Two-Dimensionally Stretchable Organic Light-Emitting Diode with Elastic Pillar Arrays for Stress Relief.

Recent advanced studies on flexible and stretchable electronic devices and optoelectronics have made possible a variety of soft and more functional electronic devices. With consumer demand for highly functional or free-form displays, high flexibility and stretchability in light-emitting devices are needed. Herein, we developed a unique structure of stretchable substrates with pillar arrays to reduce the stress on the active area of devices when strain is applied. We confirmed the advantages of the produced structures using mechanical simulation tools and determined that the structures effectively lessen the applied stress of interconnection as well as the active area in a stretched state. With this stress-relief stretchable substrate, we realized stretchable OLEDs that are compliant and maintain their performance under high strain deformation. Also, devices can be stretched in the biaxis, which is superior to only one-directional stretchable electronics; as such, devices can be used in practical applications like wearable electronics and health monitoring systems. We propose, for the first time, stretchable OLEDs patterned by the thermal evaporation fabrication process onto stress-relief substrates. These OLEDs can mitigate certain problems in previous studies of stretchable OLEDs without need to find new materials or to use a prestrained fabrication process.

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.

Highly stable 2D material (2DM) field-effect transistors (FETs) with wafer-scale multidyad encapsulation.

Field-effect transistors (FETs) composed of 2D materials (2DMs) such as transition-metal dichalcogenide (TMD) materials show unstable electrical characteristics in ambient air due to the high sensitivity of 2DMs to water adsorbates. In this work, in order to demonstrate the long-term retention of electrical characteristics of a TMD FET, a multidyad encapsulation method was applied to a MoS2 FET and thereby its durability was warranted for one month. It was well known that the multidyad encapsulation method was effective to mitigate high sensitivity to ambient air in light-emitting diodes (LEDs) composed of organic materials. However, there was no attempt to check the feasibility of such a multidyad encapsulation method for 2DM FETs. It is timely to investigate the water vapor transmission ratio (WVTR) required for long-term stability of 2DM FETs. The 2DM FETs were fabricated with MoS2 flakes by both an exfoliation method, that is desirable to attain high quality film, and a chemical vapor deposition (CVD) method, that is applicable to fabrication for a large-sized substrate. In order to eliminate other unwanted variables, the MoS2 FETs composed of exfoliated flakes were primarily investigated to assure the effectiveness of the encapsulation method. The encapsulation method uses multiple dyads comprised of a polymer layer by spin coating and an Al2O3 layer deposited by atomic layer deposition (ALD). The proposed method shows wafer-scale uniformity, high transparency, and protective barrier properties against adsorbates (WVTR of 8 × 10-6 g m-2 day-1) over one month.

Microcavity effect using nanoparticles to enhance the efficiency of organic light-emitting diodes.

In this paper, in contrast with previously reported approaches, we suggest exploiting a microcavity effect using nanoparticles to improve the optical efficiency of organic light-emitting diodes (OLED). The method to input the nanoparticles inside the OLED device is simple and cost effective by virtue of employing a solution process using a spin coating fabrication method. Titanium dioxide (TiO2) nanoparticles were used to improve the reflection by its high refractive index. In tandem with optimized heights of the organic layers, the increased light reflectance at the anode side, which includes the TiO2 nanoparticle layer, improved the optical efficiency of the OLED device via the microcavity effect. In order to prove that the enhancement of the optical efficiency was due to an enhanced microcavity effect caused by TiO2 nanoparticles, a microcavity simulation was conducted. The electrical characteristics were not affected by the nanoparticles and a clear pixel image was maintained. The results in this paper show that a nanoparticle based microcavity effect can be exploited to enhance the optical efficiency of OLEDs.

Poly-periodic hole arrays for angle-invariant plasmonic filters.

We suggest a plasmonic filter with novel hole arrays for an angle-invariant optical response. The suggested patterns consist of randomly distributed polycrystalline domains in which nano sized holes are arranged with the same period. While the microscopic area of periodicity determines the center wavelength and transmission intensity, the broken periodicity of each domain contributes to restrain the angle dependency. The results increase the possible use of nanohole-based filters in practical area.

