Color gamut change by optical crosstalk in high-resolution organic light-emitting diode microdisplays.

Herein, the color gamut change by optical crosstalk between sub-pixels in high-resolution full-color organic light-emitting diode (OLED) microdisplays was numerically investigated. The color gamut of the OLED microdisplay decreased dramatically as the pixel density of the panel increased from 100 pixels per inch (PPI) to 3000 PPI. In addition, the increase in thickness of the passivation layer between the bottom electrode and the top color filter results in a decrease in the color gamut. We also calculated the color gamut change depending on the pixel structures in the practical OLED microdisplay panel, which had an aspect ratio of 32:9 and a pixel density of 2,490 PPI. The fence angle and height, refractive index of the passivation layer, black matrix width, and white OLED device structure affect the color gamut of the OLED microdisplay panel because of the optical crosstalk effect.

Importance of Purcell factor for optimizing structure of organic light-emitting diodes.

The ratio of spontaneous emission inside a diode structure to that in free space is called the Purcell factor (F(λ)). The structure of organic light-emitting diodes (OLEDs) has a significant influence on the spontaneous emission rate of dipole emitters. Therefore, to describe the optical properties of OLEDs, it is essential to incorporate F(λ) in the description. However, many optical studies on OLEDs continue to be conducted without considering F(λ) for simplicity's sake. Hence, in this study, using carefully designed bottom- and top-emitting OLEDs, we show that the external quantum efficiency obtained without considering F(λ) can be over- or under-estimated, and in some cases, the margin of error may be significant. We also reveal that the subtle distribution of the electroluminescence spectrum can be explained properly only by including F(λ). Both these results stipulate the importance of including F(λ) to maintain a quantitative agreement between theoretical and experimental data. Hence, the inclusion of F(λ) is important for designing OLEDs with enhanced efficiency or desired spectral characteristics.

Color-tunable organic light-emitting diodes with vertically stacked blue, green, and red colors for lighting and display applications.

We demonstrate independently and simultaneously controlled color-tunable organic light-emitting diodes (OLEDs) with vertically stacked blue, green, and red elements. The blue, green, and red elements were placed at the bottom, middle, and top positions, respectively, forming color-tunable OLEDs. The independently driven blue, green, and red elements in the color-tunable OLEDs exhibited low driving voltages of 5.3 V, 3.0 V, and 4.6 V, as well as high external quantum efficiencies of 11.1%, 10.9%, and 9.6%, respectively, at approximately 1000 cd/m2. Each element in the color-tunable OLEDs showed high-purity blue, green, and red colors with little parasitic emission owing to the delicately designed device structure resultant from optical simulations. The color-tunable OLEDs could produce any colors inside the triangle formed with blue (0.136, 0.261), green (0.246, 0.697), and red (0.614, 0.386) Commission Internationale de l'éclairage (CIE) 1931 color coordinates. In addition, the correlated color temperatures (CCTs) of white colors in the color-tunable OLED can be easily changed from the warm white to the cool white by controlling the red, green, and blue emissions simultaneously. The white colors in the color-tunable OLED have the CIE 1931 color coordinate of (0.304, 0.351), with a CCT of 6289 K and (0.504, 0.440), with a CCT of 2407K at the driving voltage of 5 V (blue), 2.8 V (green), 4.4 V (red), and 4.6 V (blue), 3 V (green), 5 V (red), respectively. Furthermore, the white color in the color-tunable OLED exhibited a high color rendering index (~88.7) due to vertically stacked three color system. Moreover, we successfully fabricated a large-sized, 14 × 12 pixel array of the color-tunable OLEDs to demonstrate lighting and display applications, respectively.

Stable angular emission spectra in white organic light-emitting diodes using graphene/PEDOT:PSS composite electrode.

In this work, we suggest a graphene/ poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) composite as a transparent electrode for stabilizing white emission of organic light-emitting diodes (OLEDs). Graphene/PEDOT:PSS composite electrodes have increased reflectance when compared to graphene itself, but their reflectance is still lower than that of ITO itself. Changes in the reflectance of the composite electrode have the advantage of suppressing the angular spectral distortion of white emission OLEDs and achieving an efficiency of 16.6% for white OLEDs, comparable to that achieved by graphene-only electrodes. By controlling the OLED structure to compensate for the two-beam interference effect, the CIE color coordinate change (Δxy) of OLEDs based on graphene/PEDOT:PSS composite electrodes is 0.018, less than that based on graphene-only electrode, i.e.,0.027.

