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Page 2 of 16 Hong et al. Soft Sci 2023;3:29 https://dx.doi.org/10.20517/ss.2023.20

INTRODUCTION
Thermoelectric materials, which can directly convert heat energy into electricity, have gained significant
[1-3]
attention because of their potential for sustainable energy harvesting from waste heat sources . With
increasing concerns on energy efficiency and renewable energy sources, thermoelectric materials and
devices have been explored for a wide range of applications, including waste heat recovery in industrial
processes, automotive waste heat recovery, and powering wearable devices . The performance of
[4-6]
thermoelectric materials is determined by the dimensionless figure of merit, zT , expressed by zT = S σT/κ,
2
[7-9]
in which S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity
[comprising electron (κ ) and lattice (κ) thermal conductivities] [10-12] .
e
l
Significant progress has been achieved to enhance zT through increasing S σ and decreasing κ. Since S, σ,
2
l
and κ are coupled via the carrier concentration (n ), optimising n is the prerequisite to maximise zT .
[13]
e
H
H
2
Electrically, to increase S σ, band engineering to align multiple band valleys has been established to ensure a
large band degeneracy (N ) [14,15] . Improving the carrier mobility (µ ) is also a method to increase S σ .
2 [3,16-18]
H
v
Thus, engineering light bands to reduce the band effective mass can effectively enhance thermoelectric
performance. Other strategies that have also been proven to elevate S σ include introducing resonant energy
2
levels into the density of states (DOS), minority charge carrier filtering, and quantum confinement [19-21] .
Thermally, κ can be decreased by enhancing phonon scatterings . This includes strengthening inherent
[22]
l
phonon-phonon interactions and introducing external scattering centres [23-26] . To identify the intrinsically
strong phonon-phonon interactions, the acoustic and optical phonon branches should be overlapped in
frequency to allow sufficient scattering channels [27-29] . The slope of acoustic branches should be as low as
possible to ensure a small phonon group velocity [28,30,31] . Further reinforcing phonon scatterings is realised by
introducing various types of nanostructures and lattice imperfections [32-34] .
Additionally, the fabrication technique used to create thermoelectric materials and devices plays a crucial
role in determining their performance [35-38] . Bulk processing techniques involve the synthesis of
thermoelectric materials in a solid-state form, such as solid-state reaction, ball milling, or melt processing,
followed by shaping and consolidation processes to obtain bulk materials with desired properties [39-42] .
Vacuum deposition methods, such as sputtering and thermal evaporation, realize the deposition of thin
films of thermoelectric materials onto substrates using vacuum chambers and specialized equipment [43-47] .

While these traditional fabrication methods have been widely used in the field of thermoelectrics and have
[48]
resulted in significant advances, they also have limitations . For instance, bulk processing methods may
have limitations in terms of achieving high-resolution patterning or complex geometries, and vacuum
[49]
deposition methods may be costly and complex for large-scale production . Additionally, these methods
may require high processing temperatures or harsh conditions, which can limit the selection of materials
and substrates .
[36]
To harness the full potential of thermoelectric materials and devices, efficient and scalable fabrication
methods are crucial . Among various fabrication techniques, printing methods have emerged as promising
[50]
approaches for the production of thermoelectric materials and devices, offering advantages such as low cost,
high throughput, and design flexibility [51-53] .

Printing methods for thermoelectric materials and devices can be categorized into different types [Table 1],
[64]
[62]
such as screen printing [54-57] , extrusion printing [58-61] , inkjet printing , aerosol jet printing , drop casting ,
[63]