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Page 2 of 26 Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25

is an unprecedented demand for portable, reliable, ultra-thin, and sustainable wearable electronic power
devices. The application of conventional batteries is limited because of their frequent replacement/recharge
and extra maintenance. Therefore, developing a maintenance-free and self-powered sensor system is
critically important. Thermoelectric (TE) conversion technology can convert heat from the body or the
[1-3]
environment into electricity, providing a viable self-powered solution for flexible electronics .
Conventional inorganic bulk TE materials, such as Bi Ti and PbTe, usually show high TE performance;
3
2
however, brittle and rigid properties of inorganic materials and the complicated process have limited their
applicability, particularly when in contact with curved or irregular heat sources . In this situation, film-
[4,5]
based flexible TE materials and generators show significant advantages and application prospects since their
conformability enables effective contact with all kinds of curved or irregular heat sources to maximize heat
harvesting. Besides, a thin-film thermoelectric generator (TEG) may use fewer materials compared to a bulk
[6,7]
TEG and provide easy integration with integrated circuits . Figure 1 shows the different structures of
traditional inorganic TEGs with a cross-plane structure in Figure 1A and thin-film TEGs with an in-plane
structure in Figure 1B. Traditional inorganic TEGs are composed of bulk P/N legs tiled in two dimensions
over a ceramic substrate patterned with electrical contacts, in which the temperature gradient across the
material is perpendicular to the substrate. Unlike cross-plane structures, the temperature gradient across the
material is parallel to the substrate for the in-plane structure. In this structure, the temperature gradient
[8,9]
across the TE legs is usually minimal; creative strategies, such as corrugated, origami, and rolled designs ,
can convert the 2D arrays of thin-film TE legs into 3D architectures. Thus, the temperature gradient will
parallel the substrate to obtain a more significant thermal gradient. A flexible TEG has the advantages of
all-weather continuous operation, no moving parts, small size, and high reliability. Developing
high-performance, low-cost, and easy-to-process materials is the pursuit of the TE field. The properties are
measured primarily by the dimensionless TE value (ZT): ZT = σS T/κ. High-performance TE materials
2
require high electrical conductivity σ, a high Seebeck coefficient S, and low thermal conductivity κ.

Research on flexible TE technology generally consists of two strategies. One is to directly use organic TE
materials with good flexibility and plasticity, and the other is to integrate brittle inorganic TE materials into
the flexible substrate. [10-13] . The former has lower electrical performance, resulting in output voltage and
power much inferior to inorganic materials; the latter is difficult to produce ultra-thin flexible devices due
to its complex structure and technology. The discovery of silver sulfide–based ductile semiconductors has
extensively promoted the development of flexible inorganic TE materials [14-16] . Significantly, Yang et al.
reported high-performance p-type ductile TE materials of AgCu(Se, S, Te) pseudo ternary solid solutions,
[14]
with a ZT value of 0.45 at 300 K . They developed ultra-thin flexible TE devices with a conventional
π-shape and demonstrated ultra-high normalized power density and reasonable service stability. On the
other hand, organic TE materials, while fully leveraging their advantages of flexibility, solution-processable,
and low intrinsic thermal conductivity, have been steadily addressing their limitations in common overall
TE properties and have achieved rapid development over the past decade, with ZT values of multiple
material systems exceeding 0.1 at room temperature, constantly expanding the application range of
wearable devices [17-20] .

The preparation process of flexible TE films is crucial in constructing wearable TE devices. Inorganic plastic
semiconductors are prepared first by traditional melting and annealing methods, and then the prepared
inorganic material is cut into thin sheets. In addition, Jin et al. prepared high-performance Bi Te -single-
3
2
walled carbon nanotube (SWCNT) composite flexible self-supported TE thin films with carbon nanotubes
(CNTs) as the skeleton using independently improved magnetron sputtering deposition equipment .
[21]
Vacuum filtration [22-24] , vapor deposition [25-28] , printing [29-31] , and coating [32-34] methods effectively prepare