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Intrinsically High Thermoelectric Performance in AgInSe2 n-Type Diamond-Like Compounds Pengfei Qiu, Yuting Qin, Qihao Zhang, Ruoxi Li, Jiong Yang,* Qingfeng Song, Yunshan Tang, Shengqiang Bai, Xun Shi,* and Lidong Chen* 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 = S2σT/(κL+κe). 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 S2σ, 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 κL and the electronic thermal conductivity κe. In order to achieve high TE performance, it is essential to enhance the electronic properties (S2σ) and minimize the total thermal conductivity (κ = κL +κe). 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, κe = 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] Bi2Te3,[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 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 AgInSe2based 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.

1. Introduction Nowadays, advanced technologies based on high-performance energy materials have triggered a worldwide attention Dr. P. Qiu, Dr. Y. Qin, Q. Zhang, Q. Song, Y. Tang, Dr. S. Bai, Prof. X. Shi, Prof. L. Chen State Key Laboratory of High Performance Ceramics and Superfine Microstructure Shanghai Institute of Ceramics Chinese Academy of Sciences Shanghai 200050, China E-mail: [email protected]; [email protected] Q. Zhang, Q. Song University of Chinese Academy of Sciences Beijing 100049, China R. Li, Prof. J. Yang Materials Genome Institute Shanghai University Shanghai 200444, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/advs.201700727. © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/advs.201700727

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© 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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electronic transport properties are especially interesting.[9–30] There are about 1000 types of diamond-like compounds and several of them exhibit very good TE performances. Examples include Cu2ZnSn0.90In0.10Se4 with a zT of 0.95 at 850 K,[9] Cu2Sn0.90In0.10Se3 with a zT of 1.14 at 850 K,[10] Ag0.95GaTe2 with a zT of 0.77 at 850 K,[11] Cu3Sb0.97Ge0.03Se2.8S1.2 with a zT of 0.89 at 650 K,[12] CuGaTe2 with a zT of 1.4 at 950 K,[13–17] Cu2.075Zn0.925GeSe4 with a zT of 0.45 at 670 K,[18] CuInTe2 with a zT of 1.18 at 850 K,[19,20] Cu2TM (TM = Mn, Fe, Co)SnSe4 with a zT of 0.7 at 850 K,[21] and many others.[22–25] 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: AgInSe2 with a zT of 0.34 at 724 K[26,27] and Cu0.92Zn0.08FeS2 with a zT of 0.26 at 630 K[28–30] (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 AgInSe2 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 AgInSe2 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.

2. Results and Discussion AgInSe2 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 AgInSe2 are shown in Figure S1 (Supporting Information). The XRD data match well with the PDF card (#No. 35-1099) for AgInSe2 compounds. The scanning electron microscopy (SEM) results for AgInSe2 are shown in Figure S2 (Supporting Information) and demonstrate that a small amount of Ag2Se second phase (98% of the theoretical density) were obtained. The X-ray diffraction (XRD) analysis (D8 ADVANCE, Bruker Co. Ltd.) was employed to characterize the structure and the purity of the samples. The chemical composition and the microstructure analysis were examined by scanning electron microscopy (SEM, ZEISS Supra 55). The cell parameters were estimated by the Fullprof software (Version February-2015). The HRTEM images were obtained by JEOL 2100F by using powder samples to investigate the microstructures. The electrical conductivity and the Seebeck coefficient were measured by ZEM-3 (ULVAC Co. Ltd.) apparatus under helium atmosphere from 300 to 900 K while the thermal diffusivities were measured by the laser flash method (NETZSCH LFA 427) in argon atmosphere. The Neumann–Kopp law was applied to estimate the heat capacity for all samples and the Archimedes method was used to measure the density. The thermal conductivity κ was calculated by κ = λCpρ. The experimental sound velocities vt and vl were obtained by using a ultrasonic measurement system UMS-100 based on the resonance interference method. The measurements were carried out on the pellet samples with the diameter of 10 mm and the height of 6 mm. The Hall coefficients (RH) were measured by the Quantum Design Physical properties measurement system (PPMS) from 10 to 300 K with the magnetic field sweeping up to 3 T in both positive and negative directions. The electron concentration (n) and the Hall carrier mobility (μH) were calculated by n = 1/RHe and μH =RHσ, respectively, where e is the elementary charge. The thermal expansion coefficient was measured by Netzsch DIL 402 C.

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Lattice Dynamics Calculations: First-principles calculations were performed with Vienna ab initio simulation package (VASP). The generalized gradient approximation (GGA) of the form Perdew– Burke–Ernzerhof and projected augmented wave (PAW) method[47,48] were adopted. The lattice dynamics calculations for the diamondlike compounds were carried out by the frozen phonon method, as implemented in the Phonopy package.[49] 3 × 3 × 3 unit cell (containing a total of 216 atoms in the supercell) was constructed to ensure the convergence of Hellmann–Feynman forces. Accurate convergence criteria, that is, 5 × 10−5 eV Å−1 for structural relaxation of the unit cell and 10−7 eV for static calculation of displaced supercell were used. Thermoelectric Modules: Two n-p couple module was assembled with n-type Ag0.9Cd0.1InSe2 and p-type Cu0.8Ag0.2In0.5Ga0.5Te2 compounds. Both n-type and p-type hot pressing sintered cylinder samples with a size of Φ12.7 mm × 4 mm were processed into 4 mm × 4 mm × 8 mm rectangles with electrospark wire-electrode cutting. PEM-2 testing system (ULVAC-RIKO, Inc.) was employed to evaluate the power output and internal resistance of the module. The electrodes were stable throughout the measured temperature range from 300 to 873 K.

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This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 11234012 and 51625205, the Key Research Program of Chinese Academy of Sciences (Grant No. KGZDEW-T06), and the Shanghai Government (Grant No. 15JC1400301). P.F. thanks the support from the Youth Innovation Promotion Association, CAS (Grant No. 2016232). The authors thank Prof. Ctirad Uher in the University of Michigan for helpful discussions and language polishing.

Conflict of Interest The authors declare no conflict of interest.

Keywords ab initio calculations, diamond-like materials, thermoelectric modules

compounds,

thermoelectric

Received: October 12, 2017 Published online:

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