Intrinsically Stretchable OLEDs with a Designed Morphology-Sustainable Layer and Stretchable Metal Cathode

Recently, the growing desire to conformally integrate electronics with the human body in the form of wearable devices has spurred the need for additional form factors, skin-like softness, and stretchability of organic light-emitting diodes (OLEDs). Traditional intrinsically stretchable OLED (is-OLED) approaches have focused on improving the luminance and stretchability through methods such as blending materials to endow the component layers with stretchability and complex lamination processes. However, the designed microstructure of the blended layer cannot be maintained due to the different orthogonality between the solvents of subsequently coated layers. In addition, the lamination method often leads to degradation of the performance due to delamination induced by formed defects. To overcome these challenges, we developed a sequentially coated is-OLED and con�rmed the maintenance of the designed morphologies of each layer and a highly stretchable metallic is-cathode. Our is-OLEDs achieved a maximum total luminance of 3,151 cd m -2 and a total current e�ciency of 5.4 cd A -1 (on both the anode and cathode sides). Furthermore, our is-OLEDs exhibited a higher static stretchability of up to 70% than previous work and a notable cyclic stretchability, maintaining 80% of the luminance at 0% strain after 300 stretching cycles under 40% strain. This breakthrough in the fabrication process, coupled with the use of novel stretchable materials, represents a signi�cant step forward in the �eld of is-OLED technology, potentially leading to a new era of highly durable and e�cient soft electronic devices.

Previous is-OLED studies have focused on improving the stretchability of inherently rigid organic layers by blending various materials, such as Triton X-100 (TX) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPG-PEO) additives and polyurethane (PU) and polystyreneblock-polybutadiene-block-polystyrene (SBS) elastomers, which change the microstructure from rigid to stretchable.12-18 To fabricate is-OLEDs with organic layers, solution coating, such as spin-coating, is usually performed.However, in many cases, the lack of solvent orthogonality between layers destroys the designed microstructure of the underlying layer, which signi cantly reduces the stretchability. 19,20ence, in previous works by others, lamination was used to build up the layers in is-OLEDs.In Zhang et al., emissive layer (EML) and electron transport layer (ETL) surfaces were joined, and in Liu et al., the ETL and cathode surfaces were laminated.In many cases, a lack of solvent orthogonality occurred between the EML, ETL, and cathode layer.3][14][15] During this process, misalignment and pressure-induced defects and voids induce insu cient layer contact, leading to delamination, dark spots, reduced fabrication yield, and decreased performance, especially under repeated stretching. 11 conventional OLEDs, a metallic cathode is used due to the high density of free electrons and high charge carrier mobility.However, due to the inherent rigidity of metals, these cathodes were not considered as stretchable electrodes for use in is-OLEDs.In previous works on is-OLEDs, silver nanowires (AgNWs), graphene, and poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) were used as stretchable cathodes, which are very limited candidates.AgNWs were embedded in an elastomer matrix to impart stretchability.Hence, they had to be fabricated on a separate substrate, followed by a physical lamination process onto the precoated organic layers. 13,15Graphene cannot be directly synthesized on ETL surfaces; hence, its integration also involves a complex transfer process onto cathode surfaces, necessitating a subsequent lamination process. 13While the PEDOT:PSS cathode can be directly spin-coated on organic surfaces, this approach risks damaging the underlayer due to the effect of the polar solvent on the ETL and the post-patterning process.As a result, PEDOT:PSS cathodes were also fabricated by a lamination process after the separate fabrication of is-cathodes. 12In summary, in addition to their inferior conductivity compared to that of conventional metallic cathodes, these materials inevitably require a manual lamination process during the fabrication of is-OLEDs.
In this study, we developed a hybrid fabrication process employing sequential solution coating and thermal evaporation, eliminating the need for a lamination process.We developed a newly designed morphology-sustainable is-ETL and a highly stretchable metallic is-cathode.The is-ETL not only ensures a uniform surface but also sustains the designed morphology of the is-EML, thereby enhancing the electrical and mechanical properties of is-OLEDs.Furthermore, by adjusting the deposition conditions and controlling the thickness of the conventional brittle Ag metal lm, we successfully fabricated a stretchable Ag electrode with high electrical conductivity.As a result, our device achieved a maximum total luminance of 3,151 cd m -2 and a current e ciency of 5.4 cd A -1 , maintaining approximately 50% performance at 70% strain and 80% performance after 300 cycles of stretching tests.This work explores new stretchable materials and fabrication processes and discusses potential challenges and advancements in mechanical and electrical properties, paving the way for the promising future of is-OLED technology.
