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Feng Jiang, Zhengtao Wanga, Wen Zhonga, Yifan Zhoua, Zhengyang Zhou, Longzhi Wu, Jiang Chen, Yao Xua, Xiaodong Wang, Feng Cao, Qian Zhang, Jun Mao

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Towards the practical realization of high-performance Ag2Se-based thermoelectric coolersScience and Technology of Advanced MaterialsISSN: 1468-6996 (Print) 1878-5514 (Online) Journal homepage: www.tandfonline.com/journals/tsta20Towards the practical realization of high-performance Ag2Se-based thermoelectric coolersFeng Jiang, Zhengtao Wang, Wen Zhong, Yifan Zhou, Zhengyang Zhou,Longzhi Wu, Jiang Chen, Yao Xu, Xiaodong Wang, Feng Cao, Qian Zhang & JunMaoTo cite this article: Feng Jiang, Zhengtao Wang, Wen Zhong, Yifan Zhou, Zhengyang Zhou,Longzhi Wu, Jiang Chen, Yao Xu, Xiaodong Wang, Feng Cao, Qian Zhang & Jun Mao (12 Mar2026): Towards the practical realization of high-performance Ag2Se-based thermoelectriccoolers, Science and Technology of Advanced Materials, DOI: 10.1080/14686996.2026.2641882To link to this article:  https://doi.org/10.1080/14686996.2026.2641882© 2026 The Author(s). Published by NationalInstitute for Materials Science in partnershipwith Taylor & Francis Group.View supplementary material Accepted author version posted online: 12Mar 2026.Submit your article to this journal View related articles View Crossmark dataFull Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=tsta20https://www.tandfonline.com/journals/tsta20?src=pdfhttps://www.tandfonline.com/action/showCitFormats?doi=10.1080/14686996.2026.2641882https://doi.org/10.1080/14686996.2026.2641882https://www.tandfonline.com/doi/suppl/10.1080/14686996.2026.2641882https://www.tandfonline.com/doi/suppl/10.1080/14686996.2026.2641882https://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2026.2641882?src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2026.2641882?src=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2026.2641882&domain=pdf&date_stamp=12%20Mar%202026http://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2026.2641882&domain=pdf&date_stamp=12%20Mar%202026https://www.tandfonline.com/action/journalInformation?journalCode=tsta20 1 Publisher: Taylor & Francis & The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group. Journal: Science and Technology of Advanced Materials DOI: 10.1080/14686996.2026.2641882 Towards the practical realization of high-performance Ag2Se-based thermoelectric coolers Feng Jianga†, Zhengtao Wanga†, Wen Zhonga, Yifan Zhoua, Zhengyang Zhoua, Longzhi Wua, Jiang Chena, Yao Xua, Xiaodong Wanga,d, Feng Caoc, Qian Zhanga, b, e*, and Jun Maoa, b, e* a School of Materials Science and Engineering, and Institute of Materials Genome & Big Data, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. b State Key Laboratory of Precision Welding & Joining of Materials and Structures, Harbin Institute of Technology, Harbin 150001, P.R. China. c School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. d Institute of Special Environments Physical Sciences, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. e School of Materials Science and Engineering, Shenzhen Key Laboratory of New Materials Technology, Shenzhen 518055, P.R. China.  †These authors contributed equally to this work. *Corresponding authors, email: zhangqf@hit.edu.cn; maojun@hit.edu.cn;   mailto:maojun@hit.edu.cnhttps://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2026.2641882&domain=pdf 2 Ag2Se-based materials with promising room-temperature thermoelectric performance and excellent mechanical properties have been known for decades. However, the fabrication of the Ag2Se-based devices toward the practical application of electronic cooling has seldom been reported. Herein, the synthesis of Ag2Se material with a diameter of 25.4 mm was achieved. The homogeneous elemental distribution and similar thermoelectric properties demonstrate the high uniformity of the Ag2Se sample. In addition, ~90 Ag/Ag2Se/Ag legs can be obtained from a single Ag2Se plate, and the interfacial contact resistivity of the Ag/Ag2Se varies from 0.8 to 3.1 μΩ cm2. Four thermoelectric devices based on n-type Ag2Se and p-type (Bi, Sb)2Te3 have been fabricated, and they achieve a cooling temperature difference of ~56 K at the hot-side temperature