Enhancement of Thermoelectric Performances in

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Enhancement of Thermoelectric Performances in Topological Crys-tal Insulator Pb0.7Sn0.3Se via Weak Perturbation of the Topological State and Chemical Potential Tuning by Chlorine Doping Chan-Chieh Lin, Gareoung Kim, Dianta Ginting, Kyunghan Ahn, and Jong Soo Rhyee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00571 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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ACS Applied Materials & Interfaces

Enhancement of Thermoelectric Performances in Topological Crystal Insulator Pb0.7Sn0.3Se via Weak Perturbation of the Topological State and Chemical Potential Tuning by Chlorine Doping Chan-Chieh Lin, Gareoung Kim, Dianta Ginting, Kyunghan Ahn,* and Jong-Soo Rhyee* Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-In 17104, Republic of Korea. KEYWORDS: Thermoelectric, Topological Crystal Insulator, Topological Semimetal, Chemical potential, Power factor.

ABSTRACT: Topological insulators generally share commonalities with good thermoelectric materials because of their narrow band gaps and heavy constituent elements. Here we propose that a topological crystalline insulator (TCI) could exhibit a high thermoelectric performance by breaking its crystalline symmetry and tuning chemical potential by elemental doping. As a candidate material, we investigate thermoelectric properties of the Cl-doped topological crystalline insulator Pb0.7Sn0.3Se. The infrared absorption spectra reveal that the band gap is increased from 0.055 eV for Pb0.7Sn0.3Se to 0.075 eV for Pb0.7Sn0.3Se0.99Cl0.01, confirming that the Cl-doping can break the crystalline mirror symmetry of a topological crystalline insulator Pb0.7Sn0.3Se and thereby enlarges its bulk electronic band gap. The topological band inversion is confirmed by the extended X-ray absorption fine structure spectroscopy (EXAFS) showing that the TCI state is weakened in chlorine x = 0.05 doped compound. The small gap opening and partial linear band dispersion with massless and massive bands may have high power factor for high electrical conductivity with enhancement of Seebeck coefficient. As a result, the Pb0.7Sn0.3Se0.99Cl0.01 shows a considerably enhanced ZT of 0.64 at 823 K, which is about 1200 % enhancement in ZT compared to that of the undoped Pb0.7Sn0.3Se. This work demonstrates that the optimal n-type Cldoping tunes the chemical potential together with breaking the state of topological crystalline insulator, suppresses a bipolar conduction at high temperatures, and thereby enables the Seebeck coefficient to keep increasing up to 823 K, resulting in a significantly enhanced power factor at high temperatures. In addition, the bipolar contribution to thermal conductivity is effectively suppressed for the Cl-doped samples of Pb0.7Sn0.3Se1-xClx (x ≥ 0.01). We propose that breaking the crystalline mirror symmetry in topological crystalline insulators could be a new research direction for exploring high performance thermoelectric materials.

single non-degenerate Dirac cone, experimentally confirmed by an angle resolved photoemission spectroscopy (ARPES) measurement.7,8 Interestingly, (Bi,Sb)2(Se,Te)3 materials being a solid-solution of Bi2Se3, Bi2Te3, and Sb2Te3 are well-known both p- and n-type TE materials for near-room temperature TE applications.9-22 This is not a coincidence because a highly efficient TE material generally has a narrow band gap and heavy constituent elements to possess good electrical transport properties and low thermal conductivity, which are also required by TI materials as well.23-27 Topological crystalline insulators (TCIs) are a kind of topological materials possessing even number of Dirac cones driven by a crystalline mirror symmetry.28,29 It has been recently reported that Pb1−xSnxTe,30,31 SnTe,32,33 and Pb1−xSnxSe34-36 are TCI materials having multi Dirac cones confirmed by ARPES measurements. It is experimentally well demonstrated that the increase in valley degeneracy (NV) of a TE material results in the enhancement in density-of-states effective mass (m*) without reducing the mobility of charge carriers (µ) and thereby improves its TE performance.4 In this regard, breaking the crystalline mirror symmetry of TCIs near a quantum phase transition can increase NV29,35-37 and thus enhance their TE