Design and fabrication of two-stack tandem-type all-phosphorescent white organic light-emitting diode for achieving high color rendering index and luminous efficacy.

White organic light-emitting diodes (WOLEDs) are regarded as the general lighting source. Although color rendering index (CRI) and luminous efficacy are usually in trade-off relation, we will discuss about the optimization of both characteristics, particularly focusing on the spectrum of a blue emitter. The emission at a shorter wavelength is substantially important for achieving very high CRI (> 90). The luminous efficacy of a phosphorescent blue emitter is low as its color falls in the deeper blue range; however, that does not show any significant influence on the WOLEDs. WOLEDs with different blue dopants are compared to confirm the calculation of the CRI and luminous efficacy, and the optimized WOLEDs exhibit luminous efficacy of 38.3 lm/W and CRI of 90.9.

Trap-level-engineered common red layer for fabricating red, green, and blue subpixels of full-color organic light-emitting diode displays.

We report a novel strategy to reduce one fine metal mask (FMM) step in a full-color organic light-emitting diode (OLED) display by introducing a common red layer (CRL) which replaces a hole transporting layer (HTL) with the same thickness of a red phosphorescent dye-doped layer. Because the dopant in the HTL acts as a hole trap, careful trap-level engineering is required for achieving efficient green and blue emission from the emitting layer while minimizing the red emission from the CRL. We investigated the characteristics of OLEDs depending on hole trap levels of the CRL with five different organic HTLs, and demonstrated efficient red, green and blue (RGB) emitting devices using the CRL. The electroluminescence spectrum of the devices with the CRL is nearly identical with those of the devices without the CRL. These results open up the possibility of simplified fabrication of practical full-color OLED displays with the reduced FMM steps, resulting in lower manufacturing cost.

Biometric authentication security enhancement under quantum dot light-emitting diode display via fingerprint imaging and temperature sensing.

We improved biometric authentication security using dual recognition based on fingerprint image detection and skin-temperature-change sensing under quantum dot light-emitting diode (QLED) displays. QLEDs are more advantageous than organic light-emitting diodes (OLEDs) in terms of the contrast classification of patterns such as those in fingerprint recognition, owing to their narrow full-width-half-maximum. In this work, scattered, transmitted, and reflected light was captured from the top of the QLED, improving the digital luminance by 25%, as compared with that of OLEDs, because the electroluminescence spectra of the QLED were sustained, whereas those of the OLED were distorted by the generated noise peaks. A QLED with eight apertures sized up to tens of micrometers, mimicking the actual wiring structure of commercialized smartphones, was implemented to detect human fingerprints. The QLED using reduced graphene oxide as the temperature sensor detected temperature changes instantaneously upon finger touch, showing a 2% temperature response based on the human body temperature; however, the temperature change was less than 0.1% for spoof fingerprints printed on paper. Thus, this study successfully enhanced biometric authentication security, through fingerprint recognition based on image sensing using an optical system with micrometer-sized apertures and skin-temperature detection under QLED displays.

Reconfigurable Multilevel Optical PUF by Spatiotemporally Programmed Crystallization of Supersaturated Solution.

Physical unclonable functions (PUFs) are emerging as an alternative to information security by providing an advanced level of cryptographic keys with non-replicable characteristics, yet the cryptographic keys of conventional PUFs are not reconfigurable from the ones assigned at the manufacturing stage and the overall authentication process slows down as the number of entities in the dataset or the length of cryptographic key increases. Herein, a supersaturated solution-based PUF (S-PUF) is presented that utilizes stochastic crystallization of a supersaturated sodium acetate solution to allow a time-efficient, hierarchical authentication process together with on-demand rewritability of cryptographic keys. By controlling the orientation and the average grain size of the sodium acetate crystals via a spatiotemporally programmed temperature profile, the S-PUF now includes two global parameters, that is, angle of rotation and divergence of the diffracted beam, in addition to the speckle pattern to produce multilevel cryptographic keys, and these parameters function as prefixes for the classification of each entity for a fast authentication process. At the same time, the reversible phase change of sodium acetate enables repeated reconfiguration of the cryptographic key, which is expected to offer new possibilities for a next-generation, recyclable anti-counterfeiting platform.