of 300 K, demonstrating the great potential of Ag2Se material for cooling applications. Keywords: Thermoelectric cooling; Ag2Se; Compositional homogeneity     3 1. Introduction Thermoelectric materials enabling the direct conversion between electricity and heat are crucial for heat harvesting and electronic refrigeration [1-3]. The performance of a thermoelectric device is strongly dependent on the properties of the materials, which is expressed by the dimensionless figure of merit, zT = S2T/ρκ, where S, ρ, T, and κ are the Seebeck coefficient, electrical resistivity, absolute temperature, and thermal conductivity, respectively [4-7]. Pursuing high zT values is essential for achieving a device with excellent performance. For solid-state cooling applications, materials with high room-temperature zT values will be favored [8-11]. For example, Bi2Te3-based alloys have long been employed for commercial thermoelectric cooling applications due to their excellent performance near room temperatures [12-14]. However, the low natural abundance of the Te element and the poor mechanical properties of Bi2Te3-based materials have motivated the development of novel materials with high thermoelectric performance and superior mechanical properties. Recently, Mg3Sb2-xBix-based materials have been developed and exhibit superior performance near room temperature [15-21] and excellent mechanical properties [22-26]. Moreover, thermoelectric devices with excellent cooling performance have been reported [27-33]. For example, a two-stage Mg3Bi2-based device was developed, and achieved a cooling temperature difference of 106.7 K at the hot-side temperature of 350 K [34]. In addition, a miniaturized Mg3Bi2-based cooler was also fabricated, a high cooling power density of 5.7 W cm-2 and a cooling speed of 65 K s-1 were achieved [35]. As another potential candidate, n-type Ag2Se has long been known as a high-performance thermoelectric material with a zT value of ~0.9 at room temperature [36]. Recently, Ag2Se-based thermoelectric devices have been successfully fabricated and have achieved high cooling performance [37-39]. To enable the practical application of Ag2Se in electronic cooling, it is critical to ensure the homogeneity of Ag2Se-based materials for the practical fabrication of Ag2Se-based thermoelectric coolers. However, such studies have seldom been reported. Herein, the fabrication of the Ag2Se-based thermoelectric coolers has been realized. Ag2Se materials were prepared through ball-milling and sintering. In addition, a total of ~90 Ag/Ag2Se/Ag legs can be obtained from one Ag/Ag2Se/Ag   4 plate with a diameter of 25.4 mm. The as-prepared samples not only exhibit good homogeneity but also present promising thermoelectric performance. In addition, four thermoelectric cooler devices based on the n-type Ag2Se and p-type (Bi, Sb)2Te3 have been fabricated, and they show a maximum temperature difference of ~56 K and a cooling power density of 1.56 W cm-2 at the hot-side temperature of 300 K. 2. Experimental method 2.1. Sample synthesis Ag2Se samples were prepared by the mechanical alloying method. Silver (Ag rods, with a diameter of ~1.9 mm and a length of ~5.1 mm, ZNXC, 99.995%) and selenium (Se shots, with a diameter of ~ 2.1 mm, ZNXC, 99.999%) were weighed in total 50 g according to the nominal composition of Ag2Se1.01 and loaded into a stainless jar with two stainless balls (with a diameter of ~12.6 mm and a mass of 15.9 g) in a high vacuum glove box (with both oxygen and water level below 1 ppm). The mixed raw materials were ball-milled in a high-energy ball milling machine (SPEX 8000M) for 8h (Once every 4 hours, with a 15-minute interval). The powders were then loaded in the graphite dies with a diameter of 10 mm, 12.7 mm, 20.0 mm, and 25.4 mm for densification at 523 K for 5 min using a spark plasma sintering under a pressure of 40 MPa (with a vacuum level of ~40 ppm). 