Intrinsically High Thermoelectric Performance in AgInSe2 n‐Type Diamond‐Like Compounds

Abstract Diamond‐like compounds are a promising class of thermoelectric materials, very suitable for real applications. However, almost all high‐performance diamond‐like thermoelectric materials are p‐type semiconductors. The lack of high‐performance n‐type diamond‐like thermoelectric materials greatly restricts the fabrication of diamond‐like material‐based modules and their real applications. In this work, it is revealed that n‐type AgInSe2 diamond‐like compound has intrinsically high thermoelectric performance with a figure of merit (zT) of 1.1 at 900 K, comparable to the best p‐type diamond‐like thermoelectric materials reported before. Such high zT is mainly due to the ultralow lattice thermal conductivity, which is fundamentally limited by the low‐frequency Ag‐Se “cluster vibrations,” as confirmed by ab initio lattice dynamic calculations. Doping Cd at Ag sites significantly improves the thermoelectric performance in the low and medium temperature ranges. By using such high‐performance n‐type AgInSe2‐based compounds, the diamond‐like thermoelectric module has been fabricated for the first time. An output power of 0.06 W under a temperature difference of 520 K between the two ends of the module is obtained. This work opens a new window for the applications using the diamond‐like thermoelectric materials.


DOI: 10.1002/advs.201700727
in response to the world's increasing energy crisis and deteriorating environment. One of the promising technologies is the thermoelectric (TE) energy conversion with its ability of directly converting thermal energy into electricity. [1,2] Efficient TE conversion requires high performing n-as well as p-type elements (legs) that are assembled electrically in series and thermally in parallel in a module. To ensure longterm service reliability and stability, [3] the n-and p-type TE legs should have closely matching physical and chemical properties, such as the chemical composition, melting points, and thermal expansion coefficients. The energy conversion efficiency of a TE material is specified by the dimensionless TE figure of merit zT = S 2 σT/(κ L +κ e ). The expression can be viewed as consisting of two distinct contributions: the dominator reflecting the electronic efficiency of a material as expressed via the power factor S 2 σ, where S is the Seebeck coefficient and σ is the electrical conductivity, and the denominator representing the heat conduction ability and consisting of the lattice thermal conductivity κ L and the electronic thermal conductivity κ e . In order to achieve high TE performance, it is essential to enhance the electronic properties (S 2 σ) and minimize the total thermal conductivity (κ = κ L +κ e ). However, the transport parameters are closely correlated and interdependent. For example, S generally decreases with the increasing carrier concentration, that is, with the increasing electrical conductivity. Moreover, since the electronic thermal conductivity is related to the electrical conductivity via the Wiedemann-Franz law, κ e = LσT, it increases with the increasing electrical conductivity. Therefore, it is challenging to simultaneously achieve excellent electronic properties and low thermal conductivity in a given material.
While several families of classical TE materials have been developed and improved, among them SiGe, [4] Bi 2 Te 3 , [5] PbTe, [6] and skutterudites, [7] they either contain expensive elements or are environmentally harmful. Consequently, there is a worldwide effort to identify and develop new high-performing TE materials. Recently, many new prospective TE materials have been discovered, and Cu-based compounds [8] have generated much interest. Among them, diamond-like compounds possessing a relatively low thermal conductivity and decent Diamond-like compounds are a promising class of thermoelectric materials, very suitable for real applications. However, almost all high-performance diamond-like thermoelectric materials are p-type semiconductors. The lack of high-performance n-type diamond-like thermoelectric materials greatly restricts the fabrication of diamond-like material-based modules and their real applications. In this work, it is revealed that n-type AgInSe 2 diamond-like compound has intrinsically high thermoelectric performance with a figure of merit (zT) of 1.1 at 900 K, comparable to the best p-type diamond-like thermoelectric materials reported before. Such high zT is mainly due to the ultralow lattice thermal conductivity, which is fundamentally limited by the low-frequency Ag-Se "cluster vibrations," as confirmed by ab initio lattice dynamic calculations. Doping Cd at Ag sites significantly improves the thermoelectric performance in the low and medium temperature ranges. By using such high-performance n-type AgInSe 2based compounds, the diamond-like thermoelectric module has been fabricated for the first time. An output power of 0.06 W under a temperature difference of 520 K between the two ends of the module is obtained. This work opens a new window for the applications using the diamond-like thermoelectric materials.

