Transparent Organic Light-emitting Diodes Research Articles

P‐102: Flexible Transparent Organic Light‐Emitting Diodes with Suppressed Waveguide Modes

We developed high efficiency transparent organic light‐emitting diodes (TOLEDs) with ultrathin metal electrodes. The modal calculation showed that our TOLED does not support waveguide mode, leading to potentially trapped‐energy free device. This high efficiency TOLEDs do not require complex modal outcoupler but only high‐quality metal thin film.

Journal of the Society for Information Display

Jiun-Haw Lee Editor in Chief In the May issue, JSID will publish a special issue of (part of) “Expanded Distinguished Papers of Display Week 2021.“ Those papers were nominated by the Program Committee of SID Display Week and passed the expedited review, which was somewhat stricter than normal because of time constraints. Those papers will remain open access until the end of 2021. When inviting the authors of the SID-sponsored conference to expand their results to a full paper and submit to JSID, sometimes they ask: “May I use the same context (or figures) of the SID-sponsored conference proceeding in the expanded paper?” In the Editorial published in April, we clarified this issue of novelty and plagiarism from different viewpoints: legally speaking, the author, and the journal. Take “Expanded Distinguished Papers of Display Week“ as an example. These abstracts are nominated by the Program Committee of SID Display Week with a quick review and limited content (within 4 pages). Although the results are interesting, the abstracts sometimes cannot contain all the information that is required for a full paper (typically >6 pages) published in JSID. When preparing the manuscript of JSID, the authors are allowed to use part of or all of the content in the abstracts submitted to SID Display Week by the same authors. However, it cannot apply to the case for expanding the conference papers sponsored by other societies. Evaluation and formulation of the sense of social telepresence in video-mediated communication systems: Contribution of eye contact to enhancing social telepresence | Shinji Kimura, et al. | DOI: 10.1002/jsid.976 There is a strong demand for online meetings, especially after the COVID-19 pandemic. Compared to face-to-face communication, the user experience for existing video-mediated communication (VMC) is still not satisfactory, unfortunately, even though we can use high-resolution display, together with high-speed internet. To create the “real” feeling of face-to-face communications, especially in one-on-one situations, the authors pointed out two important human factors: “eye contact” and “life-size image.” A prototype of the VMC system was demonstrated with a large-sized transparent organic light-emitting diode (OLED) display and a camera behind the scenes. Using a large display to achieve a life-sized image is straightforward. A camera behind the transparent display was employed to align the eye gaze with the image and create the feeling of “eye contact.” Early defect identification for micro light-emitting diode displays via photoluminescent and cathodoluminescent imaging | Keith Behrman, et al. | DOI: 10.1002/jsid.985 Micro light-emitting diodes (μ-LEDs) are an emerging display technology that has attracted lots of attention for different applications, such as augmented reality, small-sized displays (∼2 inches), and huge displays (more than 100 inches). In such a display, each LED is driven independently for electroluminescence (EL) emission. However, it is not easy to achieve high process yield for the millions of pixels in one display. Hence, online detection is required during manufacture. In this paper, the authors compare two different measurement methods, photoluminescence (PL) and cathodoluminescence (CL), with EL results. It was found that PL is not a satisfactory tool to reveal the defects. On the other hand, CL images can be accurate, up to ∼70%, when mapping to an EL image. Different defect modes during the process flow were also analyzed. JSID is published monthly by Wiley. Subscription fees apply, but SID members and student members have free online access via sid.org/Publications.aspx. Many universities also have an institutional subscription. JSID is indexed in the Web of Science. Submit your original manuscript online via mc.manuscriptcentral.com/sid. Author guidelines can be found on the Journal's homepage: tinyurl.com/jsidhome. Editorial board: tinyurl.com/jsideb. Please direct any questions about JSID to its editor in chief at editor@sid.org. EarlyView: accepted papers about to be published can be accessed online via tinyurl.com/jsidev. The Journal is soliciting review papers on any display-related topic. If you have a great idea for a review paper, please contact the editor at editor@sid.org. Page charges for invited review papers will be waived.