Graphene-enabled laser lift-off for ultrathin displays.

Laser lift-off (LLO) of ultrathin polyimide (PI) films is important in the manufacturing of ultrathin displays. However, conventional LLO technologies face challenges in separating the ultrathin PI films without causing mechanical and electrical damage to integrated devices. Here, we propose a graphene-enabled laser lift-off (GLLO) method to address the challenges. The GLLO method is developed by integrating chemical vapor deposition (CVD)-grown graphene at the interface between a transparent carrier and an ultrathin PI film, exhibiting improved processability and lift-off quality. In particular, the GLLO method significantly mitigates plastic deformation of the PI film and minimizes carbonaceous residues remaining on the carrier. The role of graphene is attributed to three factors: enhancement of interfacial UV absorption, lateral heat diffusion, and adhesion reduction, and experimentations and numerical simulations verify the mechanism. Finally, it is demonstrated that the GLLO method separates ultrathin organic light-emitting diode (OLED) devices without compromising performance. We believe that this work will pave the way for utilizing CVD graphene in various laser-based manufacturing applications.

High Mobility, Low Off-Current, and Flexible Fiber-Based a-InGaZnO Thin-Film Transistors toward Wearable Textile OLED Displays.

Fiber-based organic light-emitting diodes (OLEDs) are gaining attention as promising candidates to achieve truly wearable textile displays because of their favorable electrical and mechanical characteristics. However, although fiber OLEDs have been developed into passive-matrix displays, it has not been possible to achieve active OLED operation because of the difficulty of realizing fiber-based thin film transistors (TFTs) with the proper electrical and mechanical performance at the same time. Here, 1D cylindrical fiber-based IGZO TFTs, which simultaneously exhibit a high electrical performance and flexibility, are reported. To address this trade-off relationship, four key stages of a novel fabrication process and unique device structures that suitable for the thermal properties and cylindrical structure of the fiber were applied: (I) prethermal treatment, (II) partially patterned layers, (III) coplanar structure, and (IV) continuous postannealing (CPA) process. As a result, the fabricated fiber-based IGZO TFTs showed high mobility (8.6 cm2/(V s)) and low off-current (∼10-12 A), comparable to that glass-based TFTs, as well as flexibility. Furthermore, based on these valid performances, it was demonstrated that fiber phOLEDs could be driven by fiber-based IGZO TFTs using a wiring connection with Cu wire and Ag paste. The results suggest that this may allow the potential fabrication of fully textile AMOLED displays, integrated with TFTs.

Quantum-Dot Light-Emitting Fiber Toward All-In-One Clothing-Type Health Monitoring.

In the Fourth Industrial Revolution, as the connection between objects and people becomes increasingly important, interest in wearable optoelectronic device-based medical diagnosis is on the rise. Pulse oximetry sensors based on a fiber platform, which is the smallest unit of clothing, could be considered an attractive candidate for this application. In this study, red and green quantum-dot light-emitting fibers (QDLEFs) based on a 250 µm-diameter 1-dimensional fiber were successfully implemented, achieving high current efficiencies of approximately 22.46 mW/sr/A and 23.6 mW/sr/A and narrow full-width at half-maximum (FWHM) of about 33 nm, respectively. In addition, its omnidirectional flexibility was confirmed through a vertical and lateral bending test with 0.92% strain. By employing a transparent and flexible elastomer, a wearable pulse oximeter incorporating QDLEFs was successfully demonstrated for oxygen saturation level (SpO2) monitoring on finger and wrist. It was demonstrated to be washable, and could be operated for up to about 18 h. Due to the elastomer and bottom emission, it exhibited excellent wear resistance characteristics in a 50 cycle reciprocating test conducted at about 2180.43 kPa with 220-grit abrasive paper sheet. A theoretical investigation based on modified photon diffusion analysis (MPDA) modeling also determined that using narrow FWHM light sources, such as QDLEFs, improves the resolution and accuracy of SpO2 monitoring. Accordingly, the proposed QDLEF showed distinguished potential as an all-in-one clothing type pulse oximetry.

