Seungyeop Choi

Solution-processed bottom-emitting polymer light-emitting diodes on a textile substrate towards a wearable display

Proposed herein are textile-based polymer light-emitting diodes (PLEDs) prepared via a spin coating process for a wearable display. Multilayers consisting of alumina (Al2O3) and poly(vinyl alcohol) were used as a planarization layer to fabricate PLEDs on a textile, and non-inverted bottom-emitting PLEDs were fabricated on a planarized textile. Despite the light loss caused by the woven and opaque textile substrate, the fabricated device showed high maximum luminous efficiency (9.72 cd/A) and high maximum power efficiency (7.17 lm/W), and no angular dependency of the EL spectrum was observed on the textile-based PLEDs. In addition, the fabricated PLEDs showed stable operation under bending stress with a bending radius of 2.5 mm. In summary, high-performance PLEDs were developed using a solution process on a real textile, and the feasibility of realizing textile-based PLEDs for a large-area wearable display is suggested.

Highly Flexible and Efficient Fabric-Based Organic Light-Emitting Devices for Clothing-Shaped Wearable Displays

Recently, the role of clothing has evolved from merely body protection, maintaining the body temperature, and fashion, to advanced functions such as various types of information delivery, communication, and even augmented reality. With a wireless internet connection, the integration of circuits and sensors, and a portable power supply, clothes become a novel electronic device. Currently, the information display is the most intuitive interface using visualized communication methods and the simultaneous concurrent processing of inputs and outputs between a wearer and functional clothes. The important aspect in this case is to maintain the characteristic softness of the fabrics even when electronic devices are added to the flexible clothes. Silicone-based light-emitting diode (LED) jackets, shirts, and stage costumes have started to appear, but the intrinsic stiffness of inorganic semiconductors causes wearers to feel discomfort; thus, it is difficult to use such devices for everyday purposes. To address this problem, a method of fabricating a thin and flexible emitting fabric utilizing organic light-emitting diodes (OLEDs) was developed in this work. Its flexibility was evaluated, and an analysis of its mechanical bending characteristics and tests of its long-term reliability were carried out.

Functional Design of Dielectric–Metal–Dielectric-Based Thin-Film Encapsulation with Heat Transfer and Flexibility for Flexible Displays

In this study, a new and efficient dielectric-metal-dielectric-based thin-film encapsulation (DMD-TFE) with an inserted Ag thin film is proposed to guarantee the reliability of flexible displays by improving the barrier properties, mechanical flexibility, and heat dissipation, which are considered to be essential requirements for organic light-emitting diode (OLED) encapsulation. The DMD-TFE, which is composed of Al2O3, Ag, and a silica nanoparticle-embedded sol-gel hybrid nanocomposite, shows a water vapor transmission rate of 8.70 × 10-6 g/m2/day and good mechanical reliability at a bending radius of 30 mm, corresponding to 0.41% strain for 1000 bending cycles. The electrical performance of a thin-film encapsulated phosphorescent organic light-emitting diode (PHOLED) was identical to that of a glass-lid encapsulated PHOLED. The operational lifetimes of the thin-film encapsulated and glass-lid encapsulated PHOLEDs are 832 and 754 h, respectively. After 80 days, the thin-film encapsulated PHOLED did not show performance degradation or dark spots on the cell image in a shelf-lifetime test. Finally, the difference in lifetime of the OLED devices in relation to the presence and thickness of a Ag film was analyzed by applying various TFE structures to fluorescent organic light-emitting diodes (FOLEDs) that could generate high amounts of heat. To demonstrate the difference in heat dissipation effect among the TFE structures, the saturated temperatures of the encapsulated FOLEDs were measured from the back side surface of the glass substrate, and were found to be 67.78, 65.12, 60.44, and 39.67 °C after all encapsulated FOLEDs were operated at an initial luminance of 10 000 cd/m2 for sufficient heat generation. Furthermore, the operational lifetime tests of the encapsulated FOLED devices showed results that were consistent with the measurements of real-time temperature profiles taken with an infrared camera. A multifunctional hybrid thin-film encapsulation based on a dielectric-metal-dielectric structure was thus effectively designed considering the transmittance, gas-permeation barrier properties, flexibility, and heat dissipation effect by exploiting the advantages of each separate layer.