A Systematic Study of the Interactions in the Top Electrode/Capping Layer/Thin Film Encapsulation of Transparent OLEDs

Development of flexible transparent organic light-emitting devices (TOLEDs) still requires a number of advancements in transparent conducting electrodes with low reflection and absorption, a capping layer (CL) acting as refractive index-matching, and thin film encapsulation (TFE) with high water vapor barrier properties, among others. While substantial research has been reported on isolated examples in each area, there has been no detailed and systemic research related to the overall interactions of top electrodes, CLs, TFE, and their interfaces. In this work, TOLEDs have been fabricated with a thin Ag top electrode and CLs of different surface energy, which was encapsulated with high water vapor barrier property (1.35Ⅹ10−4 gm−2day−1 at 37.8 °C and 100 % RH). The encapsulation barrier was comprised of 50-nm-thick Al2O3 thin films deposited using a low-temperature (95 °C) ALD process. Once prepared, the TOLEDs were studied using a variety of techniques to determine the enhancements to electrical, optical, and water vapor barrier properties. Although the nature of the CL materials affects the film formation on the top electrode layer, there is no impact on the properties of the Al2O3 thin films. In the formation process of the Al2O3 TFE, the device incorporating a tris(8-hydroxyquinoline)aluminum (Alq3) CL showed superior performance, whereas device performance degredation was noted with the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) CL. The lack of degradation in the Alq3 CL device during ALD processing is attributed to the high thermal stability of Alq3, which exhibits a high glass transition temperature of 175 °C. However, in the HATCN CL device, it is expected that formation of aromatic radical anions [HAT(CN)6]•− and dianions [HAT(CN)6]2−, and/or band bending of fermi energy of the HATCN at the interface of the thin Ag layer, results in the observed degraded performance.

Toward See‐Through Optoelectronics: Transparent Light‐Emitting Diodes and Solar Cells

AbstractTransparent light‐emitting diodes (LEDs) and solar cells have attracted extensive attention as the most promising optoelectronic devices surpassing conventional opaque displays and photovoltaics. These transparent devices are particularly suitable for special applications including see‐through display and building‐integrated photovoltaics, for example, head‐mounted displays, navigation displays on car windshields, smart windows, roofing, skylights, and facades. This review systematically evaluates the pros and cons of representative transparent conductive electrodes (TCEs) including transparent conductive oxides, ultrathin metal films, metal nanowires/meshes, conductive flexible polymers, carbon nanotubes, and graphene. A full‐scale overview of recent studies on transparent organic LEDs, quantum dot LEDs, perovskite LEDs, dye‐sensitized solar cells, organic solar cells, and perovskite solar cells using the above‐mentioned TCEs as top electrodes, is also provided, summarizing information on state‐of‐the‐art device performances. Finally, the key challenges in current transparent optoelectronic devices and a few potential proposals for developing highly efficient transparent devices are addressed in the last part. It is believed that, with the rapid development of novel optoelectronic materials and fabrication technologies, there will be a bright prospect for transparent optoelectronic devices and their related applications in the near future.

P‐165: An Optimized Algorithm to Reconstruct the Structure of Transparent OLED Display Based on Monte Carlo Method

For current transparent OLED(TOLED) displays, a common problem is that the background image quality is decrease due to diffraction. A method to reduce diffraction is to optimize the periodic grating structure of transparent display. We propose an algorithm based on Monte Carlo method to optimize the structure of transparent display.

Transparent bi-directional organic light-emitting diodes with color-tunable top emission

• Transparent bi-directional organic light-emitting diodes were studied. • Color conversion capping layers were utilized. • The different top emission colors and efficiency were controlled. • Top emission color changed from blue to green, yellow, and white.

Emission characteristics of transparent organic light-emitting diodes with molybdenum oxide capping layers

In this work, we have studied a molybdenum oxide (MoO3) layer as an effective capping layer (CL) for TOLEDs. We varied the thickness of the capping layer in order to investigate its effects on the device performance. Both the top and bottom emission of the devices were found to be easily controlled by changing the thickness of MoO3 layer. Especially, at the CL thickness of 40 nm, the current efficiency shows 2.2 times enhancement compared to reference device which has no capping layer and exhibited the highest transmittance value. The results also revealed that the transmittance and light outcoupling efficiency are significantly dependent on the capping layer thickness and exhibit identical trend in change. In addition, the changes in the emission characteristics were optically simulated which agreed well with the experimental data.