2.2. Composition and microstructure characterization The phase composition of the Ag2Se samples was examined by X-ray diffraction (XRD) with Cu K radiation (Rigaku D/Max-2500 PC). The microstructure and surface morphology of the Ag2Se sample, as well as the interfacial morphology of the Ag2Se legs, were investigated by the scanning electron microscope (SEM, crossbeam 350) equipped with energy dispersive X-ray spectroscopy (EDS). The chemical composition of the Ag2Se sample with a diameter of 25.4 mm was evaluated by the electron probe microanalysis (EPMA, JOEL, JXA-8100) with a voltage of 20.0 kV. 2.3. Thermoelectric properties characterization The Ag2Se samples with different diameters were cut into bar-shaped samples with dimensions of about 2 mm × 2 mm × 10 mm for simultaneously measuring the electrical resistivity and Seebeck coefficient from 300 to 380 K on a commercial   5 apparatus (CTA-3, Cryoall) under a helium atmosphere. The Ag2Se samples were cut into disks with dimensions of about 6 mm × 6 mm × 1 mm for the measurement of thermal diffusivity (LFA 457, Netzsch) from 300 to 380 K. The specific heat capacity of Ag2Se was measured by a differential scanning calorimeter (DSC 404F3, Netzsch) with an uncertainty of 3%, and the results are shown in Figure S1 (Supplementary Information). The density of the sample was measured by the Archimedean method. Eventually, the thermal conductivity is calculated by the product of thermal diffusivity, specific heat capacity, and sample density. The temperature-dependent Hall coefficients (RH) were measured using the van der Pauw method and a four-probe configuration under a reversible magnetic field of 1.5 T and an electrical current of 150 mA from 300 to 380 K. The Hall carrier concentration (nH) and Hall mobility (H) were calculated based on the following relationships nH = 1/(eRH) and H = RH/, respectively. 2.4. Thermoelectric cooler fabrication and performance characterization Silver was used as the contact layer for Ag2Se-based thermoelectric coolers. The Ag2Se plate was loaded into a graphite die with a diameter of 25.4 mm, with 1.0 g Ag powder placed on each side within the glove box. The Ag/Ag2Se/Ag joint was prepared by using the spark plasma sintering at 523 K under a pressure of 40 MPa for 5 min. Importantly, an additional pressure of 10 MPa was maintained during the cooling process to ensure good contact property [38]. The commercial p-type (Bi, Sb)2Te3 (RusTec LLC) was electroplated with Ni and applied as the p-type thermoelectric material. The thermoelectric cooler, consisting of 7 pairs of n-type Ag2Se and p-type (Bi, Sb)2Te3 legs, was assembled with the ceramic substrate that has been precoated with Cu and Ni layers, and with SnBi solder (Sn42Bi58, melting point 411 K). Then the assembly was subjected to reflow soldering at 458 K for 50 s with an external load. The contact electrical resistivity of the joints was measured using a homemade apparatus with alternating electrical currents [40]. The thermoelectric cooling performance of the Ag2Se-based device was measured by a homemade apparatus in a vacuum [41]. The cooling power (Qc) was evaluated by a reference sample with known thermal conductivity by using four thermocouples based on the one-dimensional Fourier heat conduction law.   6  cdTQ Adz κ  (1) where κ and A are the thermal conductivity and the cross-sectional area of the reference sample, respectively. The cooling coefficient of performance (COP) is defined as the ratio of the Qc and the input electrical power (P).  cCOP Q P (2) The temperature differences (ΔT) across the thermoelectric cooler under various currents were obtained under a steady condition that the hot-side temperature varies within 0.15 K in 60 s by subtracting the cold-side temperature (Tc) from the hot-side temperature (Th), both measured with attached thermocouples [41]. The operational stability of the Ag2Se-based thermoelectric coolers is evaluated by cycling tests with alternating currents between 1 A and 6 A. At each current, the test duration was set to 40 s, and the cooling temperature differences at 1 A and 6 A were recorded. 2.5. Simulation of the cooling performance of the Ag2Se-based cooler The cooling performance of the Ag2Se-based thermoelectric cooler was simulated by finite element analysis software (COMSOL Multiphysics) based on the thermoelectric properties of the n-type Ag2Se and the p-type (Bi, Sb)2Te3. The simulation parameters are presented in Table S1, Supplementary Information. The thickness of the contact layer for Ag2Se and (Bi, Sb)2Te3 is set as 200 μm and 10 μm, respectively. 