Investigating the electrical crosstalk effect between pixels in high-resolution organic light-emitting diode microdisplays.

Organic light-emitting diode (OLED) microdisplays have received great attention owing to their excellent performance for augmented reality/virtual reality devices applications. However, high pixel density of OLED microdisplay causes electrical crosstalk, resulting in color distortion. This study investigated the current crosstalk ratio and changes in the color gamut caused by electrical crosstalk between sub-pixels in high-resolution full-color OLED microdisplays. A pixel structure of 3147 pixels per inch (PPI) with four sub-pixels and a single-stack white OLED with red, green, and blue color filters were used for the electrical crosstalk simulation. The results showed that the sheet resistance of the top and bottom electrodes of OLEDs rarely affected the electrical crosstalk. However, the current crosstalk ratio increased dramatically and the color gamut decreased as the sheet resistance of the common organic layer decreased. Furthermore, the color gamut of the OLED microdisplay decreased as the pixel density of the panel increased from 200 to 5000 PPI. Additionally, we fabricated a sub-pixel circuit to measure the electrical crosstalk current using a 3147 PPI scale multi-finger-type pixel structure and compared it with the simulation result.

Molecular mechanics of Ag nanowire transfer processes subjected to contact loading by a PDMS substrate.

Precise transfer and attachment of a single nanowire to a target substrate is an interesting technique in surface engineering. The spacing, which restrains the attachment of a nanowire to a substrate, and the bending strain that occurs when the nanowire detaches from the elastomeric donor are important design parameters. In this regard, in this study, all-atom molecular dynamics (MD) simulations were conducted to analyse the mechanical behaviour of a penta-twinned silver nanowire (AgNW) placed on a polydimethylsiloxane (PDMS) donor substrate to elucidate the relevant transfer process. The bow deformation of the AgNW at the delamination front of PDMS was characterized as a function of its diameter and aspect ratio. The mechanisms of dislocation slip and propagation as well as the internal stress distribution of the AgNW were then examined. The results showed that twin boundary formation during the bow deformation is a key factor affecting the strain hardening of the AgNW and leading to complete plastic strain recovery after the removal of the PDMS substrate. Furthermore, the process was demonstrated experimentally by a localized bonding and transfer of AgNWs by continuous-wave laser irradiation. Based on the computational and experimental findings, an empirical model considering the shape parameters of AgNWs that can ensure a successful transfer process was established, which is essential for high-performance AgNW electrode design.

Effect of Fermented Red Ginseng Concentrate Intake on Stool Characteristic, Biochemical Parameters, and Gut Microbiota in Elderly Korean Women.

Fermented red ginseng (FRG) has been used as a general stimulant and herbal medicine for health promotion in Asia for thousands of years. Few studies have investigated the effects of FRG containing prebiotics on the gut microbiota. Here, 29 Korean women aged ≥ 50 years were administered FRG for three weeks to determine its effect on stool characteristics, biochemical parameters, and gut microbiome. Gut microbial DNA was subjected to 16S rRNA V3-V4 region sequencing to assess microbial distribution in different stages. Additionally, the stool consistency, frequency of bowel movements, and biochemical parameters of blood were evaluated. We found that FRG intake improved stool consistency and increased the frequency of bowel movements compared to before intake. Biochemical parameters such as glucose, triglyceride, cholesterol, low-density lipoprotein cholesterol, creatinine, alkaline phosphatase, and lactate dehydrogenase decreased and high-density lipoprotein cholesterol increased with FRG intake. Gut microbiome analysis revealed 20 specific bacteria after three weeks of FRG intake. Additionally, 16 pathways correlated with the 20 specific bacteria were enhanced after red ginseng intake. In conclusion, FRG promoted health in elderly women by lowering blood glucose levels and improving bowel movement frequency. The increase in bacteria observed with FRG ingestion supports these findings.

Releasing of a Titanium Clamp (Craniofix®) Without Mechanical Defect After Craniotomy for Acute Subdural Hemorrhage.