INTRODUCTION Thermoelectric (TE) energy conversion can be applied to waste heat power generation by direct energy conversion from heat to electricity and vice versa. The performance of a TE material, predominantly determining the efficiency of a TE device, can be quantitatively described as a dimensionless TE figure-of-merit ZT, ZT = S2T/ρκ, where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and κ is the thermal conductivity, respectively. In order to get high thermoelectric figure-of-merit, we need high power factor (PF), defined by PF = S2/ρ as well as low thermal conductivity κ simultaneously. Many attempts have been devoted to decrease lattice thermal conductivity such as nanostructuring1 and all-scale hierarchical structuring2 and to increase power factor such as resonance level formation3 and band convergence.4 Topological insulators (TIs) have attracted a lot of interests mainly due to their peculiar physical properties by a gapless surface state crossing the bulk band gap originated by a band inversion via time-reversal symmetry and Z2 invariance under a strong spin-orbit interaction.5,6 Bi2Se3, Bi2Te3, and Sb2Te3 have been recently known as a three-dimensional TIs with a

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performances. It has been reported that a p-type Na-doping in a TCI Pb0.6Sn0.4Te breaks a crystalline mirror symmetry together with the optimization of hole concentration, opens up a bulk electronic band gap accompanied with an increased NV, and consequently suppresses a bipolar conduction, resulting in a high ZT of 1 at 856 K.27 In this work, we investigate the effect of Cl-doping on TE properties of a TCI Pb0.7Sn0.3Se. Herein we demonstrate that an n-type chlorine doping can break a crystalline mirror symmetry in a TCI Pb0.7Sn0.3Se followed by a band gap increase and simultaneously optimize the electron concentration for an enhanced ZT. We present a significantly improved ZT by breaking the state of TCI with a substantially enhanced ZT of 0.64 for Pb0.7Sn0.3Se0.99Cl0.01, which is about 1200 % enhancement in ZT compared to that of the undoped Pb0.7Sn0.3Se at 823 K. This is possible by a synergistic combination of a high PF of 1.23 mW m-1 K-2 and a low κ of 1.57 W m-1 K-1 mainly due to a suppressed bipolar conduction at high temperatures via breaking the TCI state as well as a heavy n-type doping by Cl-doping.

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the Cp was determined using the equation Cp/kB per atom = 3.07 + 4.7 × 10−4 (T/K − 300) by fitting experimental data.38 Hall measurement. The Hall carrier density nH and Hall mobility µH were calculated by the relation of nH = 1/(eRH) and µH = RH/ρ, respectively, where RH is the Hall coefficient under a magnetic field sweeping from -5 T to 5 T by the physical property measurement system (16 T PPMS DynaCool, Quantum Design, USA) using a four-probe contact method.