3. Results and Discussion In order to evaluate the homogeneity of the chemical composition of Ag2Se-based materials, different sizes of Ag2Se were prepared, and the optical images are shown in Figure 1(a). All samples possess a distinct metallic luster and a high densification with a relative density of ~98% (Table S2, Supplementary Information). The X-ray diffraction patterns presented in Figure 1(b) indicated that the prepared Ag2Se materials with different sizes show a single phase with an orthorhombic structure, which is indexed as PDF#24-1041. The full width at half maximum (FWHM) of the Ag2Se sample with a diameter of 25.4 mm (φ25.4) is 0.2°, further demonstrating good crystallinity. To further evaluate the uniformity of the φ25.4 sample, elemental distribution and chemical composition are characterized, and the results are shown in Figure 1(c) and Figure 1(d). The elements are distributed homogeneously without   7 obvious elemental segregations in this sample, as shown in Figure 1(c) and Figure S2 (Supplementary Information). The electron probe microanalysis results presented in Figure 1(d) show a highly uniform distribution of chemical composition in the different regions. The measured composition of the sample is Ag2Se0.93, which is close to the reported values [42]. To evaluate the homogeneity of thermoelectric properties, five bar-shaped and five disk-shaped specimens were prepared for measuring electrical properties and thermal conductivity, respectively. As shown in Figure 2(a), the electrical resistivities of Ag2Se show a monotonic decrease from ~9.3 μΩ m at 300 K to ~7.5 μΩ m at 380 K. As a narrow bandgap semiconductor [43-45], the bipolar effect induces excitation of minor carriers with the elevation of temperature, leading to an increase in carrier concentration (Figure S3, Supplementary Information) [46, 47]. Simultaneously, the Seebeck coefficient exhibited a similar trend with the electrical resistivity as shown in Figure 2(b). The negative Seebeck coefficients of Ag2Se indicate n-type semiconductor behavior, which is likely attributed to the intrinsic Se vacancy [48, 49]. The Seebeck coefficient of Ag2Se decreases with the increase of temperature from -153 μV K-1 at 300 K to -145 μV K-1 at 380 K, further supporting the onset of bipolar effect. As shown in Figure 2(c), the power factor (PF = S2/ρ) of Ag2Se shows a slight increase from ~25 μW cm-1 K-2 at 300 K to ~28 μW cm-1 K-2 at 380 K. Figure 2(d) shows the temperature-dependent thermal conductivity of Ag2Se. The total thermal conductivity shows a continuous increase from ~0.95 W m-1 K-1 at 300 K to ~1.15 W m-1 K-1 at 380 K. The total thermal conductivity is composed of the lattice thermal conductivity (κL), the electronic thermal conductivity (κe), and the bipolar thermal conductivity (κbip). Due to the bipolar conduction, the enhanced total thermal conductivity should be ascribed to the increased electronic thermal conductivity [50, 51] (κe = LσT, where L is the Lorenz number, calculated by the empirical formula, L = 1.5 + exp(|S|/115) [52]), and the additional contribution from the bipolar thermal conductivity, as shown in Figure 2(e) and Figure S4, Supplementary Information. Finally, the zT values of Ag2Se-based materials are around 0.8 at 300 K, and the average zT (zTavg) is ~0.85 at the temperature range from 300 to 380 K, as shown in Figure 2(f). The overall thermoelectric properties of five Ag2Se samples are comparable (Figure S5, Supplementary Information). These results demonstrate the homogeneous chemical composition and promising thermoelectric performance of the prepared Ag2Se sample.   8 To fabricate the Ag2Se-based thermoelectric coolers, the preparation of the Ag/Ag2Se/Ag joint was realized by spark plasma sintering, and a residual pressure was maintained during the cooling process to minimize the detrimental effect of phase transition [38]. As depicted in the inset of Figure 3(a), ~90 Ag2Se-based legs with dimensions of 1.8 mm × 1.8 mm × 2.6 mm were obtained through cutting a single φ25.4 mm Ag/Ag2Se/Ag plate [53]. To evaluate the electrical contact resistivity at the Ag/Ag2Se/Ag interface, five Ag2Se legs were characterized, as shown in Figure 3(a). The contact resistivity of the Ag/Ag2Se interface varies from 0.8 to 3.1 μΩ cm2, indicating negligible electrical parasitic loss. To be noted that the contact resistivity is lower than that of