Introduction
Nowadays, advanced technologies based on high-performance energy materials have triggered a worldwide attention Adv. Sci. 2018, 5,1700727 electronic transport properties are especially interesting.  There are about 1000 types of diamond-like compounds and several of them exhibit very good TE performances. Examples include Cu 2 ZnSn 0.90 In 0.10 Se 4 with a zT of 0.95 at 850 K, [9] Cu 2 Sn 0.90 In 0.10 Se 3 with a zT of 1.14 at 850 K, [10] Ag 0.95 GaTe 2 with a zT of 0.77 at 850 K, [11] Cu 3 Sb 0.97 Ge 0.03 Se 2.8 S 1.2 with a zT of 0.89 at 650 K, [12] CuGaTe 2 with a zT of 1.4 at 950 K, [13][14][15][16][17] Cu 2.075 Zn 0.925 GeSe 4 with a zT of 0.45 at 670 K, [18] CuInTe 2 with a zT of 1.18 at 850 K, [19,20] Cu 2 TM (TM = Mn, Fe, Co)SnSe 4 with a zT of 0.7 at 850 K, [21] and many others. [22][23][24][25] These exciting results make the diamond-like compounds a new and prospective class of TE materials. However, all the above diamond-like compounds are p-type materials. To date, only two examples of n-type diamond-like compounds have been reported on and both of them show a rather poor performance: AgInSe 2 with a zT of 0.34 at 724 K [26,27] and Cu 0.92 Zn 0.08 FeS 2 with a zT of 0.26 at 630 K [28][29][30] (as shown in Figure 1a). Therefore, the lack of high-performance n-type diamond-like TE materials has, so far, impeded the construction of efficient TE modules based on these compounds.
In this work, we report on n-type AgInSe 2 that has an intrinsically low lattice thermal conductivity and its maximal zT reaches a value of 1.1 at 900 K. Such high zT is mainly due to the ultralow lattice thermal conductivity caused by the Ag-Se "cluster vibrations" at low phonon frequencies. Furthermore, the average zT value of AgInSe 2 is optimized via substituting Cd at Ag sites to improve the TE performance in the low and medium temperature ranges. In addition, a twocouple TE module based on these diamond-like compounds has been fabricated for the first time (as shown in Figure 1b), and has achieved the maximum output power of 0.06 W under a temperature difference of 520 K.

Results and Discussion
AgInSe 2 is a semiconductor with the band gap of ≈1.19 eV, [31] which has been extensively investigated for solar energy applications, optoelectronic applications, as well as photoelectrochemical applications. [32,33] It possesses a typical tetragonal chalcopyrite structure with the space group I-42d, which is derived from the sphalerite structure with Ag/In orderly replacing Zn. [31] The powder X-ray diffraction (XRD) patterns of AgInSe 2 are shown in Figure S1 (Supporting Information). The XRD data match well with the PDF card (#No.  for AgInSe 2 compounds. The scanning electron microscopy (SEM) results for AgInSe 2 are shown in Figure S2 (Supporting Information) and demonstrate that a small amount of Ag 2 Se second phase (<3%) exists in the prepared sample. The TE properties of AgInSe 2 are shown in Figure 2. The electrical conductivity σ of AgInSe 2 is very low with the values on the order of 10 −1 S m −1 around room temperature and 10 3 S m −1 at 900 K. Correspondingly, the Seebeck coefficient S is quite large with a value of −820 µV K −1 around room temperature and −295 µV K −1 at 900 K. These data indicate that AgInSe 2 is a typical semiconductor with a very low carrier concentration. This is consistent with our Hall effect measurements, which shows that the carrier concentration of AgInSe 2 is about 1.3 × 10 13 cm −3 at 300 K. Consequently, the power factor (PF = S 2 σ) of AgInSe 2 is also very low with a maximum value module 2.92 µW cm −1 K −2 at 900 K (see Figure S5, Supporting Information). The thermal conductivity for the pristine AgInSe 2 prepared in our work is as low as 0.99 W m −1 K −1 at room temperature and decreases to 0.39 W m −1 K −1 at 900 K, which is close to the theoretical minimum lattice thermal conductivity κ min calculated by the Cahill's model (Equation (S1), Supporting Information). [34] All these lead to a zT of 0.7 at 900 K in AgInSe 2 although its electrical conductivity is low (as shown in Figure 2d).
