Realizing high thermoelectric performance in p-type SnSe through


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Realizing high thermoelectric performance in ptype SnSe through crystal structure modification Bingchao Qin, Dongyang Wang, Wenke He, Yang Zhang, Haijun Wu, Stephen J. Pennycook, and Li-Dong Zhao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12450 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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Realizing high thermoelectric performance in p-type SnSe through crystal structure modification Bingchao Qin,1,† Dongyang Wang,1,† Wenke He,1 Yang Zhang,2 Haijun Wu,2,* Stephen J. Pennycook,2 Li-Dong Zhao1,* 1School

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

2Department

of Materials Science and Engineering, National University of Singapore,

117575, Singapore

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Abstract The simple binary compound SnSe has been reported as a robust thermoelectric material for energy conversion by showing strong anharmonicity and multiple electronic valence bands. Herein, we report a record high average ZT value > 1.6 at 300-793 K with maximum ZT values ranging from 0.8 at 300 K to 2.1 at 793 K in p-type SnSe crystals. This remarkable thermoelectric performance arises from the enhanced power factor and lowered lattice thermal conductivity through crystal structure modification via Te alloying. Our results elucidate that Te alloying increases the carrier mobility by making the bond lengths more nearly equal and sharpening the valence bands, meanwhile, the Seebeck coefficient remains large due to multiple valence bands. As a result, a record high power factor ~ 55 μWcm-1K-2 at 300 K is achieved. Additionally, Te alloying promotes Sn atom displacements, thus leading to a lower lattice thermal conductivity. Our conclusions are well supported by electron localization function calculations, the Callaway model and structural characterization via aberrationcorrected scanning transmission electron microscopy. Our approach of modifying crystal structures could also be applied in other low-symmetry thermoelectric materials and represents a new strategy to enhance thermoelectric performance. Keywords: Thermoelectrics; SnSe crystals; crystal symmetry; local displacement; conversion efficiency

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INTRODUCTION Thermoelectric technology, which can recover waste heat to generate electrical power or be used for refrigeration, has drawn extensive and intensive attention to satisfy the energy demand.1-2 Excellent thermoelectric materials for thermoelectric generators should possess a high dimensionless figure of merit (ZT), defined as ZT = (S2σ/к) T, where S, σ, к and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity (a sum of electronic кele and lattice кlat thermal conductivity), and the temperature in Kelvin, respectively.3-4 It is difficult to simultaneously enhance the power factor (S2σ) and reduce the thermal conductivity (к) due to the complex interrelationships among the above parameters.5-6 However, several general strategies have emerged to achieve high thermoelectric performance in the past decades.7-17 Effective approaches include band structure engineering (which is implemented by the convergence of electronic bands,12,

18

distortions in the

electronic density of states (DOS),11, 19 and alignment of the valence or conduction bands20-22), all-scale hierarchical architecturing,10, 13-14 and microstructure engineering.20, 22-27 It should be noted that the higher thermoelectric performance could be realized through applying most of the above approaches when the thermoelectrics possess a higher crystal symmetry, which could contribute a high carrier mobility. On the other hand, the materials with low crystal symmetry usually exhibit a low lattice thermal conductivity due to the strong anharmonicity caused by non-equilibrium chemical bond coordination. Typical examples include SnSe,15-17, 28-31

SnS,32-33 BiCuSeO,34-36 etc. Therefore, a high thermoelectric performance might be

expected by optimizing the carrier mobility and lattice thermal conductivity by synergistic tuning of local symmetry through alloying, as we show below.17, 37-39 Undoped SnSe is a typical p-type semiconductor40 with a hole carrier concentration of ~ 1017 cm-3. Historically, SnSe was ignored by the thermoelectric field because of its high electrical resistivity and large band gap ~ 0.86 eV.30, 40-41 However, breakthroughs were made in SnSe crystals with high ZT values > 2.3 along the b- and c- crystallographic directions,15 which can be attributed to the low thermal conductivity due to anharmonic and anisotropic bonding.15, 42 Successful hole doping in SnSe crystals activated the multiple valence bands, and thus high power factor and average ZT (ZTave) ~ 1.34 were obtained over a wide temperature range.16 However, one claimed that the low in-plane thermal conductivity in SnSe crystals was underestimated owing to the low sample density, after which the ultralow thermal conductivity observed in fully dense SnSe crystals complicated this topic. Further researches expand the physical and chemical investigations in SnSe by elucidating that the 3