Ni/Ag2Se (ρc of ~12 μΩ cm2) [37], and comparable with our previous result (ρc of ~2.9 μΩ cm2) [38]. The Ag2Se-based legs were further bonded with the Cu electrode (deposited with a thin Ni layer) by using the SnBi solder at 458 K. Scanning electron microscopy and energy dispersive spectroscopy were applied to characterize the interface of the soldered joint. Figure 3(b) shows clear and distinct boundaries between the successive layers: Cu electrode/SnBi solder, SnBi solder/Ag contact layer, and Ag contact layer/Ag2Se. The contact resistivity of the soldered Ag/Ag2Se joint is 1.5 μΩ cm2 (Figure S6, Supplementary Information), indicating a good contact property. In addition, the corresponding energy dispersive spectroscopy mapping results (Figure 3(c-h)) reveal the homogeneous elemental distributions without obvious diffusions. Furthermore, the linear EDS scanning across the Ag/Ag2Se joint illustrates the distinct interfaces. (Figure S7, Supplementary Information). In addition, there is a narrow diffusion layer between Ag/SnBi boundary, which is beneficial to ensure a good bonding strength (Figure S8, Supplementary Information). The above results confirm that Ag is a good contact layer for Ag2Se and the feasibility for the practical fabrication of Ag2Se-based thermoelectric coolers. To further validate the thermoelectric performance of the Ag2Se sample, a cooling device based on 7 pairs of n-type Ag2Se and p-type (Bi, Sb)2Te3 was fabricated. The cooling performance of the Ag2Se-based device was characterized on a homemade apparatus at the hot-side temperature of 300 K. As shown in Figure 4(a), there is a good linear relationship between the cooling power and cooling temperature difference of the Ag2Se-based device. When the applied electrical current was 7 A, the device achieved a maximum cooling power of ~2.46 W, corresponding to a cooling power density of 1.46 W cm-2. In addition, at the optimal electrical current, the maximum cooling temperature difference is 55.4 K, which is comparable to the   9 previously reported Ag2Se-based coolers [37-39], and lower than Bi2Te3-based [14, 54] and Mg3(Sb, Bi)2-based devices [30, 32, 34] (Table S3, Supplementary Information). The relationship between cooling power and electrical current under different cooling temperature differences is presented in Figure 4(b). Correspondingly, the steady-state cooling performance of the Ag2Se-based device is simulated, and the impact of thermal radiation is considered (Figure S9, Supplementary Information). The cooling power experiences an increment with the increase in electrical current from 1 to 7 A. In addition, the Ag2Se-based device also achieved a maximum cooling power of 2.76 W and a maximum cooling temperature difference of 61.2 K at the hot-side temperature of 325 K (Figure S10-11, Supplementary Information). The cooling coefficient of performance of the Ag2Se-based device exhibits a nearly linear relationship with the cooling temperature difference, as shown in Figure 4(c). The inset image shows the side view of the Ag2Se-based cooler. The coefficient of performance decreases with the increase in cooling temperature difference. Recently, a criterion for evaluating the measurement uncertainty has been proposed: the coefficient of performance should be 0.5 at the optimum electrical current, under the condition of maximum cooling power and zero cooling temperature difference [55]. Herein, the Ag2Se-based device achieved a coefficient of performance of 0.504 at the zero temperature difference and the optimal electrical current of 7 A (Figure S12, Supplementary Information), further supporting the reliability of the thermoelectric cooling performance measurement. To evaluate the reproducibility of their cooling performance, four Ag2Se-based thermoelectric coolers were fabricated and characterized. The cooling temperature differences as a function of electrical current at the hot-side temperatures of 300 K are presented in Figure 4(d). The inset of Figure 4(d) shows the optical image of four Ag2Se-based devices. It is clear that the cooling temperature difference increases with the increase in electrical current, and reaches a maximum value of ~56 K at the optimal electrical current for four devices. In addition, the operational stability of the Ag2Se-based device