The low carrier concentration in AgInSe 2 suggests that the low zT shown above as well as that reported in the literature [26,27] should have a large room for improvement. In order to change the material's carrier concentration, we thus tried to introduce off-stoichiometry in the composition. Specifically, we added an excess amount of Ag to AgInSe 2 to generate interstitial Ag atoms, expecting to increase the density of electrons. The powder XRD patterns for Ag 1+x InSe 2 (x = 0, 0.01, 0.02) are shown in Figure S1 (Supporting Information), and they match well with the PDF card (No. 35-1099) for the AgInSe 2 compound. The SEM results for Ag 1.02 InSe 2 are shown in Figure S3    . b) Images of fabricated two-pair TE module by using high-performance Ag 0.9 Cd 0.1 InSe 2 diamond-like compound as the n-type leg and Cu 0.99 In 0.6 Ga 0.4 Te 2 [16] diamond-like compound as the p-type leg.
in Figure 2. As we expected, the electrical conductivity σ of Ag 1+x InSe 2 compounds with excess of Ag has increased to ≈10 S m −1 at room temperature, about 1-2 orders of magnitude in comparison to the stoichiometric AgInSe 2 compound. At 900 K, σ has also significantly increased. The enhancement in σ is mainly derived from the drastically increased carrier concentrations by the excess amount of Ag. The measured carrier concentration at 300 K is 2.5 × 10 15 cm −3 for Ag 1.01 InSe 2 and 1.6 × 10 16 cm −3 for Ag 1.02 InSe 2 , about 2-3 orders of magnitude enhancement as compared with the stoichiometric AgInSe 2 compound. The room-temperature Hall mobility µ H of Ag 1+x InSe 2 is in the range of 26-68 cm 2 V −1 s −1 . In contrast, the Seebeck coefficient S for the Ag-excess Ag 1+x InSe 2 compounds has decreased with the increasing content of Ag throughout the whole temperature range investigated. Around room temperature, the S for Ag 1.02 InSe 2 has greatly decreased to −355 µV K −1 . In spite of this decrease, as shown in Figure S5 (Supporting Information), the power factor has significantly increased in the entire temperature range. Compared to the stoichiometric AgInSe 2 around room temperature, the PF for Ag-excess AgInSe 2 has increased by about 50 times. Moreover, the maximum PF value of 5 µW cm −1 K −2 have been reached in Ag 1.02 InSe 2 at 900 K, an enhancement of about 72% over the value of the stoichiometric AgInSe 2 compound. The thermal conductivity κ, on the other hand, is almost unchanged upon introducing an excess amount of Ag in AgInSe 2 , as shown in Figure 2c. The thermal conductivity has a low value of 1.1 W m −1 K −1 at 300 K and decreases to 0.4 W m −1 K −1 at 900 K. Combining the significantly improved electronic transport properties with the essentially unchanged and low thermal conductivity, a high zT of 1.1 is achieved for Ag 1.02 InSe 2 at 900 K, an increase of about 62% compared to the stoichiometric AgInSe 2 compound. The value of zT stands as the best n-type diamond-like TE material reported so far, and is comparable to the best p-type diamond-like TE materials shown in Figure 1a. Having both n-and p-type high-performance diamond-like materials available bodes well for fabrication of high-efficiency TE modules.