Figure 1 Design of is-OLEDs and characterization of the is-ETL and is-cathode.a Schematic of an is-OLED structure from the is-anode to the upper ETL.b is-OLED fabrication process with sequential coating.c Device structure and components of an is-OLED.d Schematics of the is-EML morphology change after ETL solvent coating.Based on the various analysis results, by adding Triton X into the ETL solvent, the SY chains maintain an extended morphology after the is-ETL coating process, resulting in high stretchability.e Schematics of the is-cathode morphology and electron conducting mechanism under stretching conditions.

Designs of intrinsically stretchable OLED
Figure 1a shows a schematic image of the is-OLEDs fabricated in this work.Figure 1b summarizes our sequential coating processes from the is-anode to the upper ETL.As shown in Fig. 1c, for the is-OLEDs, an intrinsically stretchable hole transport layer (is-HTL) is formulated from PEDOT:PSS and the nonionic surfactant TX, and an intrinsically stretchable EML (is-EML) is composed of a commercial EML, Super Yellow (SY) and TX. 16TX affects the conformation and microstructure of PEDOT:PSS and SY, resulting in highly stretchable is-HTL and is-EML. 16 illustrated in Fig. 1c, an intrinsically stretchable ETL (is-ETL) is spin-coated on the is-EML.3][14][15][16] Among them, PFN-Br stands out due to its superior electron injection and hole blocking capabilities, as well as its ability to facilitate a uniform surface.Nonetheless, the binary ETL composed of PFN-Br and PEIE has been found to offer limited stretchability and to adversely affect the underlying is-EML during the solution coating process, highlighting the necessity for advanced material design.
Most solvents dissolve most nonionic surfactants, potentially altering the morphology of the organic layers including a nonionic surfactant during sequential coating.Furthermore, the mechanical and electrical properties of is-EMLs are strongly affected by the morphology of the conjugated polymers. 12,16ur study mitigated this by incorporating TX across all coating layers from the is-HTL to is-ETL, ensuring the presence of TX on the underlayer surface to sustain the designed morphology of conjugated polymers, as shown in Fig. 1d.The incorporation of TX into the PFN-Br and PEIE matrix has been shown to enhance both the mechanical and electrical properties of the is-OLED, leading to improved overall device performance.
Following the is-ETL coating process, an is-metal cathode is developed using evaporation, as shown in Fig. 1e.3][14][15][16] However, these stretchable electrodes often exhibit signi cant roughness, inferior electrical characteristics, or chemical instability.Furthermore, they require either a solution process or a complicated transfer process, which damages the underlying organic layers and complicates patterning. 11,21,22Hence, the development of new stretchable electrode materials is imperative.
For high-performance is-OLEDs, electrode materials such as stretchable Al or Ag lms are expected to be most effective because of their inherently high charge carrier density and mobility. 3Such materials can be deposited by thermal evaporation, minimizing damage to the underlying organic layers, and are easily patterned.4][25] To address this, modifying the interaction with the underlayer and adjusting the deposition conditions are crucial.We successfully fabricated a stretchable Ag electrode with high electrical conductivity by ne-tuning the deposition conditions and optimizing the thickness of the traditionally brittle silver metal lm.The precisely adjusted evaporation conditions for the stretchable metal cathode resulted in only microcracks on the surface under stretching conditions, enabling e cient electron movement between the microcracks (Fig. 1e).
Figure 2 Design and characterization of is-ETLs for highly stretchable is-functional layers.a UV-vis absorption spectra of is-EMLs with various TX blending ratios, and solvent coating effect on the is-EMLs.b UV-vis absorption spectra of is-EMLs before and after TX-blended solvent coating (TX1 = 1 mg ml − 1 , TX2 = 2 mg ml − 2 , TX4 = 4 mg ml − 1 ).c Photoluminescence spectra of is-EMLs before and after TXblended solvent coating.d COS of is-EMLs before and after TX-blended solvent coating.e COS of is-ETLs with various component ratios when coated on PDMS or is-EMLs.f UPS intensity of is-ETL and Ag lms with various TX ratios.g Current density of electron-only devices with various ETL component ratios.h Energy-level alignment diagram for the conventional OLED device with an is-ETL.i V-L curve, j J-V curve, and k ε c -L curve of the conventional OLED device with an is-ETL on a glass substrate.
