Authors: Xinyuan Wang | Airan Li | Takao Mori
Source: cMat. (article, 2025-07-31)
Field: Materials Science — Materials Chemistry

Thermoelectric Copper Chalcogenides: Low‐Cost, High‐Performance, and Stability Challenges Thermoelectric (TE) materials, capable of interconverting heat and electricity, exhibit significant promise for applications in heat harvesting and solid-state cooling. The conversion efficiency of TE materials can be assessed using the dimensionless figure of merit, defined as zT = α2σT/(κL + κe), where T, α, σ, κL, κe are the absolute temperature, Seebeck coefficient, electrical conductivity, lattice thermal conductivity, and electronic thermal conductivity, respectively [1]. High-performance TE materials with elevated zT values are predominantly found among compounds comprising of the elements from groups IV, V, and VI. Notable examples include Bi2Te3, which is widely used near room temperature, GeTe and PbTe, which are commonly employed at mid-temperatures, and half-Heuslers compounds, which exhibit superior performance at high temperatures [2]. However, the reliance on elements with low natural abundance and high cost significantly challenges the scalability of these materials. In contrast, copper is relatively abundant in the earth's crust and offers the most favorable abundance-to-price ratio among the elements utilized in TE applications (Figure 1A), rendering Cu-based TE technology a promising and sustainable candidate for large-scale applications. Over the past decade, considerable attention has been devoted to Cu-based TE materials. In particular, copper chalcogenides (Cu2Q; Q = S, Se, and Te) have attracted special interest due to their outstanding TE performance in the high-temperature range (700–1100 K) [2-6], which significantly outperforms most other representative TE materials (Figure 1B). (A) The abundance and the ratio of abundance/price for representative metal element; (B) current state-of-art zT of Cu2Q with data taken from literature [3-6]. The exceptional TE performance of Cu2Q is primarily attributed to its ultralow thermal conductivity coupled with decent electrical transport properties. Cu2Q exists in multiple phase structures, and the liquid-like behavior of copper in cubic phase Cu2Q imparts its intrinsically low thermal conductivity. Liu et al. [7, 8] demonstrated that the liquid-like diffusion of copper ions effectively suppresses transverse phonons, reducing the phonon mean free path to nearly the minimum wavelength observed in glasses. Consequently, the specific heat capacity of Cu2Q falls below the Dulong–Petit limit, contributing to low thermal conductivity. On the other hand, Voneshen et al. [9] pointed that liquid-like diffusion alone will not be sufficient to fully suppress transverse phonons, as the hopping time scales are too slow to affect lattice vibrations. Their analysis of the neutron-weighted phonon density of states (PDOS) revealed a broadening of the phonon spectrum and an increase in multi-phonon scattering, which indicates a pronounced anharmonic behavior associated with local copper vibrations. In addition to ultralow thermal conductivity, the electrical transport behavior of Cu2Q plays a critical role in determining its overall TE performance. All Cu2Q exhibits p-type conductivity, primarily due to the low formation energy of copper vacancies. Variations in the chalcogen element from S to Te alter the Cu–Q bonding strength, thereby influencing the formation of Cu vacancies. Moreover, the band structure evolves from Cu2S to Cu2Te: the progressive decrease in electronegativity from S to Te raises the energy of the anion valence orbitals, resulting in a reduction of the bandgap, from 1.39 eV for monoclinic Cu2S to 0.86 eV for cubic Cu2Se, and eventually to a metallic-like state with no bandgap for Cu2Te [4]. Additionally, the increase in atomic radius and mass from S to Te leads to greater interatomic distances between Cu and Q, which likely reduces the energy separation between anion and cation orbitals and enhances their overlap. This effect sharpens the band edges, resulting in a lower carrier effective mass, higher mobility, and improved electrical conductivity in Cu2Te compared to Cu2S. Based on these trends, the variation from S to Te offers ample opportunity to tune the electrical properties of Cu2Q. It is worth noting that despite the substitution of S, Se, or Te, the crystalline anionic framework of Q remains intact, preserving its favorable carrier transport properties. When combined with the liquid-like behavior of Cu, Cu2Q perfectly embodies the concept of a phonon-liquid and electrical crystal (PLEC), a highly desirable feature for advanced TE materials. In the past, numerous strategies have been employed to enhance the TE performance of Cu2Q compounds, including but not limited to alloying, off-stoichiometry, doping, and