Weavable and Highly Efficient Organic Light-Emitting Fibers for Wearable Electronics: A Scalable, Low-Temperature Process

Fiber-based wearable displays, one of the most desirable requisites of electronic textiles (e-textiles), have emerged as a technology for their capability to revolutionize textile and fashion industries in collaboration with the state-of-the-art electronics. Nonetheless, challenges remain for the fibertronic approaches, because fiber-based light-emitting devices suffer from much lower performance than those fabricated on planar substrates. Here, we report weavable and highly efficient fiber-based organic light-emitting diodes (fiber OLEDs) based on a simple, cost-effective and low-temperature solution process. The values obtained for the fiber OLEDs, including efficiency and lifetime, are similar to that of conventional glass-based counterparts, which means that these state-of-the-art, highly efficient solution processed planar OLEDs can be applied to cylindrical shaped fibers without a reduction in performance. The fiber OLEDs withstand tensile strain up to 4.3% at a radius of 3.5 mm and are verified to be weavable into textiles and knitted clothes by hand-weaving demonstrations. Furthermore, to ensure the scalability of the proposed scheme fiber OLEDs with several diameters of 300, 220, 120, and 90 μm, thinner than a human hair, are demonstrated successfully. We believe that this approach, suitable for cost-effective reel-to-reel production, can realize low-cost commercially feasible fiber-based wearable displays in the future.

Functional Design of Highly Robust and Flexible Thin-Film Encapsulation Composed of Quasi-Perfect Sublayers for Transparent, Flexible Displays

In this study, a structurally and materially designed thin-film encapsulation is proposed to guarantee the reliability of transparent, flexible displays by significantly improving their barrier properties, mechanical stability, and environmental reliability, all of which are essential for organic light-emitting diode (OLED) encapsulation. We fabricated a bioinspired, nacre-like ZnO/Al2O3/MgO laminate structure (ZAM) using atomic layer deposition for the microcrack toughening effect. The ZAM film was formed with intentional voids and defects through the formation of a quasi-perfect sublayer, rather than the simple fabrication of nanolaminate structures. The 240 nm thick ZAM-based multibarrier (ZAM-TFE) with a compressively strained organic layer demonstrated an optical transmittance of 91.35% in the visible range, an extremely low water vapor transmission rate of 2.06 × 10-6 g/m2/day, a mechanical stability enduring a strain close to 1%, and a residual stress close to 0, showing significant improvement of key TFE properties in comparison to an Al2O3-based multibarrier. In addition, ZAM-TFE demonstrated superior environmental resistance without degradation of barrier properties in a severe environment of 85 °C and 90% relative humidity (RH). Thus, our structurally and materially designed ZAM film has been well optimized in terms of its applicability as a gas diffusion barrier as well as in terms of its mechanical and environmental reliability. Finally, we confirmed the feasibility of the ZAM-TFE through application in OLEDs. The low-temperature ZAM-TFE technology showed great potential to provide a highly robust and flexible TFE of TFOLEDs.

Synergistic gas diffusion multilayer architecture based on the nanolaminate and inorganic-organic hybrid organic layer

Al2O3 films have long been widely used as inorganic encapsulation or passivation layers. The Al2O3 single layer, however, exhibits not only a relatively low barrier performance but also poor environmental stability under harsh conditions due to its hydrolysis reaction with water vapor. Thus, to further improve its environmental reliability and barrier performance as a gas diffusion barrier (GDB), the GDB should be newly designed by forming a nanolaminate structure with ultra-thin sublayers. In addition, through the use of a multilayer based on nanolaminate/organic layers, the nanolaminate film can be effectively protected by a SiO2-inserted organic layer. In this study, alternately stacked nanolaminate/silane-based organic layers are proposed. The nanolaminate-based multilayer achieved a water vapor transmission rate (WVTR) of 5.94 × 10−5 g/m2/day under 60°C/90% accelerated conditions. In addition, after a bending test, the nanolaminate-based multilayer showed a WVTR increase by a magnitude of one order under a 0.63% bending strain. The proposed environmentally and mechanically stable hybrid thin-film encapsulation offers a strong potential for the realization of washable, wearable, or flexible displays in the future.