Designed morphology sustainable electron transport layer PFN-Br, distinguished by its high lowest unoccupied molecular orbital (LUMO) level of 2.7 eV and low highest occupied molecular orbital (HOMO) level of 5.6 eV, was selected as the is-ETL (Fig. 1c).It offers signi cant advantages for electron injection and hole blocking in OLED structures. 12,26Furthermore, the addition of PEIE contributes to a notable increase in the stretchability of the material, reaching approximately 70% strain (Supplementary Fig. 1).However, the stretchability of PFN-Br:PEIE is lower than the 80% crack onset strain (COS) of the is-EML.Additionally, when PEIE or PFN-Br:PEIE solution is coated on the is-EML, a signi cant reduction in the COS to 40% occurs (Supplementary Fig. 2).This reduction suggests that the impact on stretchability is due not only to the intrinsic properties of the ETL but also to changes in the underlying is-EML layer.
For the is-EML, TX mixed with SY-conjugated polymers (Fig. 1c) alters the morphology of the polymers from coiled to extended, resulting in high stretchability and electrical properties. 16This morphological change is evident through the redshifted absorbance (Abs), blueshifted photoluminescence (PL), and changes in the atomic force microscopy (AFM) surface phase (Fig. 2a, Supplementary Figs. 3 and 4).
Here, we coated various solvents on is-EMLs to investigate the effect of dissolution of is-EMLs in the solvent on the changes in the conformation and morphology of the is-EMLs (Fig. 2a).Regardless of the TX concentration in the SY and ETL materials, applying a solvent coating on the is-EML to coat the is-ETL reverts the Abs, PL, and phase changes to those of pristine SY (Supplementary Figs. 2, 3, and 4).
Since TX, a nonionic surfactant, is soluble in most organic solvents, we can anticipate that the characteristics of the EML may change when the TX present on the SY surface is washed away.Therefore, to maintain the morphology of the is-EML, TX was also incorporated into the ETL.This addition ensures that the designed morphology from the HTL to the ETL is maintained during the continuous coating process.Figures 2b and 2c con rm that mixing TX with the solvent sustains the redshifted Abs and blueshifted PL peaks after coating, akin to the behavior of the is-EML.Moreover, increasing the TX concentration better sustain the is-EML Abs and PL peaks up to 4 mg ml − 1 .
Figure 2d shows the effect of TX on the COS of the is-EML (Supplementary Fig. 2, Supplementary Fig. 9).Mixing SY with TX increases the COS.Applying the solvent coating alone decreases the COS to 40%, whereas the solvent containing TX yields a COS of 60% at 1 mg ml − 1 , 80% at 2 mg ml − 1 , and 90% at 4 mg ml − 1 .These ndings suggest that the TX-blended solvent maintains the is-EML morphology, ensuring high stretchability.Therefore, coating of an ETL solution with TX is crucial for preserving the extended morphology of the is-EML.
Figure 2e shows the mechanical characteristics of the ternary blends of PFN-Br, PEIE, and TX.TX increases the COS of the is-ETL by increasing the free volume within the material and decreasing the crystallinity of the conjugated polymer (Supplementary Fig. 1). 16,26Hence, the ternary blend of the is-ETL with a ratio of 2:2:2 inherently exhibits high stretchability and is capable of withstanding up to 100% COS.
When this is-ETL is applied over the is-EML, the COS increases as the ratio of TX increases, matching the COS of the is-EML itself.The introduction of TX into the PFN-Br:PEIE ETL has dual effects: providing high stretchability of the is-ETL and sustaining the designed morphology of the is-EML.This stretchability alignment between the is-ETL and is-EML is a signi cant achievement, ensuring that the overall device can maintain its structural integrity and functional performance even under considerable mechanical strain.Furthermore, the ternary blended is-ETL shows a uniform surface morphology, as shown in Supplementary Fig. 10.
Figure 2f shows the electrical properties of the is-ETLs.Ultraviolet photoelectron spectroscopy (UPS) measurements, based on the TX blending ratio, reveal that the work function of 4.8 eV for the pristine Ag lm decreases to 3.3 eV following coating with a PFN-Br:PEIE (2:2) solution.Additional blending of the ETL with TX results in work functions of 3.25 eV for a 2:2:1 ratio and 3.35 eV for a 2:2:2 ratio, indicating similar electron injection characteristics (Supplementary Fig. 12, Supplementary Table 1).Triton X is an electrical insulator, but we estimate that the dipole of the nonionic surfactant TX lowers the work functions of PEIE and PFN-Br.