is also investigated with cycling tests between the electrical currents of 1 A and 6 A at the hot-side temperature of 300 K, and the results are shown in Figure 4(e). It can be noted that the cooling temperature difference at two electrical currents experiences a fluctuation within 2% after more than 2000 cycles. Furthermore, the interface of the Ag2Se-based thermoelectric cooler after the cycling is also characterized, as shown in Figure S13 (Supplementary Information). It can be   10 noted that there is a clear boundary between the various interfaces without obvious elemental diffusions, demonstrating a superior stability of the Ag2Se-based thermoelectric cooler. 4. Conclusion Herein, the fabrication of the Ag2Se-based thermoelectric coolers has been realized. Good compositional homogeneity and excellent thermoelectric performance have been achieved in the φ25.4 mm Ag2Se sample. Using the spark plasma sintering, the Ag/Ag2Se/Ag joint is prepared, and a total amount of ~90 Ag/Ag2Se/Ag legs can be obtained with a low contact resistivity ranging from 0.8 to 3.1 μΩ cm2. Four thermoelectric devices based on the n-type Ag2Se and p-type (Bi, Sb)2Te3 have been fabricated, and they show a maximum cooling temperature difference of ~56 K at the hot-side temperature of 300 K. In addition, it also shows excellent operational stability after more than 2000 cycles of electrical currents between 1 A and 6 A. Our results demonstrate that Ag2Se-based materials and devices hold great promise for practical applications of electronic refrigeration. Acknowledgments The authors acknowledge the support from Shang Gao for SEM characterization. Disclosure statement No potential conflict of interest was reported by the authors. Conflict of Interest The authors declare no conflict of interest. Fundings This work was supported by the National Natural Science Foundation of China for Distinguished Young Scholars (52425108), the Shenzhen Science and Technology Program (KQTD20200820113045081 and SYSPG20241211173609003), and the GuangDong Basic and Applied Basic Research Foundation (2024B1515040022). J.M. acknowledges the financial support from the National Natural Science Foundation of China (52473298) and the Shenzhen Science and Technology Program (RCJC20221008092725020). Q.Z. acknowledges the financial support from the   11 National Natural Science Foundation of China (52172194), and the Shenzhen Science and Technology Program (RCJC20210609103733073, JCYJ20241202123659001). F.C. acknowledges the financial support from the National Natural Science Foundation of China (52472196). F.J. acknowledges the financial support from the National Natural Science Foundation of China (12504050), the Shenzhen Excellent Science and Technology Innovation Talent Training Program (RCBS20231211090701009). ORCID Feng Jiang, https://orcid.org/0000-0002-1220-7194  Xiaodong Wang, https://orcid.org/0009-0006-4718-6705  Feng Cao, https://orcid.org/0000-0003-3140-4502  Qian Zhang, https://orcid.org/0000-0001-5975-9781  Jun Mao, https://orcid.org/0000-0001-5275-8954  Author Contributions J.M., Q.Z., and F.J. conceived the idea, F.J. prepared the Ag2Se sample and performed the thermoelectric properties measurement, F.J., J.C., and Y.X. performed the characterizations, Y.F.Z., and Z.Y.Z. performed the Hall measurement, W.Z., F.J., and Z.T.W. fabricated the Ag2Se-based device, and F.J., L.Z.W., and Z.T.W. performed the thermoelectric cooling performance measurement, F.J., J.M., X.D.W., and F.C. analyzed the data, J.M., F.J., and Q.Z. wrote and edited the manuscript, and everyone reviewed and revised it. 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The inset shows the prepared Ag/Ag2Se/Ag legs. (b) The interface of the soldered Ag2Se leg joint and (c-i) their corresponding EDS mapping for the interface.     