Perhaps the most striking feature of AgInSe 2 is its ultralow lattice thermal conductivity κ L . The temperature dependence of κ L for AgInSe 2 and other typical diamond-like compounds is shown in Figure 3. The κ L of the stoichiometric AgInSe 2 compound is about 0.99 W m −1 K −1 at 300 K, which is much lower than all other Cu-based diamond-like compounds, including Cu 2 ZnSnSe 4 (3.3 W m −1 K −1 at 300 K), [9] CuGaTe 2 (7.4 W m −1 K −1 at 300 K), [16] CuInTe 2 (5.4 W m −1 K −1 at 300 K), [19] CuFeS 2 (5.7 W m −1 K −1 at 300 K), [29] and CuInSe 2 (4.6 W m −1 K −1 at 300 K). [35] In particular, as shown in Figure 3, even though the Ag(Ga,In)Te 2 -based diamond-like compounds contain a heavy Te element, their lattice thermal conductivity at 300 K is almost twice the value of AgInSe 2 that has a lighter Se element. [11,36] In order to rationalize this abnormal situation, we performed ab initio lattice dynamics calculations for AgInSe 2 (see Figure 4a). For comparison, the results for CuInSe 2 and AgInTe 2 are also included in Figure 4a   Ag or Cu atoms. For crystalline compounds, the low sound velocity, large lattice anharmonicity, and low frequency optic phonons definitely lead to a low lattice thermal conductivity. [37] Given a phonon spectrum, the sound velocity can be easily obtained by calculating the zone center derivative of acoustic phonon dispersions. The sound velocities for AgInSe 2 , CuInSe 2 , and AgInTe 2 , including longitudinal (v l ), transverse (v t ), and average (v avg. ) sound velocities (calculated by Equation (S2), Supporting Information), as well as the Grüneisen parameters (γ) of these three compounds, are listed in Table 1. The experimental sound velocities for AgInSe 2 , CuInSe 2 , [38] and AgInTe 2 are also included for comparison. As we expected, CuInSe 2 possesses the largest sound velocities among the three compounds due to its lightest average atomic mass. The values, however, are lower than those for the CoSb 3 -based skutterudites (v l = 4590 m s −1 , v t = 2643 m s −1 ). [39] Between the two Agbased diamond-like compounds, our calculation suggests that AgInSe 2 has a slightly faster v t and v l than AgInTe 2 , which is confirmed by our experimental data shown in Table 1. However, such a small difference between sound velocities in AgInSe 2 and AgInTe 2 obviously cannot account for the much lower κ L in AgInSe 2 as compared with that in AgInTe 2 . In addition, the Grüneisen parameters obtained either from ab initio lattice dynamics calculations or from measured sound velocities for AgInSe 2 and AgInTe 2 are close to each other, and therefore cannot explain the large difference in κ L between the two compounds.
The most distinct feature of AgInSe 2 regarding its lattice dynamics is the low-frequency optic phonons, which cover the frequency range from 3.0 to 7.4 meV (see Figure 4a). These low-frequency optic phonons can introduce additional phonon scattering channels and scattering rates, or phenomenologically, the resonant scattering that impedes the normal transport of acoustic phonons with similar frequencies, leading to an extremely low heat conduction. [37] Similar phenomena have also been observed in many other compounds with low lattice thermal conductivity, such as filled skutterudites, Cu 2 Se, and α-MgAgSb. [7,40,41] As shown in Figure 4a, the low-lying optic phonons are also observed in the phonon dispersions of AgInTe 2 . However, the lowest-lying optic phonons appear already at 3 meV in AgInSe 2 but at a higher frequency of 4 meV in AgInTe 2 . Furthermore, the range of low-lying optical phonons in AgInSe 2 is wider covering 3.0-7.4 meV than that in AgInTe 2 where it covers the range of 4.0-6.3 meV. These features suggest that the low-lying optic phonons in AgInSe 2 not only scatter the acoustic phonons of lower frequencies but also scatter the acoustic phonons over a wider range of frequencies. Therefore, the wider scattering channels with lower frequencies are Adv. Sci. 2018, 5, 1700727   Figure 3. Temperature dependence of the lattice thermal conductivity κ L for AgInSe 2 -based compounds and other diamond-like compounds taken from ref. [9], [11], [16], [19], [29], [35], and [36]. the main reason for the lower κ L observed in AgInSe 2 than in AgInTe 2 .