RESULTS AND DISCUSSION

EXPERIMENTAL SECTION Synthesis and hot-press sintering. The bulk ingots of the nominal Pb0.7Sn0.3Se1-xClx (x = 0, 0.05 %, 0.1 %, 0.5 %, 1 %, 5 %, 10 %) compositions were prepared by a conventional hightemperature melting and sintering. Stoichiometric mixtures of high purity elements (> 99.999%) of Pb, Sn, Se, and PbCl2 were loaded into an evacuated quartz tube. Then, the tube was slowly heated up to 1373 K for 18 h, kept that temperature for 6 h followed by water quenching, and a solidified ingot was further annealed at 823 K for 3 days under a high vacuum. The annealed ingot was pulverized into fine powders by using a mortar and pestle and the powders were hot-press sintered in a 12.7 mm diameter graphite mold under a uniaxial pressure of 40 MPa at 773 K for 1 h. Powder X-ray diffraction (XRD) and infrared spectroscopy. X-ray diffraction (XRD) measurements of all samples were performed by X-ray diffractometer with a Cu Kα radiation (D8 Advance, Bruker, Germany). Room temperature optical diffuse reflectance measurements on powder samples of the hot-press sintered bulk pellets were performed using a Varian 670 FT-IR spectrometer with a variable angle reflectance kit in order to probe optical energy band gaps. Scanning electron microscopy (SEM). The microstructures and compositions of constituent elements for the samples were investigated by using a high-resolution field-emission scanning electron microscope (HR FE-SEM) coupled with an energy dispersive X-ray spectroscopy (EDX) (MERLIN, Carl Zeiss, Germany). Thermoelectric properties. The Seebeck coefficient S and electrical resistivity ρ were simultaneously measured under a helium atmosphere in the temperature range of 300 K−823 K by a thermoelectric measurement system (ZEM-3, ULVAC, Japan) using a four-probe contact method. The thermal conductivity κ was calculated by κ = dλCp, where d, λ, and Cp are the sample density, the thermal diffusivity, and the specific heat, respectively. The thermal diffusivity λ was measured by a laser flash method (LFA-457, NETZSCH, Germany), and

Figure 1. Powder X-ray diffraction (XRD) patterns of the Pb0.7Sn0.3Se1−xClx samples (x = 0.0, 0.0005, 0.001, 0.005, 0.01, 0.05). The red arrows indicate diffraction peaks of PbCl2. Inset shows expanded plot of XRD patterns near (200) peak with 2-θ range of 27.5 – 31.5°.

Figure 1 shows the XRD patterns for Pb0.7Sn0.3Se1−xClx samples (x = 0.0, 0.0005, 0.001, 0.005, 0.01, 0.05), indicative of a single phase of rock salt-type cubic structure with the space group Fm−3m. The x = 0.1 samples show a second phase of PbCl2 that is originated from a high chlorine concentration exceeding the solubility limit. The inset in Figure 1 indicates that there is no shift of the (200) diffraction peak with respect to the concentration of Cl, revealing that the solubility limit of Cl should be less than 5 mol % based on the XRD data.

Figure 2. The SEM image of the Pb0.7Sn0.3Se sample (a), and its elemental maps of Pb (b) and Sn (c). The SEM im-


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age of the Pb0.7Sn0.3Se0.95Cl0.05 sample (d), and its elemental maps of Pb (e) and Sn (f). The scanning electron microscopy (SEM) images of Pb0.7Sn0.3Se1−xClx (x = 0.0 and 0.05) compounds and their elemental map of Pb and Sn are presented in Figure 2. It shows clean surfaces of x = 0.0 and 0.05 compounds (Figure 2a-2f), while the x = 0.1 compound exhibits micron or several hundred nano-meter scale precipitates from the SEM and elemental mapping of Pb and Sn from the energy dispersive X-ray spectroscopy (EDX) measurement (not shown). The XRD and SEM/EDX supports that the single phase of the compounds Pb0.7Sn0.3Se1−xClx (x ≤ 0.05).

Figure 3. Infrared absorption spectra of the Pb0.7Sn0.3Se1−xClx samples (x = 0, 0.0005, 0.001, 0.005, 0.01).

Figure 4. Temperature-dependent (a) Seebeck coefficient S and (b) electrical resistivity ρ of the Pb0.7Sn0.3Se1−xClx samples (x = 0, 0.0005, 0.001, 0.005, 0.01, 0.05).