20  Figure 4. Thermoelectric cooling performance of Ag2Se-based device at the hot-side temperature of 300 K. Cooling power as a function of (a) cooling temperature difference at varying electrical currents and (b) electrical current at varying cooling temperature differences. (c) Coefficient of performance as a function of cooling temperature difference. (d) The relationship between cooling temperature difference and electrical currents for four Ag2Se-based coolers. (e) Cycling of the Ag2Se-based thermoelectric cooler for more than 2000 times between 1 A and 6 A. The insets present the cycling test data.     22 Supplementary information Towards the practical realization of high-performance Ag2Se-based thermoelectric coolers Feng Jianga†, Zhengtao Wanga†, Wen Zhonga, Yifan Zhoua, Zhengyang Zhoua, Longzhi Wua, Jiang Chena, Yao Xua, Xiaodong Wanga,d, Feng Caoc, Qian Zhanga, b, e*, and Jun Maoa, b, e* a School of Materials Science and Engineering, and Institute of Materials Genome & Big Data, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. b State Key Laboratory of Precision Welding & Joining of Materials and Structures, Harbin Institute of Technology, Harbin 150001, P.R. China. c School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. d Institute of Special Environments Physical Sciences, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, P.R. China. e School of Materials Science and Engineering, Shenzhen Key Laboratory of New Materials Technology, Shenzhen 518055, P.R. China.  †These authors contributed equally to this work. *Corresponding authors, email: zhangqf@hit.edu.cn; maojun@hit.edu.cn;  mailto:maojun@hit.edu.cn 23 Contents 1. Setting parameters for the finite element simulation of Ag2Se-based cooler 2. Sample density of Ag2Se materials with different sizes 3. Specific heat capacity of the Ag2Se sample 4. Energy dispersive spectroscopy mapping of the Ag2Se sample 5. Hall carrier concentration and mobility of the Ag2Se sample 6. The lattice thermal conductivity of the Ag2Se sample 7. Thermoelectric properties of five Ag2Se samples 8. Contact resistivity of the soldered Ag/Ag2Se joint 9. Energy dispersive spectroscopy and linear scanning of the Ag/Ag2Se interface 10. Linear energy dispersive spectroscopy scanning of the Ag/SnBi interface 11. Comparison of thermoelectric cooling performance for Ag2Se, Bi2Te3, and Mg3(Sb, Bi)2-based devices at the hot-side temperature of 300 K 12. The simulated cooling performance of the 7 pairs Ag2Se/(Bi, Sb)2Te3 thermoelectric cooler 13. Thermoelectric cooling performance of the Ag2Se-based cooler at the hot-side temperature of 325 K 14. Coefficient of performance as a function of the electrical current of the Ag2Se-based thermoelectric cooler 15. Interfacial properties of the Ag2Se-based thermoelectric cooler after cycling     24 1. Setting parameters for the finite element simulation of Ag2Se-based cooler Table S1. Finite element simulation setting parameters. Parameter Value Cross-sectional area of the Ag2Se legs  1.8 × 1.8 mm2 Height of the Ag2Se legs 2.5 mm Cross-sectional area of the (Bi, Sb)2Te3 legs  2.0 × 2.0 mm2 Height of the (Bi, Sb)2Te3 legs 2.5 mm Thickness of copper electrode 0.065 mm Thickness of Al2O3 substrate 0.38 mm The area of the Al2O3 substrate of the cold side 13.0 × 13.0 mm2 The area of the Al2O3 substrate of the hot side 13.0 × 16.0 mm2 Thermal conductivity of the substrate 20 W m-1 K-1 Thermal conductivity of the thermal grease 4.8 W m-1 K-1 Heat load 0.0-2.5 W Hot-side temperature 300 K Contact resistivity of Ag2Se legs 5.0 μ cm2 Contact resistivity of (Bi, Sb)2Te3 legs 5.0 μ cm2     25 2. Sample density of Ag2Se materials with different sizes Table S2. Sample density of Ag2Se materials with different sizes Sample diameter (mm) Measured density (g cm-3) Relative density  (%) 10.0 8.05 98.17 12.7 8.07 98.41 20.0 8.03 97.93 25.4 8.06 98.29     26 3. Specific heat capacity of the Ag2Se sample  Figure S1. Specific heat capacity of the Ag2Se sample.     27 4. Energy dispersive spectroscopy mapping of the Ag2Se sample  Figure S2. (a) Surface morphology, and (b-d) energy dispersive spectroscopy mapping of the Ag2Se sample.     28 5. Hall carrier concentration and mobility of the Ag2Se sample  Figure S3. Hall carrier concentration and mobility of the Ag2Se sample. (a) Carrier concentration. (b) Carrier mobility.     29 6. The lattice thermal conductivity of the Ag2Se sample  Figure S4. The lattice thermal conductivity of the Ag2Se sample.     30 7. Thermoelectric properties of the Ag2Se samples  Figure S5. Thermoelectric properties of the Ag2Se samples. Temperature-dependent (a) electrical resistivity, (b) Seebeck coefficient, (c) thermal conductivity, and (d) zT values.     31 8. Contact resistivity of the soldered Ag/Ag2Se joint  Figure S6. Contact resistivity of the soldered Ag/Ag2Se joint.     