compositing. As previously described, Te, Se, and S can be alloyed with one another, enabling the tuning of band structures while simultaneously enhancing phonon scattering. As a result, Cu2Q systems have demonstrated peak zT values ranging from 1.5 to 2.1 [4, 7, 8], representing a remarkable achievement during the early stages of their development. In addition to alloying, off-stoichiometry (introducing Cu deficiencies) and doping with aliovalent elements have been widely adopted to optimize the carrier concentration in these materials. More recently, compositing has emerged as a particularly effective strategy, not only for improving the TE performance of Cu2Q systems by synergistically optimizing their thermal and electrical transport properties but also for enhancing their service stability [10-12]. Ideally, advanced TE materials should exhibit high TE performance and excellent service stability, as well as low cost and high performance (Figure 2A). However, achieving this balance remains a significant challenge in the TE community. In particular, the long-term stability of Cu2Q compounds are hindered by persistent issues related to Cu+ ion migration. As proposed by Qiu et al. [11], in superionic Cu2S, Cu+ ions will migrate under external potential fields, such as temperature gradients or electric fields, alongside hole carriers. This migration results in copper deposition at interfaces when the local chemical potential of Cu approaches or exceeds that of metallic Cu (Figure 2B). Consequently, suppressing performance degradation induced by Cu+ ion diffusion is crucial to overcoming the major bottlenecks limiting the practical application of Cu2Q-based TE materials. To address this issue, interface engineering and ionic confinement strategies have been introduced by compositing with secondary phases, such as Cu2Se/BiCuSeO/graphene [5], Cu2Se/AgSbF6 [6], Cu2Se/BiCuSeO [10], Cu2S/quart [12], and Cu1.90FexS [13]. These approaches have demonstrated significant effectiveness in confining Cu+ migration and substantially improving the service stability of Cu2Q (Figure 2C). (A) Schematic representation of the triad of important aspects in copper chalcogenides; (B) schematic diagram about the service-related degradation issue in Cu2Q; (C) current density dependence of relative electrical resistance variation (R/R0) for several Cu-based mixed ionic/electronic conductors; (D) X-ray diffraction patterns of the Cu1.8S bulk before and after heat cycling under 623 K for 470 min; (E) representative device efficiency of Cu2Q-based TE modules under different temperature gradients; (F) current challenges for the Cu2Q-based TE technology. In addition to improving stability, these compositing strategies have also led to notable enhancements in TE performance, with reported zT values reaching 2.0–3.0 at 700–1000 K. For instance, in the Cu2Se/BiCuSeO/graphene system [5], different components contribute distinct functionalities: BiCuSeO primarily enhances stability, whereas graphene improves σ and introduces phonon scattering to reduce κ. This highlights the value of a multifunctional design, where each component is selected for a targeted purpose. Looking forward, the development of high-performance and stable Cu2Q-based TE materials will rely on rational composite design, integrating functional phases with complementary properties and well-understood interfacial interactions within the Cu2Q matrix. Such an approach will be key to achieving both high zT and long-term operational reliability across a broad temperature range. It should be noted that efforts to confine Cu+ migration primarily address the electrical stability of Cu2Q, improving its resistance to current-induced degradation. However, the thermal stability of Cu2Q at elevated temperatures requires further investigation, particularly considering the volatility of S, Se, and Te. For instance, Cu1.8S demonstrates markedly inferior thermal stability, even when compared to stoichiometric Cu2S. Our recent study on Cu1.8S [14] revealed noticeable sulfur evaporation behavior, with S evaporating more readily under a partial He atmosphere than under dynamic N2, resulting in variations in their composition (Figure 2D) and TE performance. Although selecting appropriate synthesis methods can help mitigate sulfur loss, addressing the thermal stability of Cu2S-based as well as its counterparts Cu2Se- and Cu2Te-based materials remains a critical and unresolved challenge. Achieving both electrical and thermal stability in Cu2Q materials is essential for ensuring overall reliability. Although compositing has proven effective in suppressing Cu+ migration and enhancing electrical stability, its role in mitigating chalcogen evaporation, and thus improving thermal stability, has been less studied. Since thermal degradation often stems from surface