Highly Transparent and Colorless Organic Light‐Emitting Diodes

Herein, transparent organic light‐emitting diodes (T‐OLEDs) are presented with excellent light‐emission performance achieved by virtue of optically transparent functional layers. In this T‐OLED, indium tin oxide (ITO) thin films are used not only as a transparent bottom anode but also as a transparent top cathode. Blue polymer poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (PFO) is used as a host material for the emission layer (EML) of T‐OLEDs, to achieve an optical light transmittance of 70% in the visible range of the light spectrum. By controlled blending of the commercial emitting material Super Yellow with PFO, the light‐emission efficiency of EML is significantly improved through a Förster resonance energy transfer (FRET) radiative recombination between electronic charge carriers. The organic functional layers of T‐OLEDs are protected from plasma damage during direct‐current magnetron sputtering deposition of the ITO cathode, using a spin‐coated bilayer electron‐transport coating of zinc oxide nanoparticle layers and cesium carbonate‐doped polyethyleneimine ethoxylated (d‐PEIE). As a result, T‐OLED exhibits a combined luminance of 1600 cd m−2 from the top ITO cathode (700 cd m−2) and bottom glass surfaces (900 cd m−2). The T‐OLED shows a device transmittance of 70% in the visible range and a current efficiency twice as high as that of pure PFO‐type T‐OLED.

Highly reliable and transparent Al doped Ag cathode fabricated using thermal evaporation for transparent OLED applications

Abstract Here, we report a highly reliable and transparent cathode (TC) for transparent organic light emitting diode (TOLED) applications. The cathode layer is fabricated by co-deposition of silver (Ag) and aluminum (Al) metals using thermal evaporation. The atomic percentages of Al and Ag are fixed at 96 at% and 4 at%, respectively. TC shows transmittance at 520 nm (T@520nm) of 83.5% and sheet resistance as low as 7.0 Ω/□. Almost no specific agglomeration of Ag:Al (4%, 14 nm) is noticed by the thermal annealing of fabricated cathode at 85 °C, 240 h. The fabricated TOLED with this Ag:Al (4%, 14 nm) cathode shows an excellent electron injection property with a T@520nm of 83.5%, and achieved current efficiencies of 36 and 18 cd/A from bottom and top sides emissions, respectively. It is argued that the Al-doped Ag film could become an effective, feasible alternative to the existing transparent conducting electrode for TOLED applications.

Optimization of Transparent Organic Light-Emitting Diodes by Simulation-Based Design of Organic Capping Layers.

In this work, we use the transfer matrix method to optimize TPBi capping layers deposited on organic light emitting diodes with respect to light extraction and transmittance. The green transparent organic light emitting diodes comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent organic light emitting diode stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The diode stack as well as the capping layer are deposited by organic vapor phase deposition. The refractive indices and extinction coefficients of all materials in the transparent organic light emitting diodes (glass, indium tin oxide, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance and reflectance measurements. With these spectrally resolved data, we calculate the transmittance of transparent organic light emitting diodes with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm-1000 nm) by a series of experiments. By choosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated transparent organic light emitting diodes with an optical transmittance which was strongly enhanced from 47% (reference without capping layer) to 65%, measured at 555 nm.