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low thermal conductivity in SnSe crystals is sensitive to the van der Waals–like Se–Sn bonding,43 off-stoichiometric defects,44 and so on. Recently, an outstanding thermoelectric performance was achieved in n-type SnSe crystals along the out-of-plane direction due to the 3D charge and 2D phonon transport mechanism.17 It was revealed that a continuous phase transition from the low-symmetry Pnma (#62) to the high-symmetry Cmcm (#63) phase starts at ~ 600 K, in which the high thermoelectric performance is ascribed to the higher carrier mobility in the high-symmetry Cmcm phase. Typically, optimizing crystal symmetry has been successfully employed to raise the superconducting temperature due to the improved carrier mobility,45 which motivates us to modify the crystal structure of SnSe and to see whether it is applicable to improve the carrier mobility and then the overall thermoelectric performance in SnSe crystals. Herein, Te is selected to alloy in SnSe to increase the crystal symmetry. Interestingly, we find that there are several outstanding contributions from Te alloying, namely, increasing carrier mobility due to improving the crystal symmetry and optimizing the band structure, increasing Seebeck coefficients due to activating multiple valence bands, and lowering low lattice thermal conductivity due to promoting the amplitude of Sn local displacement. Resulting from the above synergistic effects, we achieve a record-high power factor ~ 55 μWcm-1K-2 at room temperature, remarkably high ZT values ranging from 0.8 to 2.1 at 300-793 K, and a recordhigh average ZT value > 1.6 in Te-alloyed SnSe crystals with a theoretical conversion efficiency > 18%. Our results elucidate that modifying crystal structure is one effective strategy to enhance the overall thermoelectric performance, and also indicate that the SnSe crystal is one promising candidate in thermoelectric power generation.

EXPERIMENTAL SECTION All samples were prepared by the modified Bridgeman method with tube furnace. High-purity elemental constitutes of Sn, Se, Te and Na were weighted, loaded into silicon tubes, evacuated and flame-sealed under pressure of ~ 10-4 Torr. The tubes were then loaded into vertical furnace, heated up to 1313 K at rate of 10 Kmin-1 and soaked for 10 h, subsequently cooled to 1073 K at a slow rate of 1 Kh-1 and furnace cooled to room temperature. The obtained crystals were then used to measure phases, crystal orientations, band gaps, electrical and thermal properties, Hall coefficients and microstructure characterization. Density functional theory (DFT) calculations and microstructure characterizations were conducted to analysis thermoelectric transport properties. More experimental details can be found in Supporting Information (SI). 4

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RESULTS AND DISCUSSION Phase structure characterization. SnSe crystallizes in a layered structure with orthorhombic Pnma (#62) space group at room temperature15 and undergoes a continuous phase transition from Pnma (#62) to Cmcm (#63) starting at ~ 600 K before the critical temperature (~ 800 K).17 Figure S1a shows the powder XRD patterns of SnSe-xSnTe samples (x = 0-3), indicating that no impurity phase occurs in all crystals (in the following, the name SnSe-xSnTe refers to SnSe1-x%Tex% with 2% Na, please see Experimental details in Supporting Information). The calculated lattice parameter shown in Figure S1b for the Tefree SnSe is consistent with the theoretical data (a = 11.49 Å, b = 4.15 Å, c = 4.44 Å).46 With increasing Te alloying fraction, the lattice parameters linearly increase up to x = 2.0, and then remain constant. As depicted in Figure S1d, the obtained band energies adopt a similar trend as the lattice parameters with increasing x. These results indicate that the solid solubility limit of SnTe in SnSe is less than 3%. Electrical transport properties. Figure 1 shows the electrical transport properties of Tealloyed SnSe crystals. Figure 1a shows that the room-temperature electrical conductivity increases with increasing Te-alloying fraction up to x = 2.0, exceeds ~ 2000 Scm-1, and drops with further increase of Te-alloying fraction, which is consistent with the variation of lattice parameter and band gap. The changing trend in electrical conductivity can be well supported by the carrier concentration (nH) (Table S1), indicating that Te-alloying in SnSe introduces hole doping. Similarly, polycrystalline SnSe-SnTe also shows similar trend on electrical conductivities with increasing Te alloying.47