In order to further understand the origin of the low-frequency optic phonons in AgInSe 2 , the phonon animation analysis was performed (see such animation in the Supporting Information). Interestingly, we observed collaborative motions of Ag-Se clusters (red and yellow balls), while In atoms (green balls) stand still. This strongly suggests that the interaction between Ag and Se is quite strong, while the interaction between the Ag-Se cluster and In is quite weak in AgInSe 2 . Generally, the interactions between the neighboring atoms can be semi-quantitatively evaluated by the force constant (k) derived from the calculated potential energy variation (ΔE) as a function of the phonon amplitude (x). [42] A smaller k usually indicates a weaker interaction and thus a larger space for the motion. Figure 4b shows the calculated potential energy variation (ΔE) as a function of the phonon amplitude (x) for the first optic mode in AgInSe 2 , which is mainly contributed by the Ag-Se clusters. The ΔE versus x curve can be well fitted by the second-order polynomial with ΔE = kx 2 and k = 0.191. Likewise, Figure 4b also shows the ΔE versus x curves and the corresponding k values for AgInTe 2 and CuInSe 2 . Clearly, AgInSe 2 has the lowest k value among these three compounds, proving that the Ag-Se clusters have the weakest interactions with the neighboring In atoms. Such well-bonded Ag and Se atoms and weakly connected In atoms in AgInSe 2 lead to special Ag-Se cluster motions with large mass and thus low-frequency optic phonons. Similar phenomenon has also been found in S-and Se-filled CoSb 3 skutterudites, where the strong S(Se)Sb bonds cause "cluster vibrations" with low frequencies. [43] Furthermore, since the low-frequency optic phonons relate to the vibrations of Ag-Se clusters, it is expected that thus-caused low κ L can also be found in other diamond-like compounds containing Ag and Se. For example, the room-temperature κ L of single-crystalline AgGaSe 2 is 1.2 W m −1 K −1 , [44] much lower than that of AgGaTe 2 shown in Figure 3a, indicating the generality of the findings in this work.
In addition, low thermal conductivity also relies on strong phonon scattering by grain boundaries, impurities, and various kinds of defects. Figure 5a shows a low-magnification transmission electron microscopy (TEM) image for stoichiometric AgInSe 2 , in which electron diffraction indicates the tetragonal structure. Figure 5b,c are high-resolution transmission electron microscopy (HRTEM) images for the red ellipse area marked as I and II in Figure 5a, respectively. Several dislocations or imperfections are notable in area I, which are clearly confirmed through the inversed fast Fourier transform (IFFT) image inserted in Figure 5b. Besides, as shown in Figure 5d, the IFFT image for the square area in Figure 5c depicts obvious stacking faults. All the aforementioned dislocations, imperfections, and stacking faults enhance phonon scattering, [6] which also contributes to the intrinsically ultralow κ L observed in Figure 5a.
The high TE performance in AgInSe 2 makes it a good candidate to fabricate n-type legs of TE devices. However, the electron concentration is still too low for an efficient energy conversion in the low and medium temperature ranges. Since it is difficult to enhance the carrier concentration further by introducing more Ag ions (such extra Ag ions would form impurities with no ability to enhance the electron concentration), we have attempted to enhance the concentration of electrons and increase the electrical conductivity at low and medium temperatures by substituting Cd at the Ag sites.