Investigation of Organic LED Materials Using a Transparent Cathode for Improved Efficiency

This paper proposes a flexible transparent organic light-emitting diode (OLED) which is notable due to its high electron injection and transmittance. Though graphene-based OLEDs are promising candidates for flexible displays due to the enormous merits offered, a significant decrease in its device efficiency was reported in comparison to the other conventional indium tin oxide-based OLEDs. The device illustrated enhances the performance of the graphene-based flexible OLEDs through balanced charge recombination. A theoretical analysis has been done through performing optical simulations. The structure of the graphene-based OLED is optimized through the employment of efficient cathode and electron injection layer materials to boost the electron injection into the device. With the aid of simulations, it was found that optimum results are obtained with aluminum zinc oxide (Al:ZnO) as the cathode and Al/LiF/Alq3 as the electron injection layer. The proposed structure exhibits high transmittance of 87.14% with a current efficiency of 61.17 cd/A, and external quantum efficiency (EQE) of 21.2% at 1000 cd/m2 for the bottom side. An evident improvement in the device performance with 32.9% increase in the current efficiency, and 45.2% increase in the EQE was observed.

Optically switchable bistable guest–host displays in chiral-azobenzene- and dichroic-dye-doped cholesteric liquid crystals

Optically switchable bistable guest–host displays in chiral-azobenzene- and dichroic-dye-doped cholesteric liquid crystals (CA-DD-CLCs) are demonstrated. The CA-DD-CLCs in a LC cell treated by homeotropic alignment are initially in stable unwound textures. Given that all LC molecules in unwound textures are oriented perpendicularly to the substrates, the light absorbed by the doped DDs reaches the minimum value, which is considered the transparent state of the proposed CA-DD-CLCs. The CA-DD-CLCs are switched to fingerprint (FP) textures by illumination with ultraviolet (UV) light and can be switched back to unwound textures by illumination with green light. The optical switching between unwound and FP textures is based on the photo-tunable chirality of the doped CA via photoisomerization effect. The light absorbed by the doped DDs in FP textures reaches the maximum value, which is considered the dark state of the CA-DD-CLCs. Moreover, the performances of light absorption from the doped DDs when the CA-DD-CLCs are in unwound and FP textures is independent of the polarization of the incident light. Pattern information can be optically addressed (erased) in CA-DD-CLC cells by illumination with UV (green) light through a suitable photomask. The CA-DD-CLCs can be stabilized in FP textures for several tens of minutes in the dark environment. As such, the power is consumed only for the illumination of specific lights during the switching of CA-DD-CLCs between unwound and FP textures. Moreover, a transparent active matrix organic light-emitting diode (LED) display that is superimposed on top of a CA-DD-CLC cell embedded with a transparent light-guide film with suitable LED light sources for optical switching between unwound and FP textures, is proposed for augmented reality applications.

Symmetrical Emission Transparent Organic Light-Emitting Diodes With Ultrathin Ag Electrodes

This paper describes a way to implement a transparent electrode for symmetrical emission from the bottom and top sides of transparent organic light-emitting diodes (TOLEDs). An ultrathin (∼6 nm) and smooth Ag film was achieved by using the pretreatment of using oxygen plasma prior to deposition of a nanoscale-thin Ag film on glass. The treatment increased the surface energy, thereby enhancing the wettability of the Ag on glass substrate and consequently leading to the formation of thin and unifrom Ag film with high optical transmittance T ∼ 76% and low sheet resistance R S T = 85.6%, and R S = 9.3 Ω □−1 was designed (ITO: 83.1%, 10 Ω □−1). We conducted finite-domain time-difference simulation for this glass/Ag/WO3 electrode to design a weak microcavity structure that retained the transparency and emitting properties. This optimized structure of TOLEDs (Ag = 6 nm) showed higher T = 73.84% than those of ITO devices ( T = 73.19%), and can induce the weak microcavity effect, resuting in improved electroluminescent properties and increased the luminance value from 38.25 to 56.25 cd A−1 at 25 mA cm–2.

Design of retinal-projection-based near-eye display with contact lens.

We propose a design of a retinal-projection-based near-eye display for achieving ultra-large field of view, vision correction, and occlusion. Our solution is highlighted by a contact lens combo, a transparent organic light-emitting diode panel, and a twisted nematic liquid crystal panel. Its design rules are set forth in detail, followed by the results and discussion regarding the field of view, angular resolution, modulation transfer function, contrast ratio, distortion, and simulated imaging.

Next generation smart window display using transparent organic display and light blocking screen.