Extracting optical modes of organic light-emitting diodes using quasi-periodic WO3 nanoislands.

Quasi-periodic WO(3) nanoislands are introduced to extract two optical modes in organic light-emitting diodes. The nano-scaled and size-tunable WO(3) islands were fabricated by use of wet-etching with self-aggregated Ag mask. The improvement of light extraction efficiency originates to the recovery of light losses which are surface plasmon mode and waveguide mode. As a result, external quantum efficiency and power efficiency were increased. No changes in emission spectrum and CIE color coordinates with WO(3) nanoislands at various observation angles are desirable if this device is to be utilized in optical system. Furthermore, cost-effective fabrication makes it possible to adopt this system in large area fabrication.

Distance-dependent plasmonic enhancement via radiative transitions of europium complex.

We elucidated the distance-dependent plasmonic effects on radiative transitions from an Eu(3+) ion-doped phosphor by varying the thickness of the dielectric spacer. Magnesium oxide prepared by electron-beam evaporation was chosen for the dielectric spacer. Spectral overlap between emission from Eu(3+) ions and the plasmon band of Ag nanoparticles led to improved luminescence intensity. This luminous enhancement was effective within the area of influence by localized surface plasmon resonance. At a long distance, the plasmon-enhanced luminescence was not effective. In addition, the numerical analysis results were in good agreement with the distance-dependent decay characteristics of plasmon resonance.

Heteroleptic Ir(III)-based near-infrared organic light-emitting diodes with high radiance capacity.

Near-infrared organic light-emitting diodes (NIR OLEDs) with heavy metals are regularly reported due to the advantages of their various applications in healthcare services, veil authentication, and night vision displays. For commercial applications, it is necessary to look at radiance capacity (RC) instead of radiance because of power consumption. However, recent papers still reported only simple high radiance performance and do not look at device from the point of view of RC. To overcome this hurdle, we designed Ir(III)-based heteroleptic NIR materials with two types of auxiliary ligand. The proposed emitters achieve a highly oriented horizontal dipole ratio (Ir(mCPDTiq)2tmd, complex 1: 80%, Ir(mCPDTiq)2acac, complex 2: 81%) with a short radiative lifetime (1: 386 ns, 2: 323 ns). The device also shows an extremely low turn-on voltage (Von) of 2.2 V and a high RC of 720 mW/sr/m2/V. The results on the Von and RC of the device is demonstrated an outstanding performance among the Ir(III)-based NIR OLEDs with a similar emission peak.

Highly Air-Stable, Flexible, and Water-Resistive 2D Titanium Carbide MXene-Based RGB Organic Light-Emitting Diode Displays for Transparent Free-Form Electronics.

Flexible see-through displays are considered to be the next generation smart display, providing improved information flow, safety, situational awareness, and overall user experience in smart windows, automotive displays, glass-form biomedical displays, and augmented reality systems. 2D titanium carbides (MXenes) are promising material as electrodes of the transparent and flexible displays due to their high transparency, metallic conductivity, and flexibility. However, current MXene-based devices have insufficient air stability and lack engineering schemes to develop matrix-addressable display forms with sufficient pixels to display information. Here, we develop an ultraflexible and environmentally stable MXene-based organic light-emitting diode (OLED) display by combining high performance MXene electrodes, flexible OLEDs, and ultrathin and functional encapsulation systems. The MXene material was synthesized and used to fabricate a highly reliable MXene-based OLED that can stably operate in air condition for over 2000 h, endure repetitive bending deformation of 1.5 mm radius, and maintain environmental stability for 6 h when exposed to wet surroundings. The RGB MXene-based OLEDs were fabricated, (1691 cd m-2 at 40.4 mA cm-2 for red, 1377 cd m-2 at 4.26 mA cm-2 for green, and 1475 cd m-2 at 18.6 mA cm-2 for blue) and a matrix-addressable transparent OLED display was demonstrated that could display letters and shapes.

