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Page 20 of 35 Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32

TE properties, including a high Seebeck coefficient, electrical conductivity, and low thermal conductivity.
While traditional materials such as bismuth telluride (Bi Te ) and lead telluride (PbTe) have been
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extensively used, they face limitations at elevated temperatures, necessitating the exploration of alternative
materials. Recent studies have highlighted the potential of materials such as germanium telluride (GeTe)
and skutterudites, which have shown promising zT values in the medium-temperature range. However, the
performance of these materials can be adversely affected by bipolar conduction at higher temperatures,
which necessitates ongoing research into alternative compositions and doping strategies to enhance their TE
performance.

Innovative methods of doping and structural optimization further augment performance. For example,
entropy engineering, Te-capping, doping, etc., in Bi Te has significantly reduced tellurium sublimation
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losses, contributing to minimal zT degradation after extensive testing [274-276] . The exploration of materials
such as Ag Se, PbSe, GeTe and Sb Te , in conjunction with state-of-the-art skutterudites and Heusler alloys,
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indicates the move towards developing high-performance TE materials for various applications [219,227,238,277] .
Interface engineering and diffusion barriers: Another important bottleneck in enhancing device
performance and stability lies within the interfaces between TE materials and electrodes. A primary
challenge is preventing interdiffusion, which can lead to the formation of undesirable phases and
subsequent performance degradation. Each interface must be tailored to the specific TE material to ensure
optimal efficiency and reliability. Notably, interfaces within multilayered segmented TE legs necessitate
extensive characterization and optimization to leverage high-performance materials such as Bi Te , PbTe,
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and Mg Sb , among others, that have shown significant zT values [278,279] . These interfaces critically influence
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thermal and electrical contact resistance, directly affecting overall device efficiency. Therefore, strategies to
mitigate interdiffusion and minimize electrical resistance while minimizing thermal losses are essential for
improving device TE performance .
[280]
Innovative interface engineering techniques have been utilized to address interdiffusion challenges. The
application of diffusion barrier materials, such as Ni-P coatings or TiN/Mo in PbTe-based and Bi Te -based
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TE modules, has effectively prevented detrimental interdiffusion with solder materials, while also enhancing
bonding strength and resistance to intermetallic formation, ensuring greater reliability at elevated
[278]
temperatures . Further, Co-P diffusion barriers have been employed to stabilize joints in PbTe TE
materials, significantly reducing atomic interdiffusion and prolonging the device lifespan at high
operational temperatures . Additionally, titanium layers have demonstrated improved interfacial stability
[281]
[282]
against copper diffusion in Bi Te , maintaining low contact resistivity and notable mechanical strength .
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Moreover, recent advancements have explored novel barrier materials like Mg Ni, which prove compatible
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with TE phases such as Mg Sb 2 [280] . This approach minimizes interfacial stresses, thereby enhancing overall
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device performance [280,283] . Furthermore, ongoing research into metal-semiconductor interfaces continues to
present promising avenues for optimizing TE efficiency.

Soldering and bounding process: Soldering and bonding processes used to connect TE materials to
electrodes also present critical challenges. Traditional soldering materials may not provide adequate thermal
and electrical conductivity; thus, the exploration of new soldering materials is essential to improve
performance. The impact of soldering processes on the microstructure, including crystal orientation and
grain boundaries, must be thoroughly assessed to understand how these factors influence the TE properties
of assembled devices. Traditional Sn-based soldering materials often exhibit limited temperature stability. In
contrast, Ag-based bonding materials serve as a viable alternative, utilized in both micron-sized flakes and