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Figure 1. Temperature dependent (a) electrical conductivity, the samples are also labeled by carrier concentration, (c) Seebeck coefficient and (e) power factor of Te-alloyed SnSe crystals; (b) Theoretical plot and experimental data16 of carrier mobility as a function of hole carrier concentration; (d) Pisarenko plot and experimental data of this work and other p-type SnSe systems with different carrier concentrations;15-16,

31, 48

(f) Comparison of power factors between this work and reported p-type SnSe

crystals with different carrier concentrations16, 31, 48 and other state-of-the-art thermoelectric systems.10, 49

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In order to investigate the role of the defects introduced by Te alloying, we carried out theoretical calculations on the formation energy of possible defects in Te-alloyed SnSe. The detailed calculation can be seen in our Supporting Information. Figure S2 plots the calculated possible equilibrium chemical potentials for Te-alloyed SnSe. The shaded area is the chemical potential region to synthesize SnSe. Under Sn-rich conditions, the allowed highest chemical potentials for Sn, Se and Te are 0 eV, -0.898 eV and -0.946 eV, respectively. They are -0.822 eV, -0.076 eV and -0.124 eV, respectively, under Se-rich conditions. Thus, three models of defects were considered, i.e. a Sn vacancy (VSn), substitutional Te atoms on Se sites (TeSe), and the complex defect (VSn+ TeSe). The calculated formation energy ΔHf(VSn+ TeSe) = -4.858 eV is smaller than ΔHf(VSn) = -4.501 eV and ΔHf(TeSe) = -0.262 eV, indicating that Te alloying contributes greatly to the easier formation of Sn vacancies. With the contribution from Te alloying, the present electrical conductivities of Te-alloyed SnSe crystals are comparable with those of other heavily hole doped p-type SnSe crystals (Figure S3a).16,

31, 48

Interestingly, as shown in Figure 1b, after Te alloying, the carrier

mobility remains considerably higher than 260 cm2V-1s-1, the experimental carrier mobility data are over the theoretical line derived from the single Kane band (SKB) model.50 Our results indicate that both the increased carrier concentration and the maintained carrier mobility contribute to the high electrical conductivity of Te-alloyed SnSe. The Seebeck coefficients as a function of temperature are shown in Figure 1c. The Seebeck coefficients of all Te-alloyed samples adopt the same trend with increasing temperature, which is consistent with that of the electrical conductivities. When nH approaches 4.77 × 1019 cm-3 in the sample SnSe-2SnTe, the Seebeck coefficient is ~ 165 μVK-1 at 300 K, comparable to other heavily hole-doped SnSe crystals.16 The large Seebeck coefficients are ascribed to the increased density of states (DOS) effective mass m*, which is evidenced by the obvious deviation of the experimental Seebeck coefficients from the theoretical line (Pisarenko relationship) in Figure 1d. The Pisarenko plot is calculated using the single band model16 with m* = 1.0 me. The experimental data in this work adopts a slightly increased m*, especially in 2% Te-alloyed SnSe. Apart from the enhancement in roomtemperature Seebeck coefficients, Te-alloyed SnSe even displays larger Seebeck coefficients at higher temperatures than other reported SnSe crystals,16, 31, 48 as shown in Figure S3b. This difference can be well interpreted by variations of the temperature-dependent carrier concentration. For the Te-free SnSe, the carrier concentration decreases gradually with increasing temperature (Figure S3c). While after alloying 1.5% and 2.0% Te, one upturn in carrier concentration is observed at around 673 K, which is consistent with the jump of 7