Fiber-based quantum-dot pulse oximetry for wearable health monitoring with high wavelength selectivity and photoplethysmogram sensitivity.

Increasing demand for real-time healthcare monitoring is leading to advances in thin and flexible optoelectronic device-based wearable pulse oximetry. Most previous studies have used OLEDs for this purpose, but did not consider the side effects of broad full-width half-maximum (FWHM) characteristics and single substrates. In this study, we performed SpO2 measurement using a fiber-based quantum-dot pulse oximetry (FQPO) system capable of mass production with a transferable encapsulation technique, and a narrow FWHM of about 30 nm. Based on analyses we determined that uniform angular narrow FWHM-based light sources are important for accurate SpO2 measurements through multi-layer structures and human skin tissues. The FQPO was shown to have improved photoplethysmogram (PPG) signal sensitivity with no waveguide-mode noise signal, as is typically generated when using a single substrate (30-50%). We successfully demonstrate improved SpO2 measurement accuracy as well as all-in-one clothing-type pulse oximetry with FQPO.

Study of mechanical degradation of freestanding ALD Al2O3 by a hygrothermal environment and a facile protective method for environmentally stable Al2O3: toward highly reliable wearable OLEDs.

Al2O3 deposited via atomic layer deposition (ALD) has been used as an insulating and barrier film for thin-film transistors, organic electronics, and microelectromechanical systems. However, ALD Al2O3 films are easily degraded by hydrolysis under harsh hygrothermal conditions, owing to their poor environmental stability. In this study, the mechanical properties and water-vapor transmission rate (WVTR) of environmentally degraded Al2O3 films were investigated by varying the temperature and relative humidity (RH). The hygrothermal environment led to surface and pinhole-concentrated degradation based on aluminum hydroxide, which caused an increased WVTR and reduced elongation of the films in harsher environments. In particular, the elongation of the degraded Al2O3 films was reduced to 0.3%, which is one-third of that of as-deposited Al2O3, and their WVTR increased on the order of 10-1 g m-2 day-1, which is more than 1000 times that of as-deposited Al2O3. Therefore, we introduced a functional silane-based inorganic-organic hybrid layer (silamer) onto the Al2O3 films to improve their environmental stability. The silamer helped preserve the characteristics of Al2O3 films by forming a strong and continuous aluminate phase of Al-O-Si at their interface in hygrothermal environments. Furthermore, the silamer-capped Al2O3 was shown to be an environmentally stable encapsulation for application in wearable organic devices.

Wearable and Wavelength-Tunable Near-Infrared Organic Light-Emitting Diodes for Biomedical Applications.

Near-infrared organic light-emitting diodes (NIR OLEDs) have significant potential for wearable phototherapeutic applications because of the unique properties of the OLEDs, including their free-form electronics and the excellent biomedical effects of NIR emission. In spite of their tremendous promise, given that the majority of NIR OLEDs in previous research have relied on the utilization of an intrinsically brittle indium tin oxide (ITO) electrode, their practicality in the field of wearable electronics is inherently constrained. Here, we report wearable and wavelength-tunable NIR OLEDs that employ a high-performance NIR emitter and an innovative architecture by replacing the ITO with a silver (Ag) electrode. The NIR OLEDs permit wavelength tuning of emissions from 700 to 800 nm and afford stable operation even under repeated bending conditions. The NIR OLEDs provide a lowered device temperature of 37.5 °C even during continuous operation under several emission intensities. In vitro experiments were performed with freshly fabricated NIR OLEDs. The outcomes were evaluated against experimental results performed using the same procedure utilizing blue, green, and red OLEDs. When exposed to NIR light irradiation, the promoting effect of cell proliferation surpassed the proliferative responses observed under the influence of visible light irradiation. The proliferation effect of human hair follicle dermal papilla cells is clearly related to the irradiation wavelength and time, thus underscoring the potential of wavelength-tunable NIR OLEDs for efficacious phototherapy. This work will open novel avenues for wearable NIR OLEDs in the field of biomedical application.

