Seonil Kwon

A Review of Flexible OLEDs Toward Highly Durable Unusual Displays

Organic light-emitting diodes (OLEDs) are remarkably promising display devices that can function in mechanically flexible configurations on a plastic substrate due to various compelling properties, including organic constituents, ultrathin and simple structure, and low-temperature fabrication. In spite of successful demonstrations of flexible OLEDs, some technical issues of containing relatively thick transparent electrodes made of ceramic materials and an unstable flexible encapsulation system have impeded reaching high levels of reliability and durability toward full commercialization. This review covers recent developments in structure designs for highly durable flexible OLEDs, ranging from alternative transparent electrodes to thin-film encapsulation layers, in which solution concepts for the existing critical issues of flexible OLEDs are addressed. Emerging unusual substrates and their application strategies are additionally introduced to find intimations of future display technologies and hence to disclose nonclassic flexible OLEDs.

Reliable thin-film encapsulation of flexible OLEDs and enhancing their bending characteristics through mechanical analysis

Thin film encapsulation of flexible organic light-emitting diodes (FOLEDs) with a moisture barrier, incorporating a silica nanoparticle-embedded sol–gel organic–inorganic hybrid nanocomposite (S–H nanocomposite) and Al2O3 were demonstrated, and their reliability and mechanical characteristics were assessed. The bending stress of the multi-layer structure for both the case of moisture barriers and encapsulated FOLEDs was investigated based on nonlinear finite-element analysis (FEA). To minimize the bending stress at the desired region, the neutral axis (NA) position could be strategically adjusted by the introduction of a buffer layer of UV-curable cycloaliphatic epoxy hybrid materials (hybrimer), synthesized via a sol–gel reaction. The optimized multi-layer structure, proposed as a result of FEA was validated by related experiments. Regarding the bending characteristics of the moisture barrier structure, the water vapor transmission rate (WVTR) of the hybrimer-coated moisture barrier was much lower than that of a non-coated sample, as a result of calcium corrosion tests after bending. The structure of encapsulated FOLEDs, which are coated by the hybrimer achieved an almost identical performance to that of non-bending samples in spite of 30 days exposure to 30 °C and 90% R.H. after a bending test with a radius of 1 cm. During this period, the occurrence of dark spots caused by moisture penetration was effectively suppressed. Collectively, these results suggest that the bending characteristics of hybrimer-coated multi-layer structures are remarkably improved with the theoretical prediction of the NA position.

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.

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.