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Page 16 of 32 Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26

[84]
stretchable TFTs maintaining the drain current level at a maximum strain of 35% without degradation
[Figure 9A]. They explained that the high Young’s modulus of parylene-C (about 3.2 GPa) enabled effective
suppression of stress distribution applied to the channel region in the TFTs. The fabricated stretchable TFTs
could be operated at a low voltage of ~0.7 V and had robust stability even under biaxial stretching. In
addition, they successfully demonstrated various stretchable circuits such as inverters and ring oscillators .
[84]
Additionally, 2D MoS semiconductors have been popularly researched due to their functionality in a wide
2
range of optoelectronic devices . Recently, numerous efforts have been made to enhance the deformability
[85]
of 2D semiconductors, aimed at expanding their application in device technology. MoS films, known for
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their lamellar structure, typically exhibit limited deformability due to the strong covalent bonds within their
layers. However, Zhang et al. proposed an innovative strategy for fabricating stretchable MoS thin films
2
[85]
using multilayer structures . It was demonstrated that a 10-layer MoS film on an elastic SEBS substrate
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can withstand approximately 50% tensile strain, overwhelming the monolayer MoS . Molecular dynamics
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simulation was performed to analyze the strain distribution between MoS tetralayers and the substrate.
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Figure 9B shows the strain distribution cloud map on tetralayer MoS under applied strain. The color
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gradient from blue to red indicates increasing strain. The cloud map reveals that as strain progressively
increases, strain distribution gradually mitigates towards the top layers of MoS . Researchers have
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corroborated experimental results with simulation and demonstrated that multilayer MoS could withstand
2
greater mechanical deformation than monolayer MoS , while maintaining photoelectronic performance,
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attributed to interlayer sliding between adjacent layers of multilayer MoS compared to monolayer MoS . As
2
2
limitations of these materials, in the case of using CNTs for channel materials, it is challenging to secure
high uniformity and/or alignment of CNTs. Furthermore, mass production and process compatibility with
conventional TFT fabrication also need to be considered. In the case of 2D semiconductors such as MoS ,
2
achieving large-area uniformity with controlled layer thickness might be a critical issue. In this regard,
chemical vapor deposition methods with large-area processability should be further developed.
LIGHT-EMITTING AND LUMINESCENCE LAYERS
The light-emitting layer, one of the important core components in constructing the display, could be
classified into various types, depending on the light-emitting materials applied, such as organic-based
LEDs [97-102] , inorganic-based phosphors [103-111] , perovskites [112-115] , and quantum dots (QDs) [116-119] . In the case of
organic-based emissive materials, it is challenging to maintain stable performance over extended periods.
Appropriate passivation or encapsulation materials should be developed which have high mechanical
stretchability. Perovskite and QD-based light-emitting materials are also vulnerable to moisture and oxygen.
Therefore, to ensure high operational stability, sufficient passivation layers are required. Additionally, light-
emitting materials with high luminous efficiency should be further developed. The important performance
indicators in stretchable light-emitting layers are mechanical durability and appropriate retention of light-
emitting intensity during stretching. This section introduces several studies that demonstrate stretchable
luminescence layers using various light-emitting materials mentioned above. Similar to the electrode and
interconnect sections, intrinsically stretchable light-emitting materials and the approach of geometrical
structure design to obtain the deformability are discussed in detail.

Organic-based emissive materials
Organic-based emissive materials have been widely used as the emissive layer for stretchable displays,
particularly the polymer-type LEDs (PLEDs). Among these, conjugated polymers (CPs), primarily used in
polymer-based emissive layers, are notable candidates due to their capability for solution processing,
facilitating large-area coverage and patterning. Moreover, their chemical synthesis and blending processes
enable facile tuning of material properties, making them suitable candidates for flexible applications [99,120-122] .