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Page 14 of 32 Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26

during stretching. As a demonstration, OTFTs were fabricated using the mesh-like polymer semiconductor
films. In this case, the initial mobility increased compared to the flat-type OTFTs due to the improved
charge transport performance caused by the crystal regions of wrinkles, and the electrical performance
degradation was not significant even up to 80% stretched condition [Figure 7D].

Metal-oxide stretchable TFTs
Amorphous oxide semiconductors (AOSs) have great potential in various electronic applications due to
their high uniformity, good stability, low leakage current, and high field-effect mobility [92,93] . However, there
are significant limitations in implementing the AOS TFTs in stretchable displays due to their brittle nature.
Nevertheless, AOS-based stretchable TFTs have been extensively studied by numerous researchers through
structural design and deformation engineering, including rigid island structures, wavy-like string structures,
and application of stress-relieving dielectric materials . In this section, recent studies and achievements of
[81]
AOS-based stretchable TFTs are discussed.

Figure 8A shows stretchable InGaZnO (IGZO) TFTs and circuitry with large-area scalability utilizing a
[81]
rigid-island structure reported by Kang et al. . Their strategy was to adopt a dual-island structure on
molecular-tailored elastomeric substrates [soft PUA substrate and rigid polyepoxy acrylate (PEA) island] to
improve the mechanical properties and maintain the electrical properties at stretched conditions. Also, in
consideration of large area and mass production, a bottom-up photolithography-based patterning approach
was adopted. It is suggested that the adhesion between PUA substrate and PEA island was improved by
forming strong covalent bonds on the two polymer interfaces using the acrylic functional groups derived
from a soft PUA substrate and rigid PEA island. Consequently, they successfully demonstrated stretchable
(strain up to 50%) 7 × 7 TFT arrays including various circuits such as logic gates and 7-stage ring oscillators.
Another example of the stretchable oxide TFTs utilizing rigid-island structures was conducted by Miyakawa
et al. . A large strain deformation occurs at the interface between the stretching region (soft substrate) and
[78]
the non-stretching region (rigid island) in the stretchable device. Therefore, strong interfacial adhesion
between the rigid island region and the soft substrate is required to secure stable operation under stretching.
As shown in Figure 8B, an acrylic adhesive layer was utilized to provide strong adhesion between the PI
rigid island and the soft substrate, enabling stable switching characteristics with a field-effect mobility of
30 cm ·V ·s without significant performance degradation even at 50% tensile stretching.
2
-1 -1
Another approach to achieving stretchability is structural engineering. Oh et al. reported fabrication of
IGZO TFTs on a serpentine string structure to overcome the trade-off between device integration density
[79]
and stretchability [Figure 8C] . Particularly, to obtain mechanical stretchability, the TFT devices were
fabricated directly on the serpentine structure PI strings. After fabricating the TFTs, the devices were coated
with a second PI layer to place the TFT devices near the center of the PI cladding. The cladded TFT devices
were transferred to a stretchable substrate by using a lift-off process. This unique structural approach
2
enabled a maximum integration density of > 30,000 transistors/cm , and the stress could be minimized even
at 100% strain, maintaining the electrical properties. Another recent attempt was reported to improve the
mechanical robustness of TFT devices while maintaining their electrical performance by applying a
combination of organic/inorganic materials to the stress relief buffer layers and dielectric layers. As shown
in [Figure 8D], Kim et al. reported the utilization of organic-inorganic hybrid gate insulators and buffer
layers to fabricate metal oxide-based stretchable TFTs . An indium-gallium-tin-oxide (IGTO) channel
[73]
layer was used because the film formation could be performed at a relatively lower temperature compared to
IGZO (~150 °C). Also, the hybrid film was fabricated by the combination of 1,6-bis(trimethoxysilyl)hexane
(BTMSH) organic crosslinking agent and zirconium oxide (ZrO ). The IGTO TFTs fabricated on a PI-
x
coated glass substrate were separated from the glass substrate and transferred onto a stretchable substrate
(polyethylene, PE) in a similar process shown in [Figure 8C]. The stretchable IGTO TFTs on PI/PE