Transparent organic light emitting diodes (TOLED) have widespread applications in the next-generation display devices particularly in the large size transparent window and interactive displays. Herein, we report high performance and stable attractive smart window displays using facile process. Advanced smart window display is realized by integrating the high performance light blocking screen and highly transparent white OLED panel. The full smart window display reveals a maximum transmittance as high as 64.2% at the wavelength of 600 nm and extremely good along with tunable ambient contrast ratio (171.94:1) compared to that of normal TOLED (4.54:1). Furthermore, the performance decisive light blocking screen has demonstrated an excellent optical and electrical characteristics such as i) high transmittance (85.56% at 562nm) at light-penetrating state, ii) superior absorbance (2.30 at 562nm) in light interrupting mode, iii) high optical contrast (85.50 at 562 nm), iv) high optical stability for more than 25,000 cycle of driving, v) fast switching time of 1.9 sec, and vi) low driving voltage of 1.7 V. The experimental results of smart window display are also validated using optical simulation. The proposed smart window display technology allows us to adjust the intensity of daylight entering the system quickly and conveniently.

Dual Ag electrodes for semitransparent organic light-emitting diodes

Abstract Thick ReO3 film (>30 nm) is found to serve as an active hole-injecting layer as well as a passive buffer layer to suppress point discharge of Ag anode. This buffered Ag electrode enables fabrication of transparent organic light emitting diodes (TOLEDs). The excellent conductivity of ReO3, second only to metal amongst oxide semiconductors, is made possible to use a thick hole-injecting buffer layer. The use of thick ReO3 layer alleviates strong local electric field on a rough electrode surface. For device optical design, the transfer matrix method is used to provide theoretical guidance on electrode fabrication. Based on microcavity theory in TOLEDs, the optical path parameters are also optimized to the thicknesses of different functional organic films. To test the device performance, we fabricated normal fluorescent green Alq3 devices. The data show that TOLEDs with ReO3-buffered Ag electrodes have good photochromic uniformity on both sides of the devices.

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.

Enhanced image quality of OLED transparent display by cholesteric liquid crystal back-panel

The transparent active-matrix organic light-emitting diode (AM-OLED), which is a self-luminous polarizer-free display, is regarded as one of the most potential candidate of high-transparency transparent display. However, the image quality of the transparent AM-OLED depends strongly on the background, resulting in poor contrast and image distortion. In this work, a transparent display with high image quality that combines AM-OLED and dye-doped cholesteric liquid crystal is demonstrated. Its electro-optical properties are optimized, enabling the transparent display to be not only compatible with active-matrix driving but also the contrast and distortion are sufficiently improved without sacrificing the transparency.

Ultrahigh-luminance organic light-emitting diodes using LiF/MgAg as cathode for the application of both surface emission

ABSTRACTThe effect of different species of cathode on the performance of organic light-emitting diodes under direct current drive has been investigated in this paper. The organic light-emitting diode using LiF/Al and LiF/MgAg layer as cathode has been fabricated, and the luminance of device with cathode structure of LiF/MgAg was found to realize higher efficiency. The performance of the device was found to be improved greatly due to efficient electron injection from MgAg cathode. Moreover, a configuration of LiF/MgAg/IZO (indium zinc oxide) was developed, finding that both the top-emitting intensity and the bottom-emitting intensity achieve very-high luminance, and in this basis, transparent OLEDs with the extremely high-luminance of about 100,000 cd/m2 has been realized, which is considered as the highest luminance among the previous works.

Transparent organic light-emitting diodes with balanced white emission by minimizing waveguide and surface plasmonic loss.

It is challenging in realizing high-performance transparent organic light-emitting diodes (OLEDs) with symmetrical light emission to both sides. Herein, an efficient transparent OLED with highly balanced white emission to both sides is demonstrated by integrating quasi-periodic nanostructures into the organic emitter and the metal-dielectric composite top electrode, which can simultaneously suppressing waveguide and surface plasmonic loss. The power efficiency and external quantum efficiency are raised to 83.5 lm W-1 and 38.8%, respectively, along with a bi-directional luminance ratio of 1.26. The proposed scheme provides a facile route for extending application scope of transparent OLEDs for future transparent displays and lightings.