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Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26 Page 19 of 32

resulted in strong covalent crosslinking, providing stable mechanical robustness. This ensured the
luminescent functionality was maintained under various mechanical deformations [Figure 10D].

Perovskite and QD materials
In general, perovskites have been utilized as optically superior emissive materials due to their high
photoluminescence quantum yield and narrow full width at half maximum, along with the ease of tuning
[115]
the quantum confinement effect . However, perovskites possess high elastic modulus, making them
unsuitable for stretchable devices. Therefore, to impart the elasticity, a method of integration with a
polymer matrix can be applied, similar to the fabrication process of stretchable inorganic ZnS phosphor-
based devices. However, due to their unique ionic structure, perovskites are prone to structural deformation
[115]
when mixed with polymers, which can lead to instability in the optical properties . Lee et al. chemically
modified the surface of CsPbBr nanocrystals with silica using (3-aminopropyl) triethoxysilane with hexane
3
as a sacrificial solvent, aiming to improve the polymer dispersibility and chemical stability of CsPbBr
3
[115]
perovskite [Figure 10E(i)]. They fabricated a perovskite stretchable EL device using PDMS substrates and
AgNW electrodes and reported that the luminescent properties were maintained even under 50% tensile
strain and various mechanical deformations [Figure 10E(ii)]. QD LEDs (QLEDs) are also being actively
researched as next-generation display emitters due to their unique advantages, such as wide color gamut,
high color purity, relatively high brightness at low operating voltages, and applicability in thin-film form
factors . Kim et al. integrated a thin-film QLED fabricated on a polyethylene naphthalate/graphene
[124]
substrate and pre-strained (70%) Ecoflex elastomer to obtain stretchability through spontaneously formed
buckled structures [Figure 10F(i)]. The fabricated stretchable QLED maintained stable luminescent
[125]
performance even under 70% strain [Figure 10F(ii)]. This is a representative achievement in the
implementation of stretchable QLEDs, utilizing the QLEDs in an ultra-thin form factor. Recently, Kim et al.
successfully demonstrated an intrinsically stretchable QLED by utilizing a mechanically soft and stretchable
light-emitting layer composed of a ternary nanocomposite of colloidal CdSe/ZnS QDs, elastomer matrix
[polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene-graft-maleic anhydride, SEBS-g-MA],
and hole transport polymers (poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine)) . The
[119]
polymer-rich charge transport region provides a hole transport pathway to the embedded QDs, and the
addition of the elastomer at a concentration of 10 wt% of the QD weight in the emissive layer contributes to
maintaining a consistent interparticle distance between the embedded QDs, ensuring durability under
tensile strain. The fabricated QLED device achieved mechanical stretchability up to 50% without cracks and
-2
exhibited a maximum luminance of 15,170 cd·m at 6.2 V (turn-on voltage of 3.2 V). In addition, other
studies have reported the use of hybrid emissive layers fabricated by mixing QDs and perovskites. Li et al.
fabricated a thin-film perovskite-QLED (Pe-QLED) by assembling a flexible transparent electrode made of
PI/AgNWs and a pre-stretched adhesive elastomer film [Figure 10G(i)]. The surface of the Pe-QLED
[126]
fabricated on the pre-stretched elastomer film formed a wrinkled pattern when the pre-stretching is
released. The fabricated Pe-QLED operated at a threshold electric field of 3.2 V and exhibits a maximum
-2
luminance of 3187 cd·m at 9 V. Additionally, it maintained the optical performance under mechanical
stretching up to 50% [Figure 10G(ii)]. Table 3 provides a brief comparison of light-emitting types, emitting
layer materials, stretchability, maximum brightness, and turn-on voltages.

Geometrical structure design of emissive layers
The proposed methods for implementing the aforementioned stretchable emissive layers, whether using
intrinsically stretchable emissive materials or integrating thin-film LED devices on pre-deformed elastomer
substrates, may have limitations in the availability of high-performance emissive materials or optical
[127]
clarity . To resolve these issues, Kim et al. developed stretchable OLED interconnecting SU-8 rigid island
arrays, where the OLED is located, using serpentine bridges formed on a bilayer elastomer substrate
[127]
[Figure 11A(i)]. This structural design enabled the luminescent performance to be maintained even when