Transparent Organic Light-emitting Diodes Research Articles

Paper No 5.3: Simulation of Visual Quality for Transparent OLED Display

AbstractA transparent organic light emitting diode (OLED) display has received much attention as one of the future display technologies. While various studies have conducted to increase transparency, reduce haziness and remove image distortion in transparent OLED, fewer studies have attempted to provide the requirements of visual quality for a particular purpose of applications. In order to establish the guidelines of visual quality requirement, a method for the visualisation of seeing through a transparent OLED is required. In this paper, we present a feasible simulation method for visualising transparent OLED. We focused on three influential conditions—illumination, transparency and contrast—though numbers of conditions affect visual quality in Transparent OLED. And each condition was implemented on the assumption that the design of pixel structure may be separated into two parts, emitting and see‐through areas. All of the procedures were derived from image processing techniques. Consequentially, a simulation result can be seen as one image, which is composite of two images, image on the display and image through the display. The simulation results provide an intuitive approach to examine the visual quality. We believe that the proposed method can be used to give guidelines for designing transparent OLED.

Fabrication of an organic light-emitting diode inside a liquid crystal display

The fabrication of a hybrid device architecture fully integrating a transparent organic light-emitting diode (OLED) and a liquid crystal display (LCD) within two glass substrates is reported in this study. The transparent OLED was fabricated on the inner surface of the glass substrate. Twisted nematic liquid crystal (LC) materials were used to fill the space between the two glass substrates. The OLED was driven by an indium-tin oxide (ITO) anode on the glass substrate and a thin bi-metal (Al/Ag) cathode, which also served as the electrode of the LCD. The other electrode for the LCD-mode operation was the ITO on the other glass substrate. A commercially available ultraviolet (UV)-curable resin was spun onto the thin Al/Ag as the passivation layer to protect the OLED from attacks by the following polyimide layer (serving as the alignment layer of the LCD), rubbing process and LC materials. In this device structure, the electrical characteristic of the OLED-mode operation was almost the same as that of the control device. Current efficiency (in terms of cd/A) of the hybrid device from top-emission (towards the LCD) decreased by 26.5% due to optical interference effect, whereas efficiency from bottom-emission remained the same. The driving voltage of the LCD-mode operation increased by 1.6V due to the insertion of the passivation layer between the two electrodes. The contrast ratio decreased from 150 to 25 due to the reflection of the thin Al/Ag layer. Compared with that of the control device, the storage lifetime of the OLED increased as a result of filling the encapsulated cavity with LC materials, which helped repel ambient water and oxygen.

Transparent OLED Lighting Panel Design Using Two-Dimensional OLED Circuit Modeling

In this work, we develop a simulation method to predict a two-dimensional luminance distribution method using a circuitry simulation. Based on the simulation results, we successfully fabricate large area (90 mm × 90 mm) transparent organic light-emitting diode panels with high luminance uniformity.

49.2: A Study on Electron‐injecting and Surface‐modifying Layer for Transparent Organic Light Emitting Diodes

AbstractWe present highly transparent organic light emitting diodes (TOLED) with new transparent cathode structures. This TOLED has ∼71.7% transmittance at 550 nm including the general glass encapsulation. Our transparent cathode stacks (organic layers / cathode) show 91.2% transmittance at 550 nm and low sheet resistance (5.4Ω/□). Such excellent performances are attributed to a new good electron injection system, flat surface morphology of Ag layer, and high refractive index capping layer of WO3.

P.122: Optical Control of Surface Plasmon Loss in Transparent Organic Light Emitting Devices Coupled with Optical Compensation Layer

AbstractLight extraction of transparent OLED can be improved by using an optical compensation layer on the back‐side of semi‐transparent metal cathode. From the results of detail optical calculation based on the multi‐scale analysis including near‐field optics, surface plasmon loss in semi‐transparent metal cathode was drastically suppressed to less than 10% due to the increase of wave‐vector caused by phase control of reflective coefficient of evanescent wave on the cathode interface. Not less than 90% of optical power in the dipole emission can be successfully utilized as the propagation light. The phosphorescent green light emitting device exhibits higher light extraction by a factor of about 1.5.