In order to determine energy band gaps of Pb0.7Sn0.3Se1−xClx (x = 0, 0.0005, 0.001, 0.005, 0.01), the infrared absorption spectra are obtained as shown in Figure 3. It is noteworthy that an absorption edge of Pb0.7Sn0.3Se shifts to a higher energy with increasing chlorine doping concentration, especially from 0.1 % to 1 %. This

The n-type x = 0.005 sample having the electron concentration of 3.5 × 1019 cm-3 exhibits a negative sign of S in the measured temperature range of 300 – 800 K while there exists a bipolar conduction at T ≥ 650 K due to a very small band gap of 0.055 eV. In contrast, it is very noteworthy that the Cl-doping (x ≥ 0.01) enables the state of TCI to be broken, enlarges the bulk band gap up to 0.075 eV for Pb0.7Sn0.3Se0.99Cl0.01, and thereby a bipolar conduction is fully suppressed at high temperatures for heavily Cl-doped n-type samples, resulting in a maximum S of -140 µV/K at 823 K for the x = 0.01 sample.

demonstrates that the substitution of Cl (x ≥ 0.001) for Se in a TCI Pb0.7Sn0.3Se breaks its crystalline symmetry, resulting in widening its bulk band gap from 0.055 eV for Pb0.7Sn0.3Se to 0.075 eV for Pb0.7Sn0.3Se0.99Cl0.01, which is similar to alkali-doped Pb0.6Sn0.4Te reported in previous studies.27,39 The temperature-dependent Seebeck coefficient S and electrical resistivity ρ of the Pb0.7Sn0.3Se1−xClx samples are shown in Figure 4a and 4b, respectively. The sign of room temperature (RT) S is positive for x ≤ 0.001, indicating that the hole is a major charge carrier, while negative RT S values revealing a n-type charge carrier are observed for x ≥ 0.005. Interestingly, the sign of S for x ≤ 0.001 is positive below the temperature range of 600 – 700 K, but it is switched to a negative sign at high temperatures due to a thermal activation of carriers at high temperatures.12,13,40 For x ≤ 0.001, the S reaches a maximum at a lower temperature with a higher x and then decreases with increasing x for x = 0.05 % and 0.1 %.

The electrical resistivity ρ is increased with increasing temperature, indicative of a degenerated semiconducting behavior (Figure 4b), but a bipolar conduction, reducing ρ at high temperatures, is observed for x ≤ 0.001 due to the thermal excitation of electronhole pairs.40 For x ≤ 0.001, the Hall carrier concentration of ptype Pb0.7Sn0.3Se is reduced with increasing Cl-doping content because the Cl acts as a n-type dopant (Figure 5), which gives rise to the increase in RT ρ as the x increases. For example, the RT ρ of the x = 0, 0.0005, and 0.001 is 1.24, 1.29, and 1.59 mΩ-cm, respectively. The respective RT carrier concentration is 1.3 × 1019, 6.2 × 1018, and 5.5 × 1018 cm-3 (Figure 5). However, an opposite trend is observed at T > 600 K, indicating that the ρ decreases with increasing x due to a p-type-to n-type transition at high tem-



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7.3×1019, and 1.3×1020, respectively. The RT Hall carrier mobility µH of the Pb0.7Sn0.3Se1−xClx samples exhibits an opposite dependence on the Cl-doping content in comparison with the nH, which should be related with more charge carrier scattering for higher RT nH value (Figure 5b). Here, the increase in nH is much more than the decrease in µH and thereby the ρ is predominantly determined by the nH.

peratures. For x ≥ 0.005, the higher x gives rise to the lower RT ρ because of the higher electron concentration (Figure 5), which will be discussed just below in the part of Hall effect data. For example, the RT ρ of the x = 0.005, 0.01, and 0.05 sample is 0.51, 0.33, 0.27, and 0.19 mΩ-cm, respectively. Furthermore, it is noticeable that the ρ of the x = 0.005 sample more rapidly increases with temperature than those of the x ≥ 0.01 samples, which may be related with the bipolar conduction at T ≥ 650 K for the x = 0.005 sample.