After craniotomy, bone flap fixation can be performed using wires, sutures, microplates, and Craniofix®. Well-margined and fixed bone flaps are important not only for postoperative brain protection but also for esthetics. Herein, we report a case of cranioplasty due to bone flap dislocation by Craniofix® clamp loosening after craniotomy with acute subdural hemorrhage removal. Iatrogenic outward force during epidural drain removal adjacent to Craniofix®, insertion of the clamp around the circumference of the bone flap, increased intracranial pressure due to brain swelling and fluid collection, and external shock during postoperative patient management are thought to be the causes of bone flap dislocation. To our knowledge, this is the second reported case of craniotomy with a Craniofix® clamp release.

Bright and Stable Quantum Dot Light-Emitting Diodes.

Quantum dot light-emitting diodes (QLEDs) are one of the most promising candidates for next-generation displays and lighting sources, but they are barely used because vulnerability to electrical and thermal stresses precludes high brightness, efficiency, and stability at high current density (J) regimes. Here, bright and stable QLEDs on a Si substrate are demonstrated, expanding their potential application boundary over the present art. First, a tailored interface is granted to the quantum dots, maximizing the quantum yield and mitigating nonradiative Auger decay of the multiexcitons generated at high-J regimes. Second, a heat-endurable, top-emission device architecture is employed and optimized based on optical simulation to enhance the light outcoupling efficiency. The multilateral approaches realize that the red top-emitting QLEDs exhibit a maximum luminance of 3 300 000 cd m-2 , a current efficiency of 75.6 cd A-1 , and an operational lifetime of 125 000 000 h at an initial brightness of 100 cd m-2 , which are the highest of the values reported so far.

Identification of the Antidepressant Function of the Edible Mushroom Pleurotus eryngii.

Pleurotus eryngii produces various functional molecules that mediate physiological functions in humans. Recently, we observed that P. eryngii produces molecules that have antidepressant functions. An ethanol extract of the fruiting body of P. eryngii was obtained, and the extract was purified by XAD-16 resin using an open column system. The ethanol eluate was separated by HPLC, and the fraction with an antidepressant function was identified. Using LC-MS, the molecular structure of the HPLC fraction with antidepressant function was identified as that of tryptamine, a functional molecule that is a tryptophan derivative. The antidepressant effect was identified from the ethanol extract, XAD-16 column eluate, and HPLC fraction by a serotonin receptor binding assay and a cell-based binding assay. Furthermore, a forced swimming test (FST) showed that the mice treated with purified fractions of P. eryngii exhibited decreased immobility time compared with nontreated mice. From these results, we suggest that the extract of P. eryngii has an antidepressant function and that it may be employed as an antidepressant health supplement.

Fabrication of Perforated PDMS Microchannel by Successive Laser Pyrolysis.

Poly(dimethylsiloxane) has attracted much attention in soft lithography and has also been preferred as a platform for a photochemical reaction, thanks to its outstanding characteristics including ease of use, nontoxicity, and high optical transmittance. However, the low stiffness of PDMS, an obvious advantage for soft lithography, is often treated as an obstacle in conducting precise handling or maintaining its structural integrity. For these reasons, a Glass-PDMS-Glass structure has emerged as a straightforward alternative. Nevertheless, several challenges are remaining in fabricating Glass-PDMS-Glass structure through the conventional PDMS patterning techniques such as photolithography and etching processes for master mold. The complicated techniques are not suitable for frequent design modifications in research-oriented fields, and fabrication of perforated PDMS is hard to achieve using mold replication. Herein, we utilize the successive laser pyrolysis technique to pattern thin-film PDMS for microfluidic applications. The direct use of thin film at the glass surface prevents the difficulties of thin-film handling. Through the precise control of photothermal pyrolysis phenomena, we provide a facile fabrication process for perforated PDMS microchannels. In the final demonstration, the laminar flow has been successfully created owing to the smooth surface profile. We envision further applications using rapid prototyping of the perforated PDMS microchannel.

Double Metal Oxide Electron Transport Layers for Colloidal Quantum Dot Light-Emitting Diodes.

