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Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32 Page 17 of 35

Chalcopyrites [199,200] , high-entropy alloys [201,202] , transition metal dichalcogenides [203-206] , Tetrahedrites [207,208] ,
[214]
composites [209-211] , and organic materials [212,213] , among others . This diverse array of materials encompasses
a wide range of structures and properties, aiming to enhance efficiency and performance across different
temperature ranges [1,215] .

TRANSITIONING HIGH ZT THERMOELECTRIC MATERIALS TO DEVICES
Near-room temperature applications
Bi Te and its alloys have been the benchmark for near-room-temperature TE applications for energy
3
2
harvesting and cooling [216,217] , achieving a zT of ~ 1.4 for Bi Te with various modifications around 300
3
2
K [19,20] . However, brittleness, instability under high-temperature gradients, high material costs, and concerns
about toxicity, scarcity of the components, and environmental impact, have driven ongoing research into
alternative materials. In this sense, Ag Se has emerged as a more cost-effective and environmentally friendly
2
candidate, achieving near room temperature zT values ~1.2 [218-223] . However, the challenge remains to
enhance the stability and performance of these materials under operational conditions, particularly
regarding thermal cycling, when operating at temperatures above the phase transition, and mechanical
stress.

Other TE materials have also been tested for near-room temperature applications. PbTe [224-226] ,
[232]
[231]
Sb Te 3 [178,227,228] , Mg (Sb/Bi) 2 [229,230] , chalcogenide PbSe , and the half-Heusler AgCuTe have shown
2
3
promising potential for achieving high TE performance (zT) near room temperature.
Medium-temperature applications
For medium-temperature applications, materials such as Skutterudites and Heusler alloys have
demonstrated significant potential, with zT values reaching ~ 2. Skutterudites, particularly those based on
CoSb 3 [233,234] , have been extensively studied for their ability to achieve high zT values due to their complex
crystal structures, which facilitate low thermal conductivity while maintaining good electrical conductivity.
Similarly, both half and full-Heusler alloys have shown considerable potential in this temperature range,
with several compositions yielding zT of ~ 1.5 [58,167] . Another well-known material in this temperature range
is Mg Si, which boasts the advantages of being low-cost and non-toxic, showing a zT of approximately 1-1.5
2
in several studies [235-237] .

However, a notable challenge faced by these materials is bipolar conduction, which adversely affects their
performance at elevated temperatures. Unlike narrow-bandgap semiconductors or semimetals that excel at
lower temperatures, the presence of both holes and electrons in these materials leads to a reduction in the
Seebeck coefficient and an increase in thermal conductivity. This interplay ultimately results in a
deterioration of the overall zT.

In addition to Skutterudites and Heusler alloys, other materials such as GeTe , Cu Se and PbTe have
[240]
[238]
[239]
2
also demonstrated zT ~ 2 in the medium-temperature range. Among these, SnSe [18,241-244] has garnered
significant attention due to its exceptional TE performance, achieving zT values exceeding 3.1 for
[12]
polycrystalline SnSe , underscoring its potential as a simple and cost-effective binary compound with
increasing high-performance capabilities. This remarkable performance is attributed to its ultra-low thermal
conductivity.
Furthermore, the semiconductor nature of SnSe allows for tunability in both p-type [245,246] and n-type [46,247-249]
conduction through the introduction of external dopants, enabling the maintenance of a high zT across
different compositions. However, the practical application of SnSe and other selenides is hindered by their