32 9. Energy dispersive spectroscopy and linear scanning of the Ag/Ag2Se interface  Figure S7. (a) The Ag/Ag2Se interface and (b-d) corresponding EDS mapping for the interface. (e) Linear EDS scanning across the Ag/Ag2Se interface.     33 10. Linear energy dispersive spectroscopy scanning of the Ag/SnBi interface  Figure S8. Linear EDS scanning across the Ag/SnBi solder interface.     34 11. Comparison of thermoelectric cooling performance for Ag2Se, Bi2Te3, and Mg3(Sb, Bi)2-based devices at the hot-side temperature of 300 K Table S3. Comparison of thermoelectric cooling performance for Ag2Se, Bi2Te3, and Mg3(Sb, Bi)2-based devices at the hot-side temperature of 300 K n-type leg p-type leg Cooling temperature difference (K) Cooling power density (W cm-2) Reference Ag2Se Bi2Te3 alloys 55.4 1.5 This work Ag2Se Bi2Te3 alloys 57.7 1.5 Jiang et al.[1] Ag2Se Bi2Te3 alloys 56.0 1.4 Liu et al.[2] Ag2Se MgAgSb 52.0 0.8 Zhao et al.[3] Bi2Te3 Bi2Te3 alloys 70.1 1.6 Sun et al.[4] Bi2Te3 Bi2Te3 alloys 73.9 2.2 Zhao et al.[5] Mg3(Sb, Bi)2 Bi2Te3 alloys 69.0 1.3 Ma et al.[6] Mg3(Sb, Bi)2 MgAgSb 52.0 0.8 Xie et al.[7] Mg3(Sb, Bi)2 MgAgSb 61.0 - Zhang et al.[8]      35 12. The simulated cooling performance of the 7 pairs Ag2Se/(Bi, Sb)2Te3 thermoelectric cooler  Figure S9. The simulated cooling performance of the 7 pairs Ag2Se/(Bi, Sb)2Te3 thermoelectric cooler. Cooling power as a function of (a) the temperature difference at different electrical currents and (b) electrical current at different temperature differences. (c) The coefficient of performance as a function of the temperature difference at different electrical currents. (d) The maximum cooling temperature difference at different electrical currents.     36 13. Thermoelectric cooling performance of the Ag2Se-based cooler at the hot-side temperature of 325 K  Figure S10. Cooling power as a function of (a) cooling temperature difference at different electrical currents, and (b) electrical current at different cooling temperature differences.  Figure S11. The relationship between cooling temperature difference and electrical current at the hot-side temperature of 300 and 325 K.     37 14. Coefficient of performance as a function of the electrical current of the Ag2Se-based thermoelectric cooler  Figure S12. Coefficient of performance as a function of the electrical current of the Ag2Se-based thermoelectric cooler at the hot-side temperature of 300 K.     38 15. Interfacial properties of the Ag2Se-based thermoelectric cooler after cycling  Figure S13. (a) Interfacial morphology, and (b-h) energy dispersive spectroscopy mapping of the interface of the Ag2Se-based thermoelectric cooler.     39 References: [1] Jiang F, Lin CH, Cheng JX, et al. Prefer-oriented Ag2Se crystal for high-performance thermoelectric cooling. Adv Funct Mater. (2024);35(6):2415000. doi: 10.1002/adfm.202415000 [2] Liu M, Zhang XY, Zhang SX, et al. Ag2Se as a tougher alternative to n-type Bi2Te3 thermoelectrics. Nat Commun. (2024);15(1):6580. doi: 10.1038/s41467-024-50898-6 [3] Zhao SY, Shi XL, Zhou Q, et al. Substitution energy-guided screening of diffusion barrier materials for Ag2Se-based thermoelectric coolers. Nano Res. (2025);18(10):94907903. doi: 10.26599/NR.2025.94907903 [4] Sun YX, Wu H, Dong XY, et al. High performance BiSbTe alloy for superior thermoelectric cooling. Adv Funct Mater. (2023);33(28):2301423. doi: 10.1002/adfm.202301423 [5] Zhang Y, Xu G, Nozariasbmarz A, et al. Thermoelectric cooling performance enhancement in BiSeTe alloy by microstructure modulation via hot extrusion. Small Sci. (2023);4(2):2300245. doi: 10.1002/smsc.202300245 [6] Ma XJ, Lin CH, Yang HY, et al. Elevating thermoelectric performance in the sub-ambient temperature range for electronic refrigeration. Innovation. (2025);6(5):100864. doi: 10.1016/j.xinn.2025.100864 [7] Xie LJ, Yang JW, Liu ZY, et al. Highly efficient thermoelectric cooling performance of ultrafine-grained and nanoporous materials. Mater Today. (2023);65(4):5-13. doi: 10.1016/j.mattod.2023.03.021 [8] Zhang XF, Zhu HT, Dong XJ, et al. High-performance MgAgSb/Mg3(Sb,Bi)2-based thermoelectrics with η = 12% at T ≤ 583K. Joule. (2024);8(12):3324-3335. doi: 10.1016/j.joule.2024.08.013    Graphical Abstract    AMO_TSTA_A_2641882 Graphical Abstract