Powder XRD patterns for the Ag 1−x Cd x InSe 2 (x = 0.08, 0.1) compounds are shown in Figure S1 (Supporting Information). XRD patterns match well with the PDF card (No. 35-1099) for the AgInSe 2 compound. There is also a trace of a secondary Ag 2 Se phase detected in SEM-EDS of the Ag 0.9 Cd 0.1 InSe 2 compound (see Figure S4, Supporting Information). The temperature dependence of the electrical conductivity σ, the Seebeck coefficient S, the thermal conductivity κ, and the figure of merit zT for Ag 1−x Cd x InSe 2 (x = 0.08, 0.1) compounds is shown in Figure 2. The electrical conductivity σ around 300 K has increased by 3-4 orders of magnitude through doping Cd at the Adv. Sci. 2018, 5, 1700727  Ag sites. Even at high temperatures, the σ values for Cd-doped compounds are still larger by an order of magnitude than for the stoichiometric AgInSe 2 compound. Correspondingly, the Seebeck coefficient S at 300 K is decreased to a value of −387 and −253 µV K −1 in Ag 0.92 Cd 0.08 InSe 2 and Ag 0.9 Cd 0.1 InSe 2 , respectively. Finally, the PFs for the Cd-doped AgInSe 2 compounds are further increased as compared with the Ag-excess AgInSe 2 compounds (see Figure S5, Supporting Information). All these features are clearly a result of the much increased electron concentration. In Ag 0.9 Cd 0.1 InSe 2 , the n is increased to 2.2 × 10 18 cm −3 at room temperature, about five orders of magnitude higher than the value of ≈10 13 cm −3 for AgInSe 2 and two orders of magnitude higher than the value of ≈10 16 cm −3 for Ag-excess AgInSe 2 . In addition, the thermal conductivity of Cd-doped AgInSe 2 is also reduced at low and medium temperatures due to the extra point defect phonon scattering by Cd dopants. Especially, the thermal conductivity for Ag 0.9 Cd 0.1 InSe 2 has been reduced to 0.69 W m −1 K −1 at 300 K, a decrease of 46% compared to the stoichiometric AgInSe 2 compound. As a result, the zT is more optimized through the whole temperature range. The average zT between 300 and 900 K has increased to 0.46, an ≈188 and 92% enhancement as compared to the stoichiometric AgInSe 2 compound and the Ag-excess AgInSe 2 compound, respectively (see the inset in Figure 2d). Figure S6 (Supporting Information) shows the reproducibility test of the electronic transport properties of the Ag 0.9 Cd 0.1 InSe 2 sample. The data are almost reproducible during three independent runs. Figure S7 (Supporting Information) shows the stability test for the Ag 0.9 Cd 0.1 InSe 2 sample under the current density of 12 A cm −2 , which is usually used for the test of superionic TE materials. [45,46] The relative resistance (R/R 0 , where R 0 is the sample's initial resistance) for AgInSe 2 is almost unchanged after about 50 000 s (14 h) test, proving that the Ag 0.9 Cd 0.1 InSe 2 sample has a good stability under large current. The discovered high TE performance in n-type AgInSe 2based diamond-like materials makes them suitable, in conjunction with high-performance p-type structures, for fabrication of diamond-like TE modules. Here, we have chosen Ag 0.9 Cd 0.1 InSe 2 as the n-type leg and Cu 0.99 In 0.6 Ga 0.4 Te 2 [16] as the p-type leg. A two-couple diamond-like TE module was successfully fabricated for the first time. As shown in Figure 6a, the cylinder-shaped samples with a diameter of 12.7 mm and a height of 4 mm were obtained by hot-pressing (HP) sintering. The ingots were cut into 4 mm × 4 mm × 8 mm rectangles by electrospark wire-electrode cutting. The two square lateral faces of the samples were electroplated with nickel in a NiCl 2 solution with an ampere density of 10 mA cm −2 for 90 s. Subsequently, the electroplated samples were welded to Mo 50 Cu 50 alloy blocks with the Cu-P brazing filler metal (T melt ≈ 580 °C) at the hot side and the copper clad ceramic substrates with Sn 42 Bi 58 (T melt ≈ 138 °C) at the cold side. Figure 6b shows schematically the structure of our fabricated diamond-like TE module. The two π-pairs were assembled to attain a two-couple diamond-like TE module (see Figure 1b). The gap between the two couples was filled with a thermally insulation material to reduce the heat loss. PEM-2 testing system (ULVAC-RIKO, Inc.) was used to measure the performance of this module. As shown in Figure 6c, a maximum output power of 0.06 W was obtained with a temperature difference (ΔT) of 520 K between the two ends of the TE module. This is the first diamondlike TE module, clearly showing that it is feasible to fabricate efficient TE devices and perhaps even systems based on the Adv. Sci. 2018, 5, 1700727 Figure 6. a) Schematic of the fabrication process for diamond-like modules, including cutting the hot-pressed cylinder samples into rectangular samples, and the following electroplating process. b) Schematic of the structure of the fabricated diamond-like module. c) Output voltage (black) and power (blue) as a function of the current for the TE module based on the diamond-like materials. The temperature difference between the cold side and the hot side of the module is 420, 470, and 520 K, respectively.