Figure 6. The absolute Seebeck coefficient |S| as a function of Hall carrier concentration nH of the Pb0.7Sn0.3Se1−xClx samples (x = 0, 0.0005, 0.001, 0.005, 0.01, 0.05). The dashed and dotted line is theoretically calculated with m* = 0.17me and m* = 0.34me, respectively, based on a single parabolic band (SPB) model. Figure 6 shows RT S as a function of Hall carrier density, called as Pisarenko plot, for the Pb0.7Sn0.3Se1−xClx samples. The dashed and dotted lines are the calculated Pisarenko plot for the effective masses m* = 0.17 me and m* = 0.34 me based on the single parabolic band (SPB) model, respectively. The RT S values of the p-type samples (x ≤ 0.001) are located near the dashed line while those of the n-type samples (x ≥ 0.005) roughly follow the dotted line. Here, the m* values are calculated as given by:41

Figure 5. Temperature-dependent (a) Hall carrier concentration nH and (b) the room temperature nH and Hall carrier mobility µH with respect to the Cl-doping content of the Pb0.7Sn0.3Se1−xClx samples (x = 0, 0.0005, 0.001, 0.005, 0.01, 0.05).

∗ ⁄

⁄

⁄ ⁄

(1) (2)

where n and rH are the chemical carrier density and Hall factor, respectively. This reveals that the higher m* values for the n-type samples should be attributed to more non-parabolic band structure and higher band degeneracy caused by merging multi nondegenerate Dirac surface states via breaking the TCI state by Cldoping, than the p-type samples, resulting in a significant enhancement in S for the x = 0.01 sample.

To further understand electrical transport properties of the Pb0.7Sn0.3Se1−xClx samples, their Hall effect measurements are performed in Figure 5. All the samples show negligible temperature variations of Hall carrier concentration nH in the measured temperature range of 5 – 300 K (Figure 5a). As mentioned before, the carrier concentration of hole in p-type Pb0.7Sn0.3Se firstly decreases with increasing Cl-doping content until x = 0.001. The Cldoping switches from p-type into n-type carrier, and then the carrier concentration of electron increases, which is consistent with the trend in ρ (Figure 4b). For example, the carrier concentration of electron for the x = 0.005, 0.01, and 0.05 samples are 3.5×1019,


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Table 1. Electrical resistivity ρ, Seebeck coefficient S, Lorenz number L, electronic thermal conductivity κe, total thermal conductivity κtot, lattice thermal conductivity κph by single parabolic band (SPB) model, and minimum lattice thermal conductivity, obtained by the plot of κtot versus σ for the compounds of Pb0.7Sn0.3Se1-xClx (x = 0.005, 0.01, and 0.05).

823 K x = 0.5% x = 1% x = 5%

ρ mΩ-cm 6.30 1.60 1.17

S µV/K -39.85 -140.01 -108.71

L (SPB) 10−8 WΩ/K2 2.29 1.74 1.87

κe (SPB) 0.30 0.90 1.32

κtot

κph (SPB) W m-1 K-1 1.32 1.02 1.57 0.67 1.80 0.48

κph (min.) 1.23 0.85 0.66

conductivity. The lattice thermal conductivity can be obtained by typical thermal conductivity relation κtot = LT/ρ + κlat, where L is the Lorenz number calculated by a single parabolic band (SPB) model with an assumption of acoustic phonon scattering mechanism as expressed by:41

(3)

where Fx(η) is the Fermi integral, η is the reduced chemical potential obtained from experimental S data: !

