The increasing demand for wearable electronics necessitates the development of flexible and stretchable organic light-emitting diodes (OLEDs) that can seamlessly integrate with the human body. Traditional OLEDs are rigid and unsuitable for such applications. While various strategies exist to create stretchable OLEDs from rigid materials, these often compromise performance, stretchability, or resolution. Intrinsically stretchable OLEDs (is-OLEDs), using stretchable polymer materials, offer a promising alternative. However, challenges remain in refining manufacturing processes and designing materials to ensure both high performance and significant stretchability. Previous research has focused on modifying the emissive layer (EML) to enhance stretchability through material blending and molecular design. However, insufficient attention has been given to other layers and fabrication processes. Conventional fabrication methods, like solution coating, often suffer from solvent orthogonality problems, leading to the destruction of the underlying layer's morphology. Lamination, an alternative technique, faces issues with alignment, defects, and delamination, which negatively impact the device's performance and yield. This research addresses these challenges by developing a novel fabrication process and improved materials for is-OLEDs.

Existing research on is-OLEDs has primarily focused on improving the stretchability of the inherently rigid EML. Techniques include blending the EML with materials like Triton X-100 (TX), PEO-PPG-PEO, polyurethane (PU), and SBS elastomers to modify the microstructure. Molecular design incorporating soft substances like alkyl chains or butadiene has also been explored. However, these studies often neglect the challenges associated with other layers (e.g., electron transport layer, ETL, and cathode) and the fabrication process. The lack of solvent orthogonality between layers in solution-coated is-OLEDs often causes damage to the underlying layers and compromises stretchability. Lamination, while used in some studies to overcome solvent issues, introduces new challenges including alignment difficulties, defects, and delamination, resulting in reduced performance and yield. The choice of cathode material also plays a crucial role; although high-performance materials like aluminum or silver offer high charge carrier mobility, they lack intrinsic stretchability. Existing alternatives like silver nanowires (AgNWs), graphene, or PEDOT:PSS require complex lamination processes, which are less efficient and can negatively affect performance.

This study employs a hybrid fabrication process combining sequential solution coating and thermal evaporation to create high-performance is-OLEDs. The is-EML is prepared by blending a commercial EML (Super Yellow, SY) with TX in toluene. The is-HTL uses a 1:1 solution of PEDOT:PSS and isopropyl alcohol (IPA) with 5 wt% TX. The is-ETL incorporates PFN-Br, PEIE, and TX in methanol, where the ratio of PFN-Br:PEIE:TX is optimized (2:2:2). A d-PEIE solution is used as an upper ETL. The stretchable substrate is fabricated from PDMS cured on a PET substrate. The mechanical characterization of the is-EML, is-ETL, and is-cathode is performed using contact angle (COS) analysis and stretching tests. For the is-cathode, a highly stretchable Ag electrode is developed by carefully controlling the thermal evaporation parameters. The Ag deposition process is optimized to obtain a partially connected film morphology with improved conductivity and stretchability. The is-anode is fabricated using spray-coated AgNWs embedded in PDMS. The is-OLEDs are fabricated via sequential solution coating and thermal evaporation, avoiding lamination. Characterization techniques include atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), ultraviolet-visible (UV-Vis) spectrophotometry, and current-voltage-luminance (J-V-L) measurements. Electron-only devices are also fabricated to study electron transport properties.

The incorporation of TX into the is-ETL solution is crucial for maintaining the extended morphology of the is-EML during the sequential coating process. The addition of TX to the PFN-Br:PEIE ETL blend increases the COS and enhances the stretchability of the is-ETL, while also maintaining the morphology of the is-EML. The optimized ternary blend of PFN-Br:PEIE:TX (2:2:2) in the is-ETL exhibits high stretchability (up to 100% COS) and improved electron transport properties. The stretchable Ag cathode is fabricated by precisely controlling the deposition conditions, resulting in a partially connected film morphology with microcracks that accommodate strain without significantly compromising conductivity. The Ag60 cathode (60 nm thick, deposited at 1 Å s⁻¹) demonstrates a low sheet resistance of 6 Ω/sq, high luminance, and maintains R_s/R_s0 < 10 under 70% strain. The is-OLEDs achieve a maximum total luminance of 3151 cd m⁻² and a current efficiency of 5.4 cd A⁻¹. The device retains approximately 50% performance at 70% strain and 80% performance after 300 cycles of stretching tests at 40% strain. This surpasses the performance of is-OLEDs fabricated using lamination methods, demonstrating enhanced durability and simplified fabrication. The improved performance is attributed to the synergistic effects of the morphology-sustainable is-ETL and the stretchable Ag cathode. The devices demonstrate consistent light emission under various deformations, including stretching, bending, folding, and crumpling.

The findings demonstrate the successful fabrication of high-performance, highly stretchable is-OLEDs using a novel sequential coating method that avoids the limitations of traditional lamination techniques. The designed morphology-sustainable is-ETL and the highly stretchable Ag metal cathode are key to this achievement. The incorporation of TX in the ETL not only maintains the extended morphology of the is-EML but also improves the electron transport properties. The optimized deposition conditions for the Ag cathode result in a unique partially connected film morphology that allows for reversible crack formation and closure under strain, preserving conductivity. The results significantly advance is-OLED technology, offering a simpler fabrication process compared to lamination while achieving superior cyclic stretchability and comparable luminance and efficiency. This work contributes to the broader field of flexible and wearable electronics by providing a practical pathway for producing durable and high-performance soft displays.

This study successfully demonstrates the fabrication of intrinsically stretchable OLEDs using a novel sequential coating method. The key advancements are the development of a morphology-sustainable is-ETL and a highly stretchable Ag metal cathode. The resulting is-OLEDs show superior performance and durability compared to those made with lamination processes. Future research should focus on improving the device lifetime and efficiency, which can be addressed by incorporating encapsulation layers and improving the anode design.

The current study's is-OLEDs exhibit a relatively short operational lifespan (approximately 1 minute) and lower efficiency compared to conventional OLEDs. The lack of encapsulation contributes to device degradation due to oxygen and moisture exposure. The use of AgNW-embedded PDMS as the anode substrate introduces limitations due to the AgNWs' one-dimensional morphology and low work function, leading to reduced contact area, compromised hole injection, and increased leakage current. Further improvements are needed to fully realize the potential of this technology for practical applications.