As shown in Fig. 2g, we fabricated electron-only devices using the is-ETLs, the detailed structure of which is presented in Supplementary Fig. 13.Compared to PFN-Br, the PFN-Br:PEIE ETL exhibits a higher current density.Furthermore, as the TX blending ratio increases, the electron current density correspondingly increases.This nding implies that the TX-blended is-ETL improves the electron transport properties by maintaining the extended morphology of the is-EML.Then, we fabricated devices with a conventional OLED structure using the is-ETLs (Fig. 2h). Figure 2i demonstrates the improved performance obtained with the TX-blended is-ETLs compared to that obtained with the 2:2 ratio, and the increase in the e ciency is shown in Fig. 2k.Hence, the TX-blended is-ETLs not only exhibit inherently high mechanical and electrical properties but also help sustain the designed morphology of the is-EML, thereby enhancing its mechanical and electrical characteristics.Although the 2:2:1 ratio results in a slightly better performance, we chose a 2:2:2 ratio of the is-ETL for the is-OLED fabrication to enhance the is-EML stretchability.Fig. 3 Fabrication and characterization of is-OLEDs.a Schematics of the is-cathode surface morphologies from a thin Ag to thick Ag lm.b Sheet resistance, stretchability of the is-cathode, and luminance of the is-OLED at various is-cathode thicknesses.c Optimal Ag evaporation conditions for the is-cathode.d Resistance change in Ag cathode lms with various thicknesses ranging from 30 nm to 70 nm during static stretching tests.e Resistance change in the 60 nm is-cathode during cyclic stretching tests at 20% and 40% strain.f TEM cross-sectional image of the is-OLED.g SEM image of the is-cathode surface at 0% and 40% strain.h SEM image of the is-cathode surface when stretched to 60% strain and released to 0% strain from 40% strain.

Intrinsically stretchable Ag metal cathode
To render the Ag lm stretchable, we optimized the deposition parameters (evaporation rate and thickness) and the substrate.The structure and properties of the lm largely depend on the substrate.8][29][30] The deposition surface must have (1) a smooth surface, (2) high compatibility (adhesion) with Ag atoms, and (3) appropriate energy levels for electron injection.PFN-Br:PEIE:TX meets all these requirements because of its low surface roughness and adequate energy level (Supplementary Fig. 10 and Fig. 2f ).Additionally, the amine groups in PEIE are compatible with Ag atoms through coordination bonds, functioning as seed layers for ultrathin Ag lms. 23e evaporation rate and thickness were optimized to achieve both good optoelectrical properties and stretchability.Ag lms deposited on the elastomer substrate form three-dimensional islands according to the Volmer-Weber growth mechanism and eventually connect to form a continuous lm with increasing thickness (Fig. 3a). 31A greater thickness ensures high bulk conductivity but can also make the lm brittle.A high evaporation rate leads to continuous lms by suppressing Volmer-Weber growth, while a lower rate enhances stretchability due to the interlocking interface between the metal and substrate and the optimal grain size. 29,32Therefore, we aimed to scale down the deposition thickness and rate until superior optoelectrical properties could be maintained (Figs.3b, c, and d).
We selected Ag lms with a thickness of 60 nm deposited at a rate of 1 Å s − 1 as our stretchable cathode.To determine the optimal deposition parameters, we fabricated our stretchable cathode on the is-OLEDs (Fig. 1a), We then characterized its optoelectrical properties, focusing on maximum luminance (Fig. 3b, c) and cathode conductivity under static and cyclic stretching conditions (Fig. 3d, e).The conductivity rapidly decreases below 40 nm in thickness due to the poor connectivity between Ag grains (Figs.3b, d and Supplementary Fig. 14).Ag lms thicker than 60 nm have high conductivity and show a superior luminance of over 3,200 cd m − 2 when used as a cathode in is-OLEDs but cannot endure strains above 60% due to the brittle nature of the dense, continuous Ag grains (Figs.3b, d, Supplementary Fig. 14, and Supplementary Fig. 16).A 60 nm stretchable cathode exhibits a low sheet resistance of 6 Ω sq − 1 and a maximum luminance of more than 2,300 cd m − 2 and maintains a resistance change of R R 0 − 1 < 10 under 70% strain, simultaneously ensuring high electrical conductivity and mechanical stretchability (Figs.3b, c, d, and Supplementary Fig. 16).Additionally, the material maintains its conductivity during 200 cycles of 40% strain stretching (Fig. 3e).
We investigated the relationship between the microstructures and electrical properties of Ag lms via scanning electron microscopy (SEM), transmission electron microscopy (TEM), and time-of-ight secondary ion mass spectrometry (TOF-SIMS).Partially connected morphologies of Ag grains are observed, as shown in Figs.3f and g.The actual grain size of the deposited lm is approximately 35 nm (note that we describe the deposition thickness values measured by a quartz crystal sensor during thermal evaporation).Due to the slow deposition process, an intermixed region of Ag and organic layers is observed in the TOF-SIMS and TEM cross-section images (Fig. 3f and Supplementary Figs.17-19).No observable channel cracks appear on the lm in the optical microscopy (OM) images until 60% strain (Supplementary Fig. 20).Instead, microcracks develop throughout the lm during the stretching process.Under stretching, strain is released by widening of preexisting cracks while conducting paths are preserved through connected grains (Figs.3d, g, and h).The cracks reversibly close after strain release; thus, the Ag cathode also shows high electrical and mechanical stability under cyclic stretching (Figs.3e, h and Supplementary Fig. 23).
