, the lattice thermal conductivity is significantly reduced measured average longitudinal (vL=3210 m/s) and transvers (vS=1952 m/s) sound velocities for polycrystalline samples obtained in this work, the theoretical minimal lattice thermal conductivity (κLmin) is estimated to be ~0.4 W/m-K at high temperatures, based on the Cahill model[35]:
down to ~0.4 W/m-K at 700 K. With the MANUSCRIPT
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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 Ge0.79Ag0.01Sn0.2Se. 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 ~5×1019 cm-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 ~1020 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.
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conduction due to its low carrier concentration (only up to ~1018 cm-3), although the band calculation ACCEPTED 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 Ge1-xAgxSe 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 T>600 K is believed to be due to the bipolar conduction.
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Temperature dependent total thermal conductivity and its lattice component for Ge1-xAgxSe and its alloys are shown in Fig. 4c and 4d. The lattice thermal conductivity (κL) is obtained by subtracting the electronic thermal conductivity (κE) from the total thermal conductivity. The electronic thermal conductivity is estimated by κE=LT/ρ according to the Wiedemann–Franz law, where L is the Lorenz factor determined by the single parabolic band (SPB) model discussed above. Both κ and κL 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 1/T 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
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,ACCEPTED 11474219 and 51401147) and the national Recruitment Program of Global Youth Experts (1000 Plan).
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Table 1. Room temperature transport properties of GeSe doped with Cu, Na, La, and As.
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Table 2. Dulong-Petit limit of heat capacity (Cp) and density (d) of Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe.
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. Fig. 2. (a) Temperature dependent Hall carrier concentration for Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe. (b) Temperature dependent Hall mobility for Ge1-xAgxSe. Fig. 3. Hall carrier concentration dependent Seebeck coefficient (a) and Hall mobility (b) for Ge1-xAgxSe 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-xAgxSe 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).
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Fig. 1. (a) Crystal structure of orthorhombic GeSe, in which the blue and red atoms represent Ge and Se respectively. (b) XRD
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patterns for Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe.
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Fig. 2. (a) Temperature dependent Hall carrier concentration for Ge1-xAgxSe and Ge0.99Ag0.01Se-based alloys with PbSe and SnSe. (b) Temperature dependent Hall mobility for Ge1-xAgxSe.
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dependent zT at 700K (b).
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Professor Yanzhong Pei, Tongji University Email:
[email protected] 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.
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