∞

"0

1&exp#*

-

#! $#

(4)

* (5)

The lattice thermal conductivity κph (SPB) from the single parabolic band model is presented in Table 1. The decrease of lattice thermal conductivity come from the increase of point defect with the increase of chlorine doping. However, we believe that the κph value is under-estimated. When we plot the κtot versus electrical conductivity σ for the compounds showing monotonic increase behavior of ρ(T), the extrapolation to zero electrical conductivity limit indicates the minimum lattice thermal conductivity as shown in Figure 7b. The minimum lattice thermal conductivity is higher than the lattice thermal conductivity obtained by the single parabolic band assumption, as presented in Table 1. It supports that the single parabolic band model does not be applied in the compounds. It is reasonable because the parent compound Pb0.7Sn0.3Se exhibits the topological crystal insulator state.34 The linear band contribution, even in the vicinity of TCI phase transition, violates Wiedemann-Franz law with conventional Lorenz number. The κtot of p-type samples (x ≤ 0.001) slightly decreases with increasing temperature until the temperature range of 350 – 450 K and then increases with a further rise in temperature up to 823 K. This can be explained by the bipolar conduction behavior,40 which is consistent with the temperature variations of ρ and S for the x ≤ 0.001 samples. The evolution from the bipolar conduction behavior of κtot(T) for x ≤ 0.001 to the Umklapp scattering dominated thermal conductivity for x ≥ 0.005 come from the chemical potential shift from the top of valence band (p-type) to the bottom of the conduction band (n-type) which is consistent with the Seebeck coefficients S(T) in Figure 4a.

Figure 7. Temperature-dependent (a) total thermal conductivity κtot and (b) total thermal conductivity κtot versus electrical conductivity of the Pb0.7Sn0.3Se1−xClx samples as indicated the compositions.

Figure 7a shows the temperature-dependent total thermal conductivity κtot of the Pb0.7Sn0.3Se1-xClx samples. It is noticeable that there exists a quite different temperature variation of κtot between p-type and n-type samples. For the n-type samples (x ≥ 0.005), the κtot decreases with increasing temperature mainly due to the ρ(T) behavior with acoustic phonon contribution of thermal



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and 0.05) compounds. The L1-band edge near 15.865 keV photon energy is the transition from filled 2s state to the empty p states of conduction band. Here we define the band transition widths of L1band edge ∆µ(L1) and L3-band edge ∆µ(L3), respectively. In the normal state, p-state is the filled state of the valence band, and sstate is the empty conduction band in PbTe compound. On the other hand, because of the band inversion in the TCI state, the sstate becomes valence band, and the p-state becomes conduction band in TCI state. Therefore, the absorption probability of L3band edge is significant in a normal state, in other words the bandedge transition rate is changed with respect to the topological band inversion.

Figure 9. Ratio of the absorption jump from the XAS spectra at Pb L1- to L3-edge with respect to Cl concentration. Figure 8. X-ray absorption spectra for Pb L1-edge (a) and L3-edge (b) of the Pb0.7Sn0.3Se1-xClx (x = 0.0, 0.01, and 0.05) compounds. ∆µ represents the edge jump for each spectra.

Figure 9 presents the the ratio of the absorption edge jump of L3edge to that of L1-edge, ∆µ(L3)/∆µ(L1), as a function of Cl concentration (%). It shows that the compounds with low chlorine concentration exhibits low transition rate of L3-edge to that of L1edge, indicating the topological band inversion (lower filled sstate and upper empty p-state). For high chlorine concentration (x = 0.05), it significantly increases the relative transition rate ∆µ(L3)/∆µ(L1), implying the normal state with lower filled p-state and upper empty s-state. It strongly supports the TCI state is broken by chlorine doping due to the increase of crystalline disorder and breaking inversion symmetry.