Our stretchability of the Ag cathode seems to be based on two unique structural features of is-OLEDs.First, the multilayers under the cathode (from the is-HTL to is-ETL) provide a gradient in the Young's modulus between the soft elastomer substrate and the stiff metal cathodes.This enhances the stretchability of the cathode compared to structures without multilayer con gurations (Supplementary Fig. 24).Second, the partially interconnected Ag morphologies allow additional layers to be overcoated on the cathodes.We overcoated cesium carbonate-doped PEIE (d-PEIE) on a Ag cathode, which possesses superior electron injection properties. 33If d-PEIE is placed between our is-ETL and is-cathode, it could hinder electron transport due to its electrically insulating nature and disrupt the intended Ag nucleation and growth processes by altering the surface properties.However, by applying it as an overcoating layer above the cathode, it primarily in uences the lowering of the energy barrier for electron injection through channels within Ag lms.Therefore, the d-PEIE overcoating layer was considered to contribute to the high luminance and improved stretchability of the is-OLEDs (Supplementary Fig. 25). 1 changes of the is-OLEDs during 40% strain cyclic stretching tests.f Radar plot comparing the electroluminescence (EL) performance, stretchability, and fabrication complexity of is-OLEDs produced using the sequential coating or lamination method.The fabrication complexity corresponds to the number of transfer and lamination processes during the is-OLED fabrication process.g Optical images of the is-OLEDs with an original emission area of 3.0 × 3.0 mm 2 operated under various strains.h Optical photographs of 3X2 is-OLED array devices operated under various strains and deformations.

Fully intrinsically stretchable OLED
After detailed characterization of the is-ETL and is-cathode, is-OLEDs were fabricated.The fabrication involved sequential coating of the is-ETL, is-cathode, and an upper ETL over the is-EML and is-HTL based on AgNW-embedded polydimethylsiloxane (PDMS).The structural design and energy-level alignment of these devices are illustrated in Figs.4a and 4b. Figure 4c shows the characteristic current densityvoltage-luminance (J-V-L) curves for the is-OLEDs.These devices display a turn-on voltage of 6 V, achieving a maximum brightness of approximately 2,340 cd m − 2 at 9 V, for a measurement taken from the AgNW side.The maximum current e ciency recorded for this side is approximately 4.2 cd A − 1 .
On the opposite side, where a semitransparent Ag cathode lm is used, the performance is approximately 40% of that observed on the AgNW side.This side shows a luminance of 811 cd m − 2 and a current e ciency of 1.2 cd A − 1 .The diminished performance on this Ag side is primarily due to its lower transmittance and re ectance compared to those on the AgNW side.When considering the total performance of the device, combining both the anode and cathode sides, the overall luminance reaches an impressive 3,151 cd m − 2 , with a corresponding current e ciency of 5.4 cd A − 1 .This indicates the high overall performance of the is-OLEDs, demonstrating the effectiveness of the sequential coating process and the synergy between the is-ETL and is-cathode in enhancing the device e ciency and brightness.
The stretchability of the is-OLEDs was tested to evaluate their durability and adaptability under mechanical stress.These tests involved both static and cyclic stretching tests, with the outcomes presented in Fig. 4d, focusing on the relative luminance change in response to various levels of strain at 8 V.At 0% strain, the average luminance across ve samples was recorded as 791 cd m − 2 .As the strain increases, the devices demonstrate remarkable resilience.Up to a 20% strain at 8 V, there is a negligible reduction in performance, indicating a high level of stretchability.However, as the strain is further increased, the luminance gradually decreases.At 40% strain, the devices maintain approximately 75% of their original luminance.This value decreases to approximately 55% at 60% strain and further decreases to approximately 45% at 70% strain.14]16 This performance underlines the signi cant potential of metal-based stretchable cathodes in future applications.
