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Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22 Page 19 of 28

Recent trends in flexible devices aim to enhance energy efficiency, minimize thickness, and reduce
manufacturing costs by utilizing multifunctional layers. A notable example is the replacement of polarizers
with color filters in new OLED displays [Figure 9A]. However, removing the polarizer exposes the OLED to
UV light, potentially accelerating the degradation of emitters. This underscores the need for UV-blocking
adhesives. These adhesives are also in high demand for automotive displays, which are exposed to strong
sunlight for extended periods. Traditional UV-cured polymerization processes face challenges when
incorporating UV-cut additives, as UV light is essential for curing.

To address this, Kwon et al. utilized a visible-light-driven polymerization method to successfully create a
flexible adhesive with UV-blocking capabilities . This adhesive not only completely blocks UV light below
[27]
400 nm but also uses minimal amounts of a photocatalyst that absorbs visible light, ensuring optical
transparency and colorlessness. Additionally, the rapid polymerization process makes it economically
viable. The UV-blocking adhesive effectively blocks UV light, preventing luminescence loss and voltage
drop in blue OLEDs [Figure 9B]. This study is significant for its industrial applicability and for
incorporating additional functions without compromising the characteristics of existing flexible adhesives.

PERSPECTIVES
With the advancement of information technology (IT) infrastructure and increasing demand for real-time
information sharing, display form factors are expected to become more diverse and complex. The
development of innovative designs such as foldable, rollable, and transparent displays enhances consumer
convenience and introduces new user experiences. As these devices evolve, the adhesives used in them are
also being developed to meet various requirements, with ongoing efforts to push existing material properties
to their limits or incorporate new functionalities. While adhesives for foldable phone and laptop/tablet
displays are relatively well-developed, those for automotive displays still require improvements due to more
[210]
stringent reliability conditions. According to the temperature standards for existing automotive displays ,
adhesives must maintain their rheological properties over a broader range, ensuring reliability down to
-40 °C . Additionally, as smartphone display structures are simplified - such as through the removal of
[42]
polarizers and the implementation of CoE - further research is needed to incorporate additional properties
such as UV protection into adhesives.

Rollable displays, which offer significant enhancements in space utilization and portability, are particularly
promising for automotive applications and thus demand high environmental reliability. Adhesives for these
displays need to endure considerable deformation without tearing and recover effectively when unrolled,
requiring a blend of existing techniques and innovative approaches. Similarly, adhesives for stretchable
displays, such as those used in attachable medical sensors or textile displays, necessitate additional
functionalities. These adhesives should exhibit well-defined properties tailored to specific applications,
including the capacity to fill micro LED step heights and considerations for biocompatibility and water
resistance. Textile displays, in particular, demand adhesives with fiber compatibility, enhanced wash
durability, and chemical resistance, necessitating a sophisticated set of properties. As evidenced, the
properties required for adhesives used in rollable or stretchable displays are not as well-defined as those
used in foldable displays. Therefore, in-depth research is essential to identify the target properties crucial for
ensuring the durability and reliability of these devices.

Additionally, adhesives used in all types of flexible displays must ensure long-term stability to maintain the
durability of the display and the device in which it is utilized. The repeatability and stability of adhesives are
crucial for the practical application of flexible devices. Given that flexible devices such as foldable
smartphones, rollable TVs, wearable sensors, and human-computer interaction displays are used over long