The performance of colloidal quantum dot light-emitting diodes (QD-LEDs) have been rapidly improved since metal oxide semiconductors were adopted for an electron transport layer (ETL). Among metal oxide semiconductors, zinc oxide (ZnO) has been the most generally employed for the ETL because of its excellent electron transport and injection properties. However, the ZnO ETL often yields charge imbalance in QD-LEDs, which results in undesirable device performance. Here, to address this issue, we introduce double metal oxide ETLs comprising ZnO and tin dioxide (SnO2) bilayer stacks. The employment of SnO2 for the second ETL significantly improves charge balance in the QD-LEDs by preventing spontaneous electron injection from the ZnO ETL and, as a result, we demonstrate 1.6 times higher luminescence efficiency in the QD-LEDs. This result suggests that the proposed double metal oxide ETLs can be a versatile platform for QD-based optoelectronic devices.

Tailoring the Electronic Landscape of Quantum Dot Light-Emitting Diodes for High Brightness and Stable Operation.

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m2, suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m2), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

Thin-film transistor-driven vertically stacked full-color organic light-emitting diodes for high-resolution active-matrix displays.

Thin-film transistor (TFT)-driven full-color organic light-emitting diodes (OLEDs) with vertically stacked structures are developed herein using photolithography processes, which allow for high-resolution displays of over 2,000 pixels per inch. Vertical stacking of OLEDs by the photolithography process is technically challenging, as OLEDs are vulnerable to moisture, oxygen, solutions for photolithography processes, and temperatures over 100 °C. In this study, we develop a low-temperature processed Al2O3/SiNx bilayered protection layer, which stably protects the OLEDs from photolithography process solutions, as well as from moisture and oxygen. As a result, transparent intermediate electrodes are patterned on top of the OLED elements without degrading the OLED, thereby enabling to fabricate the vertically stacked OLED. The aperture ratio of the full-color-driven OLED pixel is approximately twice as large as conventional sub-pixel structures, due to geometric advantage, despite the TFT integration. To the best of our knowledge, we first demonstrate the TFT-driven vertically stacked full-color OLED.

High-performance thin H:SiON OLED encapsulation layer deposited by PECVD at low temperature.

Highly moisture permeation resistive and transparent single layer thin films for the encapsulation of hydrogenated silicon oxynitrides (H:SiON) were deposited by plasma-enhanced chemical vapor deposition (PECVD) using silane (SiH4), nitrous oxide (N2O), ammonia (NH3), and hydrogen (H2) at 100 °C for applications to a top-emission organic light-emitting diode (TEOLED). Addition of H2 into the PECVD process of SiON film deposition afforded the hydrogenated SiON film, which showed not only improved optical properties such as transmittance and reflectance but also better barrier property to water permeation than PECVD SiON and even SiN x . The H:SiON film with thickness of only 80 nm exhibited water vapor transmission rate (WVTR) lower than 5 × 10-5 g per m2 per day in the test conditions of 38 °C and 100% humidity, where this WVTR is the measurement limit of the MOCON equipment. An additional coating of UV curable polymer enabled the H:SiON films to be flexible and to have very stable barrier property lower than 5 × 10-5 g per m2 per day even after a number of 10k times bending tests at a curvature radius of 1R. The mild H:SiON film process improved the electrical properties of top-emission OLEDs without generating any dark spots. Furthermore, single H:SiON films having high water vapor barrier could maintain the original illumination features of TEOLED longer than 720 hours. These excellent properties of the H:SiON thin films originated from the structural changes of the SiON material by the introduction of hydrogen.

Unraveled Face-Dependent Effects of Multilayered Graphene Embedded in Transparent Organic Light-Emitting Diodes.

With increasing demand for transparent conducting electrodes, graphene has attracted considerable attention, owing to its high electrical conductivity, high transmittance, low reflectance, flexibility, and tunable work function. Two faces of single-layer graphene are indistinguishable in its nature, and this idea has not been doubted even in multilayered graphene (MLG) because it is difficult to separately characterize the front (first-born) and the rear face (last-born) of MLG by using conventional analysis tools, such as Raman and ultraviolet spectroscopy, scanning probe microscopy, and sheet resistance. In this paper, we report the striking difference of the emission pattern and performance of transparent organic light-emitting diodes (OLEDs) depending on the adopted face of MLG and show the resolved chemical and physical states of both faces by using depth-selected absorption spectroscopy. Our results strongly support that the interface property between two different materials rules over the bulk property in the driving performance of OLEDs.