Figure 4d illustrates the changes in the performance of the is-OLEDs under cyclic stretching conditions at 40% strain.A special jig for the cyclic test is shown in Supplementary Fig. 26.In the early stages of the cyclic stretching test, speci cally up to 50 cycles, the performance of the devices decreases, and the luminance decreases to approximately 85% of its original value.However, when the test is extended to 300 cycles, the luminance of the devices remarkably stabilizes, maintaining approximately 80% of the initial value.This indicates a signi cantly greater cyclic stretchability than that reported in previous studies using lamination processes, which exhibited a performance below 80% after 100 cycles at 15% and 40% strain. 12,14Notably, our devices maintain stable performance for up to 300 cycles, surpassing the 200 cycles achieved in similar studies using sequential coating. 16ter the cyclic test, the top surface of the is-OLEDs was observed via SEM (Supplementary Fig. 27).This analysis reveals the formation of microcracks on the surfaces of the is-OLEDs due to stretching.However, microcracks and crack closure are vital characteristics, as they allow for continued ow and injection of charge across the affected areas, ensuring sustained device functionality.]16 Figure 4f presents a radar plot comparing the performance and fabrication complexity of is-OLEDs made using traditional lamination processes and our sequentially coated is-OLEDs (Supplementary Table 2).While the luminance and e ciency are similar or slightly lower than those achieved with lamination methods, our is-OLEDs exhibit higher static stretchability and notably superior cyclic stretchability.In terms of fabrication complexity, our approach shows the most advantageous characteristics, indicative of the reduced need for lamination and transfer processes during the fabrication process.The sequentially coated is-OLEDs, made using the designed morphology-sustainable is-ETL and is-metal cathode, simplify the traditionally complex processes associated with lamination methods, achieving high levels of cyclic stretchability.
Figure 4g depicts the light emitted from an is-OLED when it is stretched.The image shows that the device emits light consistently across its entire surface area both in a neutral state (0% strain) and under signi cant stretching (up to 70% strain).Further expanding on the capabilities of these devices, Fig. 4h shows a 3×2 array device fabricated using the is-OLEDs subjected to stretching.The detailed structure of this array is illustrated in Supplementary Fig. 28.This array device shows stable light emission up to 50% strain.Furthermore, the array device also demonstrates resilience to various other forms of deformation, including bending, folding, and crumpling.This ability to maintain stable light emission even under deformation conditions is a testament to the mechanical robustness and versatility of these devices.
In summary, this study has made signi cant strides in the eld of is-OLEDs by innovating and developing new stretchable component materials, such as designed morphology-sustainable is-ETLs and is-Ag metal cathodes.These advancements have been crucial in enhancing the overall performance of is-OLEDs.The meticulously adjusted formulation of PFN-Br, PEIE, and TX blended into the is-ETL has been instrumental in enhancing both the mechanical and electrical properties of is-OLEDs.The blending of PFN-Br and PEIE has demonstrated the ability to form a stretchable matrix simultaneously exhibiting high electron injection characteristics.Additionally, the incorporation of TX into this blend has been effective in sustaining the morphology of the underlying is-EML, leading to simultaneous improvements in both the mechanical and electrical properties of the is-EML and is-ETL.The is-cathode, engineered under controlled deposition conditions, has achieved signi cant advancements in terms of morphology control and adhesion with the is-ETL.This has led to the creation of a metal cathode that not only is highly stretchable but also maintains excellent electrical conductivity, which is a critical attribute for effective is-OLED operation.
Furthermore, this research represents a departure from more complex lamination techniques through rational material engineering and optimized device construction.This approach signi cantly contributes to simplifying the fabrication process, making it more accessible for the development of next-generation stretchable optoelectronic devices.While acknowledging that the current performance of these is-OLEDs still lags behind that of conventional OLEDs, this study underscores the immense potential for future enhancements.In particular, the prospects of improving the performance through ne-tuning aspects such as the Ag stretchable cathode as well as the thickness and combination of the ETL are promising.Looking ahead, the continued development and utilization of stretchable metallic cathodes are expected to lead to signi cant performance improvements in is-OLEDs, opening up new possibilities and applications in the realm of exible and wearable electronics.

Preparation of is-EML and is-HTL
For preparation of the is-EML solution, TX (Sigma-Aldrich) was rst dissolved in toluene at a concentration of 2.5 mg ml -1 .Then, SY (PDY-132, Sigma-Aldrich) was blended in the TX-dissolved toluene at a concentration of 5 mg ml -1 .The blended solution was stirred at 300 rpm for 12 h at 60 ℃.
After stirring, the is-EML was spin-coated on the is-HTL composed of a 1:1 solution of PEDOT:PSS (Al 4083, Heraeus Clevios) and isopropyl alcohol (IPA) with 5 wt% TX.