Q loss, incorporating stable composite materials (e.g., quartz) and applying surface coatings such as BN sprays may help reduce volatilization and enhance thermal robustness. Although promising, these strategies may introduce added fabrication complexity. Beyond these approaches, atomic-scale doping or alloying can further strengthen Q-framework bonding and inhibit Cu+ mobility. An integrated design combining doping, compositing, and coating may offer a comprehensive solution for improving overall stability of Cu2Q. Alongside these material-level improvements, high-performance and stable Cu2Q-based TE devices have also been fabricated and demonstrated (Figure 2E). It is worth noting that Qiu et al. [15] achieved a high conversion efficiency of 9.1% under a 680 K temperature difference using a Cu2Se/Yb0.3Co4Sb12 module, which maintained stability during long-term aging tests at Thot = 823 K (ΔT = 520 K) by optimizing the geometry of the legs to ensure that the voltage across Cu2Se remained below its safe operational threshold. In another example, Hu et al. [6] incorporated AgSbF6 to confine Cu+ in Cu2Se and fabricated a TE module combined with Mg3(Sb,Bi)2, achieving a remarkable efficiency of ∼13.4% for a temperature difference of 518 K without noticeable degradation after 120 thermal cycles. Despite these promising achievements, challenges remain at the device level (Figure 2F). The impact of Cu2Q's phase transitions at low temperatures on device stability is still not well understood, and the relatively poor TE performance of Cu2Q in the low-temperature regime continues to limit its full potential. Strategies such as optimizing carrier concentration and stabilizing the cubic phase at lower temperatures may help improve performance in this range. Currently, segmented TE leg combining p-type Bi2Te3 with Cu2Se seems to provide a practical solution, achieving a conversion efficiency of 9.0% at a 516 K temperature difference, even after 110 thermal cycles. Besides the limits at low temperatures sides of Cu2Q, barrier layer stability at high temperature sides is another critical issue requiring more thorough investigation. Although Mo and Ni have been used as barrier layers in Cu2Q modules, the detailed microstructural evolution and long-term property variation under operational conditions remain largely unexplored. Beyond Mo and Ni, more chemically inert and thermally robust materials may warrant investigation as potential barrier layers to improve contact stability. Moreover, surface engineering approaches, such as atomic layer deposition and protective coatings, may help suppress chalcogen loss and enhance high-temperature durability. Furthermore, as a whole, the lack of a high-performance n-type counterpart within the same Cu-based system will limit the overall cost-effectiveness and design flexibility of full Cu-based TE devices. Future research should focus on controlling native defects, identifying suitable donor dopants, and exploring band and phonon engineering to enable n-type behavior. In parallel, high-throughput computational screening and machine learning approaches may accelerate the discovery of viable n-type Cu-based thermoelectric materials. In summary, low-cost Cu2Q-based TE materials have made remarkable progress in achieving high TE performance and device efficiencies, renewing interest in their application for high-temperature energy conversion. However, service stability remains a central challenge. Beyond addressing electrical instability from Cu+ migration, thermal instability caused by chalcogen evaporation must be carefully investigated and managed to guarantee long-term material and device reliability. Additionally, enhancing the low-temperature performance of these materials, developing durable barrier layers, and discovering high-performance n-type counterparts are the essential next steps. Tackling these issues through fundamental understanding, materials design, and device engineering will be key to further unlocking the potential of Cu-based thermoelectrics and advancing the broader TE field. This work was financially supported by JST (Japan Science and Technology Agency) Mirai Program JPMJMI19A1. Xinyuan Wang gratefully acknowledges financial support from the China Scholarship Council (CSC). This work was supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” (No. JPMXP1223NM5150) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The authors declare no conflicts of interest. All data analyzed during this study are included in this published article. Copper ; Thermoelectric effect ; Materials science ; Engineering physics ; Nanotechnology ; Metallurgy ; Engineering ; Physics ; Thermodynamics Advanced Thermoelectric Materials and Devices ; Chalcogenide Semiconductor Thin Films ; Magnetic and transport properties of perovskites and related materials