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Page 8 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

Optical properties
In addition to the previously mentioned key characteristics, adhesives used in displays must exhibit optical
clarity, especially in the front layers, and are classified as OCAs. Optical transparency and refractive index
are critical to ensure the efficient transmission of light emitted from OLEDs without additional loss. To
maintain the integrity of the OLED’s color, specifications related to color, such as the yellow index and Lab,
are also crucial. Furthermore, driven by the trend towards minimizing components, enhancing light
transmittance, and reducing power consumption in flexible devices, innovative technologies such as color
filter on encapsulation (CoE) and touch on encapsulation (ToE) have been developed. These advancements
have enabled the elimination of polarizers. In OLED devices, the role of the polarizer has diminished
compared to its application in LCDs, yet it still performs critical functions such as blocking external light
reflections, enhancing contrast ratio, and providing UV-blocking ability to prevent OLED
degradation [110-112] . Since the removal of the polarizer in foldable devices, technological developments have
been underway to address the reduction of external light reflections and enhancement of contrast ratio
through the application of anti-reflection coatings on the cover window and the black matrix on color
filters, respectively. However, since other layers cannot compensate for the UV-blocking ability required to
prevent OLED degradation, it necessitates incorporating UV-block capabilities into the adhesive [27-29] .
Reflecting these needs, recent research has advanced to incorporate the evolving requirements of display
technology.

The aging resistance
Given that electronic devices are typically used over extended periods, the aging resistance of adhesives is
[113]
critically important for maintaining their long-term functionality and reliability . Aging resistance is the
ability of an adhesive to preserve its physical and chemical properties over time, which is essential for
ensuring the long-term reliability of flexible displays. Aging of adhesives typically results from exposure to
factors such as heat, humidity, UV, oxygen, and various chemicals. These factors could change or degrade
the polymer structure of the adhesive, thereby reducing its mechanical and rheological properties. For
example, at high temperatures, the chemical bonds within the adhesive could be degraded by oxidation,
reducing mechanical strength and adhesion . Humidity and oxygen could accelerate the oxidation and
[114]
hydrolysis reactions in the adhesive, weakening its flexibility and tensile strength . To counteract these
[115]
issues, adhesives could be formulated with stabilizers such as antioxidants, UV absorbers, and thermal
stabilizers, to enhance aging resistance. Additionally, selecting monomers and crosslinkers with high
thermal and moisture resistance could further improve aging resistance. To evaluate aging resistance,
various accelerated aging tests can be conducted. These tests expose the adhesive to extreme conditions such
as high temperatures, high humidity, and UV exposure to observe the long-term aging effects within a short
period [116,117] . Beyond evaluating the adhesive itself, the aging resistance of adhesives can be further assessed
by applying adhesives to display modules and conducting stability tests under varying temperature,
humidity, and UV exposure conditions [Table 1]. These tests can identify phenomena such as bleaching,
color changes, or bubbles resulting from adhesion loss. Collectively, ensuring the aging resistance of
adhesives is essential to maintain the mechanical and optical reliability of flexible displays.

RECENT ADVANCES IN ADHESIVES FOR FLEXIBLE DEVICES
Employing finite element analysis for adhesive design
Finite element analysis (FEA) is a method that uses partial differential equations to numerically and
mathematically simulate and analyze specific physical phenomena. This technique is particularly valuable
for predicting the behavior of individual components, layered structures, or assemblies under various
conditions. Researchers actively use FEA software in adhesive design to optimize layered components and
determine the necessary physical properties of adhesives, thereby reducing trial-and-error experimentation
and lowering costs. In the field of flexible devices, FEA with the modulus of the adhesives effectively