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Seebeck coefficients in Figure S3b. The upturns in carrier concentrations and Seebeck coefficients are related to the involvement of more valence bands at elevated temperatures, which is promoted by Te alloying. The carrier concentration upturn is a typical signal of a multi-band transport mechanism, which has been generally investigated in p-type PbTe and PbSe systems.4, 22 The combination of increased electrical conductivities and considerably large Seebeck coefficients results in high power factors (PFs), as shown in Figure 1e. A record high PF of ~ 55 μWcm-1K-2 at 300 K is achieved in the 2% Te-alloyed SnSe crystal. This value rivals that of the optimized p-type BiSbTe system51 and is comparable with those of half-Heuslers52 and Skutterudites.53 Although the PFs decrease sharply with rising temperature, high-temperature PFs remain as high as ~ 17 μWcm-1K-2 and 15 μWcm-1K-2 at 773 K and 793 K, respectively. Figure 1f shows that the PF values obtained in Te-alloyed SnSe are higher than those reported in p-type SnSe crystals16, 31, 48 and other state-of-art thermoelectrics,10, 49 especially in the 300 to 500 K range. To reveal the origin of the high power factor achieved in this work, we investigated the modifications on the crystal structure of SnSe after Te alloying. Figure 2 schematically shows the low-symmetry crystal structure of the Pnma phase and the high-symmetry Cmcm phase Figure 2a, b. The low-symmetry phase has 4 long Sn-Se bonds and 3 short Sn-Se bonds.30 When Te takes the short bond position, as shown in Figure 2d, the angle 1 in Figure 2a shows a decreasing trend with increasing Te content, from 89.80° to 88.38°, while for Te in the long bond position, the angle 2 increases from 78.71° to 80.78°, as shown in Figure 2e. The corresponding angles of the high-symmetry Cmcm phase of SnSe are equal to be ~ 86.07°, as shown in Figure 2b. Such changes in crystal structure reveals that both Te-alloying sites increase the crystal symmetry, which contributes to an enhanced carrier mobility, consistent with the reported improved carrier mobility in the high-symmetry Cmcm phase.17 Our results also showed good agreement with the induced superconductivity in BaFe2As2 through reducing the As-Fe-As bond angle via high pressure.45

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Figure 2. Schematic of the crystal structure and bond structure of Te-alloyed SnSe: Continuous phase transition from (a) low-symmetry Pnma phase to (b) high-symmetry Cmcm phase with increasing temperature;17 Schematic diagram of the bond structure for (c) Te-free SnSe and (c), (d) Two different situations for Te substituting for Se sites in Te-alloyed SnSe. The dark red bond refers to 3 short bonds while the light blue represents the 4 long bonds. The Te-alloying angles are calculated from the nearest neighbors of Te in a supercell Sn32Se31Te.

Using the experimentally derived structural data, we next show that the electronic structure evolves similarly in the crystal structure. The high mobility also can be attributed to the modified band structure and decreased single effective mass mb*. Detailed discussion is given in the Supporting Information and Figure S4. The electronegativity difference of the Sn-Te bond (χSn ~ 1.96, χTe ~ 2.10, Δχ ~ 0.14) is notably lower than that of Sn-Se bond (χSe ~ 2.55, Δχ ~ 0.59), therefore Te-alloying may significantly increase the bond covalency. The increased bond covalency in the conductive layers would lead to a decreased carrier effective mass,54-55 which is consistent with our DFT calculations. Therefore, the main origin of the anomalously increased carrier mobility in Te-alloyed SnSe crystals is attributed to the increased crystal symmetry and bond covalency, and thus a decreased single band effective mass. Thermal transport properties. The total thermal conductivities (κtot) as a function of temperature are shown in Figure 3a. The room temperature κtot shows a decreasing trend with increasing Te alloying content and decreases from ~ 2.4 Wm-1K-1 for Te-free SnSe to ~ 1.5 9