In order to identify the topological crystal insulator (TCI) state, we measured the X-ray absorption spectroscopy (XAS) using Xray absorption near edge structure (XANES) analysis for the Pb0.7Sn0.3Se1-xClx (x = 0.0, 0.01, and 0.05) compounds. It is relatively easy way to identify the topological band inversion comparing to the conventional angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) experiments because XAS uses not single crystalline compounds but polycrystalline powder. The X-ray absorption spectra measured the electronic transitions from filled atomic core levels to empty final states with the Fermi’s golden rule: ./01 23|5|67 89: * 9; * < and the selection rule (∆l = ±1), where ./01 is the probability of a transition, 23|5|67 is the matrix element of a transition, 9: (9; ) is the energy of final (initial) states and = is the quantum number of the core state. The L1edge is related with electronic transition from filled 2s states to the empty p states of conduction band while the L3-edge involves the electronic transition from filled 2p (j = 3/2) states to the empty s states in conduction band.42 Figure 8a and 8b represent the X-ray absorption spectra for Pb L1-edge (a) and L3-edge (b) of the Pb0.7Sn0.3Se1-xClx (x = 0.0, 0.01,


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ACS Applied Materials & Interfaces In summary, in order to tune the chemical potential in a TCI Pb0.7Sn0.3Se by Cl-doping, we prepared polycrystalline Pb0.7Sn0.3Se1−xClx (x = 0, 0.0005, 0.001, 0.005, 0.01, 0.05) bulk samples by melting and hot-press sintering. We demonstrate that the optimal Cl-doping (x ≥ 0.01) breaks the crystalline mirror symmetry of a TCI state as well as optimize the n-type carrier concentration, enlarges the band gap from 0.055 eV for Pb0.7Sn0.3Se to 0.075 eV for Pb0.7Sn0.3Se0.95Cl0.05. The topological band inversion is investigated by the extended X-ray absorption fine structure spectroscopy (EXAFS) showing that the TCI state is weakened in chlorine x = 0.05 doped compound. The small gap opening and partial linear band dispersion with massless and massive bands may have high power factor for high electrical conductivity with enhancement of Seebeck coefficient. The bipolar conduction of carriers at high temperatures is suppressed by breaking the TCI state, resulting in a considerably improved PF at high temperatures. From the synergistically combined effect with the enhancement of PF (1.23 mW m-1 K-2 at 823 K) and the reduction of κ (1.57 W m-1 K-1 at 823 K), the Pb0.7Sn0.3Se0.99Cl0.01 sample shows a significantly enhanced ZT of 0.64 at 823 K, which is about 1200 % enhancement in ZT compared to that of the pristine TCI Pb0.7Sn0.3Se compound. We propose that a TCI material could be promising for a high-performance TE material when its crystalline symmetry is optimally broken by chemical doping.

AUTHOR INFORMATION Corresponding Author * Corresponding authors should be addressed to [email protected] (K. Ahn); [email protected] (J.-S. Rhyee).

Figure 10. Temperature-dependent (a) power factor PF (= S2/ρ) and (b) dimensionless thermoelectric figure-of-merit ZT of the Pb0.7Sn0.3Se1−xClx samples (x = 0, 0.0005, 0.001, 0.005, 0.01, 0.05).

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-TA1403-02.

The temperature-dependent power factor PF and dimensionless TE figure-of-merit ZT of the Pb0.7Sn0.3Se1−xClx samples is shown in Figures 10a and 10b, respectively. The insufficient Cl-doped samples (x ≤ 0.005) show relatively low PF and ZT values mainly due to the bipolar conduction behavior at high temperatures. In contrast, the heavily Cl-doped samples (x ≥ 0.01) enlarges the band gap by breaking the crystalline mirror symmetry of a TCI Pb0.7Sn0.3Se accompanied with the increases in electron concentration, and thereby the bipolar conduction is fully suppressed at high temperatures. As a result, the Pb0.7Sn0.3Se0.99Cl0.01 sample shows a significantly improved ZT of 0.64 at 823 K due to a synergistically combined effect of the enhancement of PF 1.23 mW m-1 K-2 and the reduction of κtot 1.57 W m-1 K-1 at 823 K. The suppression of the bipolar conduction at high temperatures via breaking a TCI state gives rise to the significant enhancement of thermoelectric performance as much as 13 times higher than that of the pristine Pb0.7Sn0.3Se compound.

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CONCLUSION



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Table of Content Artwork