Preparation of is-ETLs and Upper ETL
For the is-ETL and upper ETL, a PFN-Br:PEIE:TX solution and a d-PEIE solution were prepared.For the d-PEIE solution, PEIE (Sigma-Aldrich, 37 wt%) and Cs 2 CO 3 (Sigma-Aldrich) were codissolved in 2methoxyethanol (for which the weight ratio of PEIE to Cs 2 CO 3 was 10:1) at a 4 wt% concentration.After sonication of the d-PEIE solution via ultrasonication, the d-PEIE solution was stirred at 80°C for 12 hours.
For the PFN-Br:PEIE:TX solution, PEIE and TX were dissolved in methanol at a concentration of 4 mg ml - 1 .Then, 2 mg of PFN-Br (Ossila) was dissolved in 0.5 ml of PEIE and 0.25 ml of a TX-blended solution (weight ratio of PFN-Br:PEIE:TX of 2:2:1) or 0.5 ml of a TX-blended solution (weight ratio of PFN-Br:PEIE:TX of 2:2:2).Mechanical characterization of is-EMLs, is-ETLs, and is-cathode For the stretchable substrate, a PDMS substrate was cured on a polyethylene terephthalate (PET) substrate.The PDMS (Sylgard 184, Dow Corning) was prepared by mixing the base and agent at a 10:1 weight ratio and spin-coating the mixture on the PET substrate at 500 rpm for 30 s.The spin-coated PDMS was cured at 120 ℃ for 12 h.Then, the cured PDMS was peeled off from the PET substrate and used as a stretchable substrate.
For mechanical characterization of the is-EMLs, COS analysis was performed for various solvent treatments.On the cured PDMS substrate, oxygen plasma was applied at 140 W for 90 s, and the is-HTL was spin-coated at 500 rpm for 60 s and 1000 rpm for 10 s.PDMS with the is-HTL was annealed on a hot plate at 120 ℃ for 15 min.Then, the sample was transferred to a glove box lled with nitrogen gas.The is-EML was spin-coated on the is-HTL at 1500 rpm for 30 s and annealed at 90 ℃ for 10 min.Then, MeOH, IPA, and 2-methoxyethanol were spin-coated on the is-EML at 4000 rpm for 30 s and annealed at 100 ℃ for 10 min.In the case of the is-ETL, the is-ETL was spin-coated at 5000 rpm for 30 s on the plasma-treated PDMS substrate and annealed on the hot plate at 90 ℃ for 10 min in the glove box.The is-EMLs and is-ETLs on the PDMS substrate were loaded on a homemade stretching jig.Strain was applied in increments of 10% from 0-100%.

Fabrication of the ITO anode based rigid OLEDs
The slide glass substrate (Paul Marienfeld & Co. KG, Germany) was cut to 2.5 cm × 2.5 cm and sequentially washed in the sonication baths of acetone, IPA, and deionized water for 5 min each.On precleaned glass, ITO (150 nm thick) was deposited using direct current (DC) magnetron sputtering and annealed at 300 ℃ for 30 min inside a glove box lled with nitrogen gas.On the ITO-coated glass, oxygen plasma was applied at 140 W for 90 s.On the plasma-treated ITO glass, an is-HTL solution was spin-coated at 2000 rpm for 60 s after being ltered through a 1 µm polytetra uoroethylene (PTFE) syringe lter and annealed at 120 ℃ for 20 min.The sample was then transferred into a glove box lled with nitrogen gas.On the is-HTL, the is-EML was spin-coated at 1500 rpm for 30 s and annealed at 100 ℃ for 10 min.Then, the PFN-Br, PEIE, and TX-blended solution was spin-coated on the is-EML at 5000 rpm for 30 s and annealed at 100 ℃ for 10 min.Finally, the Al cathode was thermally evaporated on the is-ETL with a 150 nm thickness.

Fabrication of electron only device
For electron only device fabrication, 150 nm Ag anode was deposited on the pre-cleaned glass substrate using thermal evaporator.On the Ag anode, 0.5 wt% d-PEIE solution was spin coated at 5000 rpm for 30 s and annealed at 100 ℃ for 10 min.It was then transferred into a glove box lled with nitrogen gas.On the d-PEIE, is-EML was spin-coated at 1500 rpm for 30 s and annealed at 100 ℃ for 10 min.Then, PFN-Br, PFNBr:PEIE, PFNBr:PEIE:TX blend solution was spin-coated on the is-EML at 5000 rpm for 30 s and annealed at 100 ℃ for 10 min.Finally, Al cathode was thermally evaporated on the is-ETL with 150 nm thickness.

Preparation of FOTS on Glass substrate
A precleaned glass substrate was rst treated by O2 plasma at 140 W for 90 s.Then, the glass substrate was vapor annealed in a desiccator with trichloro(1H,1H,2H,2H-per uorooctyl)silane (FOTS, Sigma-Aldrich) solution for 30 min and annealed at 150 ℃ for 30 min.