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Wm-1K-1 with 3% Te-alloyed SnSe. The κtot of all samples show the same trend with rising temperature and approaches ~ 0.6 Wm-1K-1 at 793 K. The κtot in the whole measurement temperature range for 2% Te-alloyed SnSe sample is comparable to the values of other reported p-type SnSe crystals,16, 31, 48 as depicted in Figure S5a. The same reducing trend is also observed in lattice thermal conductivities (κlat), as shown in Figure 3b. The κlat ranges from ~ 1.9 Wm-1K-1 to ~ 1.1 Wm-1K-1 at 300 K and decreases sharply to 0.3-0.4 Wm-1K-1 at 793 K. The measured densities of all SnSe-SnTe crystals are listed in Table S2. The thermal diffusivity (D), specific heat (CP, calculated from the Debye model, see Supporting Information), Lorenz number (L) and electronic thermal conductivity (κele) are shown in Figure S6. To further understand the effect of Te alloying on the reduction of κlat, we carried out theoretical calculations based on the Callaway model56; the calculation details can be found in the Supporting Information. As depicted in the inset of Figure 3b, the theoretical line for Callaway model lies in good agreement with the experimental data, which means that the reduction of κlat in Te-alloyed SnSe is due to the point defect scattering derived from both mass and strain field fluctuations,57 especially for 1% - 2% Te alloyed SnSe crystals, in which the minimum lattice thermal conductivity approaches 0.3 Wm-1K-1 at 793 K ( Figure S5b). Similarly, the lattice thermal conductivity reduction could be well revealed in SnSe-SnTe polycrystals.47

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Figure 3. Temperature dependent (a) total thermal conductivity and (b) lattice thermal conductivity of Tealloyed SnSe crystals; the inset figure shows that the experimental data can be well described by the Callaway model, indicating that point defect scattering produced by Te alloying is predominant in lowering the lattice thermal conductivity. The electronic localization function (ELF) of (c) Te-free SnSe and (d) Tealloyed SnSe in the b-c plane. The dotted lines represent the chemical bond length (in Å) between Sn and Se, the arrows refer to the displacements of Sn atoms after Te alloying, δ representing the value in Angstroms. The related 3D structures of (e) Te-free SnSe and (f) Te-alloyed SnSe. The grey dashed circles show the Sn atom positions before Te alloying and the arrows show the displacement directions.

To well understand the low lattice thermal conductivity and reveal the fluctuations from Callaway model in SnSe caused by Te alloying, we carried out electron localization function 11

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(ELF) calculations. The ELF has often been used to characterize the degree of electron localization and to quantitatively identify the character of chemical bonds between atoms.58 The electron sharing can be well visualized using the 2D ELF map as shown in Figure 3c, d. Figure 3c shows that the charge density and bond lengths around Sn atoms in Te-free SnSe, with two equal short bonds and two equal long bonds. After Te alloying, the lower electronegativity difference between Sn and Te changes the balance of the chemical force between atoms and leads to deviation of the Sn atom from its original position. Figure 3d demonstrates the changing bond length and offset directions after Te alloying. Our calculation indicates that the displacement δ approaches 0.14 Å, ~ 10% of the Sn atomic radius (1.58 Å), when introducing 3% Te into SnSe lattice. In addition, we present the schematic structure in Figure 3e, f. Our results indicate that Te-alloying promotes the displacement of Sn to increase the short bond and decrease the long bond, which will change the original force balance and become a crucial factor to the greatly decreased lattice thermal conductivity. Our present results are consistent with the recent discovery that the Pb atoms displace off the octahedron centers in the rock salt structure of PbTe, rather than staying in the centers. As the displacement increases with rising temperature, such effect contributes to the low thermal conductivity in PbTe, especially at elevated temperatures.59 Microstructure characterization. To reveal the structural origin of the enhanced thermoelectric properties of p-type SnSe after Te alloying, we employed aberration-corrected scanning transmission electron microscopy (STEM) to investigate the microstructures, especially for the substitution of Te on Se sites and its effluence on atom bonds. STEM highangle annular dark-field (HAADF) imaging mode produces contrast interpretable by massthickness (the number of atoms) or atomic number (Z) contrast, which is powerful to differentiate atoms with different Z.60 Figure 4a is an atomically-resolved STEM HAADF image along the [100] zone axis (a axis) (Figure 4b), showing a dumbbell-like atomic arrangement. Each atom column is not round, but slightly elongated due to the overlapping of half of Sn and Se atoms with a slight mismatch, as shown in Figure 4c. Actually, the two atom columns of one dumbbell are equivalent in intensity. To better observe the substitution of Te on Se sites, we turn to the b or c axis, along which the Sn and Se atom columns are well separated. Figure 4d and 4e is an atomically-resolved STEM HAADF image along the [001] zone axis (c axis). As clearly seen, the Sn and Se atoms are well differentiated through the clear intensity difference in the STEM HAADF image, as shown in Figure 4f. The respective electron diffraction pattern in Figure 4e shows superlattice reflections due to the multiple periods in the atom arrangement, which is different 12