Nanoplasmon-enhanced transparent plasma display devices.

Plasmon-enhanced transparent plasma display devices are demonstrated via the resonant interface between Ag nanoparticles and a Eu(3+)-doped phosphor. Enhanced emission from the phosphor by metallic nanoparticles leads to an increase of the luminous efficacy in the transparent plasma display device. This is a prototype of the plasmon-enhanced transparent plasma display device.

Optical characteristics of YVO4:Eu3+ phosphor in close proximity to Ag nanofilm: emitting layer for mirror-type displays.

We demonstrate the optical characteristics of YVO4:Eu3+ phosphor in close proximity to Ag nanofilm to create a highly efficient emitting layer in mirror-type self-emissive displays. The propagating surface plasmon mode induced between the dielectric layer (MgO) and the Ag nanofilm activates the electric dipole transition of Eu3+ ions. The transmittance of a 100 nm-thick Ag nanofilm is zero in the visible wavelength range, making this nanofilm a good reflector in the visible wavelength range and capable of fulfilling a mirror function. The emission of an YVO4:Eu3+ phosphor layer with a 100 nm-thick Ag nanofilm was enhanced to the point that it was eight times higher than that of a reference sample without Ag nanofilm. Therefore, the present work shows potential for application to mirror-type displays with high luminous efficacy.

Improved light extraction efficiency in organic light emitting diodes with a perforated WO3 hole injection layer fabricated by use of colloidal lithography.

We present an organic light emitting diode with a perforated WO3 hole injection layer to improve the light extraction efficiency. The two-dimensionally perforated WO3 layer was fabricated by use of colloidal lithography. The light extraction efficiency was improved due to Bragg scattering of waveguide modes and surface plasmon polaritons, and the operating voltage was also decreased. As a result, the external quantum efficiency and the power efficiency were increased as compared with those of conventional organic light emitting diodes without WO3 layer. The angular dependence of emission characteristics was investigated by measuring radiant intensity profiles for emission angles and azimuthal angles.

Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots.

We investigated surface plasmon-waveguide hybrid resonances for enhancement of light emission in polymer light-emitting diodes (PLEDs). Hybrid waveguide-plasmon resonances in the visible range for waveguide mode and near IR range for surface plasmons were observed by incorporation of hexagonal Ag dot arrays. Considerable overlap between the emission wavelength of the PLEDs and the waveguide mode by an Ag dot array with a lattice constant of 500 nm was observed. Because of enhanced light extraction by Bragg scattering of waveguide modes, photoluminescence (PL) and electroluminescence (EL) were increased by 70% and 50%, respectively.

High-Performance and Reliable White Organic Light-Emitting Fibers for Truly Wearable Textile Displays.

Light-emitting fibers have been intensively developed for the realization of textile displays and various lighting applications, as promising free-form electronics with outstanding interconnectivity. These advances in the fiber displays have been made possible by the successful implementation of the core technologies of conventional displays, including high optoelectronic performance and essential elements, in the fiber form-factor. However, although white organic light-emitting diodes (WOLEDs), as a fundamental core technology of displays, are essential for realizing full-color displays and solid-state lighting, fiber-based WOLEDs are still challenging due to structural issues and the lack of approaches to implementing WOLEDs on fiber. Herein, the first fiber WOLED is reported, exhibiting high optoelectronic performance and a reliable color index, comparable to those of conventional planar WOLEDs. As key features, it is found that WOLEDs can be successfully introduced on a cylindrical fiber using a dip-coatable single white-emission layer based on simulation and optimization of the white spectra. Furthermore, to ensure durability from usage, the fiber WOLED is encapsulated by an Al2 O3 /elastomer bilayer, showing stable operation under repetitive bending and pressure, and in water. This pioneering work is believed to provide building blocks for realizing complete textile display technologies by complementing the lack of the core technology.