Fabrication of is-OLEDs
For the is-anode, 0.5 ml of a AgNW (with a diameter of 30 nm and a length of 30 µm; Novarials) solution (1 mg ml -1 concentration) was spray-coated on the FOTS-treated glass substrate using a patterned metal mask.On the AgNWs, an aerogel solution with 4 wt% ethanol was spin-coated at 1000 rpm for 30 s and annealed at 100°C for 10 min.Then, a PDMS solution in tetrahydrofuran (THF) solvent (base and agent ratio of 10:1 at 20 mg ml -1 ) was spin-coated on the aerogel surface at 1000 rpm for 30 s.Additionally, PDMS (base and agent ratio of 10:1) was spin-coated at 300 rpm for 30 s and cured at 120°C for 12 hours.After the curing process, the AgNW-embedded PDMS was released from the glass substrate.
The released AgNW-embedded PDMS substrate was reattached to a glass substrate with the AgNW electrode facing up.Before coating with the is-HTL, oxygen plasma was applied at 140 W for 90 s.Then, an is-HTL solution was spin-coated at 500 rpm for 60 s and 1000 rpm for 10 s after being ltered through a 1 µm PTFE syringe lter and annealed at 120 ℃ for 15 min.The is-HTL-coated sample was transferred to a N 2 -lled glove box.In the glove box, the is-EML was spin-coated at 1500 rpm for 30 s and annealed at 90°C for 5 min.On the is-EML, is-ETLs was sequentially coated.An is-ETL solution (PFN-Br:PEIE:TX) was spin-coated at 5000 rpm for 30 s and annealed at 90°C for 5 min.Then, a Ag cathode was evaporated on the is-ETL at 1 Å s -1 up to 60 nm thickness.After the evaporation process, 4 wt% d-PEIE solution was spin-coated on the Ag surface at 1000 rpm for 30 s and annealed at 90°C for 5 min.Finally, the is-OLED was released from the glass substrate.The 3x2 array device fabrication process was almost the same as the is-OLED fabrication process.Only the anode and cathode patterns and glass size were different from those for the is-OLEDs.

Declarations Figures
Design of is-OLEDs and characterization of the is-ETL and is-cathode. of an is-OLED structure from the is-anode to the upper ETL.b is-OLED fabrication process with sequential coating.cDevice structure and components of an is-OLED.d Schematics of the is-EML morphology change after ETL solvent coating.Based on the various analysis results, by adding Triton X into theETL solvent, the SY chains maintain an extended morphology after the is-ETL coating process, resulting in high stretchability.
e Schematics of the is-cathode morphology and electron conducting mechanism under stretching conditions.
Design and characterization of is-ETLs for highly stretchable is-functional layers.a UV-vis absorption spectra of is-EMLs with various TX blending ratios, and solvent coating effect on the is-EMLs.b UV-vis absorption spectra of is-EMLs before and after TX-blended solvent coating (TX1 = 1 mg ml -1 , TX2 = 2 mg ml -2 , TX4 = 4 mg ml -1 ).c Photoluminescence spectra of is-EMLs before and after TX-blended solvent coating.d COS of is-EMLs before and after TX-blended solvent coating.e COS of is-ETLs with various component ratios when coated on PDMS or is-EMLs.f UPS intensity of is-ETL and Ag lms with various alignment diagram for the conventional OLED device with an is-ETL.i V-L curve, j J-V curve, and k ε c -L curve of the conventional OLED device with an is-ETL on a glass substrate.
Fabrication and characterization of is-OLEDs.a Schematics of the is-cathode surface morphologies from a thin Ag to thick Ag lm.bSheet resistance, stretchability of the is-cathode, and luminance of the is-OLED at various is-cathode thicknesses.c Optimal Ag evaporation conditions for the is-cathode.dResistance change in Ag cathode lms with various thicknesses rangingfrom 30 nm to 70 nm during static stretching tests.e Resistance change in the 60 nm is-cathode during cyclic stretching tests at 20% and 40% strain.f TEM cross-sectional image of the is-OLED.g SEM image of the is-cathode surface at 0% and 40% strain.h SEM image of the is-cathode surface when stretched to 60% strain and released to 0% strain from 40% strain.

Figure 4
Figure 4 Device performance of is-OLEDs and deformable displays.a Schematics of the is-OLED structure and thickness.b Energy-level alignment diagram of the is-OLED.c J-V-L curve of the is-OLEDs.d Relative L change (L•L 0 − 1 ) of the is-OLEDs during static stretching tests when operated at 8 V. e L•L 0 −

Figure 4 Device
Figure 4