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from the case along the a axis (Figure 4b). To get the information about Te substitutions and bonds, we performed a quantitative analysis of Figure 4d for atom position and intensity, with the method of peaking finding.61 Here, we divide atom columns into four types, Sn1, Sn2, Se3 and Se4. Figures 4g1, g2 are intensity maps of the two types of Sn atom columns. The inhomogeneous intensity distribution suggests the existence of Te substitution (abnormally brighter columns) on Se sites. Based on the identified atom positions, the bond lengths of SnSn and Sn-Se could be obtained. Figures 4h1, h2 and 4i1, i2 are calculated lattice parameters of Sn1 and Sn2 atom columns, which are corresponding to the bond lengths of Sn-Sn along the X ([020]) and Y ([400]) directions. After identifying the positions of all atom columns, the bond lengths of Sn-Se could be reflected from their projections along X and Y directions, as shown in Figures 4j1, j2. All these maps reflect one common feature: inhomogeneous contrast with a slight deviation. The difference in intensity and bond length resulting from the substitution of Te on Se sites is quite small, since very few substitutional atoms are buried in a thick matrix-atom column (~tens of atoms). As discussed in Figure 3, the substitution of Te on Se sites induces a change of the local bond lengths and angles, and the local strain field, which contributes much to both the electrical and thermal transport.

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Figure 4. Structure of Te-alloyed SnSe crystals. (a) Atomically-resolved STEM HAADF image along the [100] zone axis, with enlarged image inset. (b, c) Electron diffraction pattern and structural model along the [100] zone axis. (d) Atomically-resolved STEM HAADF image along the [001] zone axis, with enlarged image inset; four types of atom columns are marked as Sn1, Sn2, Se1 and Se2. (e, f) Electron diffraction pattern and structural model along the [001] zone axis. (g-j) Maps of intensity and lattice parameter of different types of atom columns are obtained from (d), where the unit of lattice parameters is Å and the intensity values are relative. (g1, g2) Intensity map of the two types of Sn atom columns, Sn1 and Sn2. (h1, h2) Lattice parameters (LP) of Sn1 and Sn2 atom columns along the X direction. (i1, i2) Lattice parameters of Sn1 and Sn2 atom columns along the Y direction. (j1, j2) Lattice parameters of all atom columns along the X and Y directions.

Remarkable average ZT and calculated efficiency. Based on the above discussions, we demonstrate the ZT values as a function of temperature in Figure 5a. Both record-high power 14

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factors and depressed lattice thermal conductivities lead to high ZT values over a wide temperature range. The room temperature maximum ZT value exceeds 0.8 in 2% Te-alloyed SnSe crystal while it exceeds 2.0 as temperature rises above 673 K. These ZT values rival the former p-type SnSe crystals with different carrier concentrations16, 31, 48 and outperform most of current state-of-art p-type thermoelectrics10, 51, 62-63 over 300–793 K, as shown in Figure 5b. The conversion efficiency of heat to electricity for potential applications requires a higher average ZT value (ZTave) over a wider range of temperature. ZTave is given by:64 1

𝑇

(1)

𝑍𝑇𝑎𝑣𝑒 = 𝑇ℎ ― 𝑇𝑐∫𝑇ℎ𝑍𝑇𝑑𝑇 𝑐

where Th and Tc represent the hot and cold side temperature, respectively. Here, an ultrahigh ZTave ~ 1.58 is realized for 300–773 K. This value outperforms other previously studied ptype SnSe crystals16, 31, 48 within the same temperature range, as demonstrated in Figure 5c. If we consider the temperature range at 300-793K, a record-high ZTave > 1.6 could be obtained. According to the average ZT values, the thermoelectric conversion efficiency (η) can be calculated by:16, 65 𝜂=

𝑇ℎ ― 𝑇𝑐

1 + 𝑍𝑇𝑎𝑣𝑒 ― 1

𝑇ℎ

1 + 𝑍𝑇𝑎𝑣𝑒 + 𝑇𝑐 𝑇 ℎ

(2) Calculated results of 2% Te-alloyed SnSe crystal for Tc = 300 K and Th ranging from 323 – 793 K are shown in Figure 5d. The projected conversion efficiency exceeds 18% for Tc = 300 K and Th > 773 K, which is higher than those of other high-performance SnSe systems.16, 31, 48 It is worth to mention that the obtained average ZT and conversion efficiency conducted with other calculation methods66-67 remain still higher than other systems. Importantly, the high thermoelectric performance realized in this work shows excellent thermal stability., the reversibility (heating- cooling) data for the 2%Te-alloyed SnSe sample in Figure S7 well reveal the stability and reliability.

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Figure 5. Comparisons of ZT values and calculated efficiency in SnSe crystals: (a) Temperature dependent ZT values for Te-alloyed SnSe cystals; (b) ZT values comparison of Te-alloyed SnSe in this work and other reported p-type SnSe crystals with different carrier concentrations16, 31, 48 and other state-of-the-art p-type thermoelectrics;10, 51, 62-63 (c) Average ZT values at 300-773K (to be comparable to other reports, 300-773K range is selected for comparisons) of this work and other p-type SnSe crystals;16, 31, 48 (d) The calculated efficiency as a function of hot side temperature (cold side temperature is 300 K) of Te-alloyed SnSe crystals and other p-type SnSe crystals with different carrier concentrations.16, 31, 48 It should be noted that the carrier concentration of SnSe crystals was enhanced from 2.16 to 4.77  1019 cm-3 through Te alloying, results in high thermoelectric performance that is comparable to those high-nH SnSe crystals.16, 31, 48

CONCLUDING REMARKS We successfully synthesized high-performance p-type Te-alloyed SnSe crystals. After Te alloying, the thermoelectric performance of p-type SnSe is enhanced through modification of the crystal structure: 1) the crystal symmetry of SnSe is increased, leading to a high carrier mobility; 2) the formation energy of Sn vacancies is reduced and the carrier concentration is increased, and thus the Seebeck coefficient is enhanced through activating multiple valence 16

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bands; 3) Te-alloying lowers the intrinsically low lattice thermal conductivity of SnSe through scattering phonons via Sn displacements. After Te alloying, the electrical and thermal transport properties of SnSe are synergistically optimized, a record-high average ZT value > 1.6 is obtained at 300–793 K, with a calculated high thermoelectric conversion efficiency > 18%. The present results indicate that modifying crystal structure is an effective method to enhance thermoelectric performance in low symmetry thermoelectric materials. We have demonstrated that SnSe is a promising thermoelectric candidate for energy conversion through harvesting waste heat.

ASSOCIATED CONTENT Supporting Information Experimental details;; XRD patterns and band energy measurements of Te-alloyed SnSe crystals (Figure S1); Calculated equilibrium chemical potential for Te-alloyed SnSe (Figure S2); Comparison between this work and other p-type SnSe crystals for electrical conductivity and Seebeck coefficient and high temperature hall measurements (Figure S3); Band structure calculations (Figure S4); Comparison between this work and other p-type SnSe crystals for thermal conductivity (Figure S5); Temperature dependence of thermal transport properties in Te-alloyed SnSe crystals (Figure S6); Room temperature hall data (Table S1); Sample densities of SnSe-xSnTe crystals (Table S2);Calculation on the formation energy; Discussion on the band structure and effective mass; Calculation on the Callaway model; Calculation for the specific heat (Cp) based on Debye model. This material is available free of charge via the Internet at http://pub.acs.org.

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AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

ORCID Li-Dong Zhao: 0000-0003-1247-4345 Hai-Jun Wu: 0000-0002-7303-379X Author Contributions †B.Q.

and D.W. contribute equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China under Grant No. 2018YFB0703600, the National Natural Science Foundation of China under Grant No. 51571007, 51632005 and 51772012, the Beijing Municipal Science & Technology Commission under Grant No. Z171100002017002, the Ministry of Education of Singapore under its Tier 2 Grant No. MOE2017-T2-1-129, the 111 Project under Grant No. B17002. The authors are grateful for the discussion and help on the DFT calculations from Prof. Guangtao Wang.

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