Organic/metal hybrid cathode for transparent organic light-emitting diodes

We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron transport layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications. Particular attention is paid to the surface morphology on the Ag film and its influence on the optical transparency and electrical conductivity. With the use of Cs-ETL, a smooth and continuous surface morphology of Ag film was achieved, leading to a high transmittance of ∼75% in TOLED with a low sheet resistance of 4.5Ω/Sq in cathode film. We successfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparent OLEDs. Our approach suggests a new electrode structure for transparent OLED applications.

Actively transparent display with enhanced legibility based on an organic light-emitting diode and a cholesteric liquid crystal blind panel

Transparent display is one of the most promising concepts among the next generation information display devices. Nevertheless, conventional transparent displays have two inherent problems: low forward light efficiency due to the light being emitted also in a backward direction; and low legibility due to the visual interruption caused by the light coming from the background. In this work, a cholesteric liquid crystal (Ch-LC) based, actively operational blind panel is combined with transparent organic light-emitting diodes (TR-OLEDs) to recycle the light wasted by backward propagation in transparent displays while blocking the light from behind the display, pursuing both improved forward light efficiency and enhanced image legibility. By tuning the reflectance spectrum of the Ch-LC panel to match the emission spectrum of TR-OLEDs, we achieved luminous efficiency increase by as large as 21% (85%) when the top metal cathode side (the bottom ITO side) of the OLEDs fa'transparent OLED' ces the blind panel. Maximum transmittance of the proposed device reached a high value of 60%, successfully demonstrating a new window-like transparent display concept.

Controlling the optical efficiency of the transparent organic light-emitting diode using capping layers

The performance of transparent organic light-emitting diodes (TOLEDs) that use a combination of organic and inorganic capping layers (CLs) is reported. The first organic layer on top of the transparent cathode was used to increase the light output of the normally directed emission toward the top direction. The second layer, which had high-refractive-index inorganic materials on top of the first organic layer, was used as a reflector to enhance the efficiency of the bottom direction by taking advantage of the discrepancy in the refractive index against the air. By incorporating these CLs, control of the optical properties of the TOLEDs was achieved to allow the application of the transparent lighting-source.

Highly Enhanced Light Extraction from Surface Plasmonic Loss Minimized Organic Light‐Emitting Diodes

Extremely high light out-coupling efficiency from a transparent organic light-emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well-matched analysis of optical simulations.

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2nm)/Cu (18)/TcTa (60nm). The TEWOLED shows an efficiency of 25.6cd/A, 20.1Lm/W at 1000cd/m2, and low voltage of 4.2V for 1222cd/m2. Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6cd/A, 24.9Lm/W.

Achieving High Efficiency and Improved Stability in ITO‐Free Transparent Organic Light‐Emitting Diodes with Conductive Polymer Electrodes

AbstractEfficient transparent organic light‐emitting diodes (OLEDs) with improved stability based on conductive, transparent poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes are reported. Based on optical simulations, the device structures are carefully optimized by tuning the thickness of doped transport layers and electrodes. As a result, the performance of PEDOT:PSS‐based OLEDs reaches that of indium tin oxide (ITO)‐based reference devices. The efficiency and the long‐term stability of PEDOT:PSS‐based OLEDs are significantly improved. The structure engineering demonstrated in this study greatly enhances the overall performances of ITO‐free transparent OLEDs in terms of efficiency, lifetime, and transmittance. These results indicate that PEDOT:PSS‐based OLEDs have a promising future for practical applications in low‐cost and flexible device manufacturing.

Effect of Cathode and Anode Fabrication on Flexible Transparent OLED Based on PEN Film

Among many display devices, organic light-emitting diodes (OLEDs) have been regarded most promising for the transparent flexible displays. In this work we fabricated transparent flexible OLEDs by adopting ITO/Ag/ITO multilayer as anode and Ag/Al thin film as cathode on PEN film and examined their performances.

Optical and barrier properties of thin-film encapsulations for transparent OLEDs

We investigate the optical and barrier properties of thin-film encapsulations (TFEs) for transparent organic light-emitting diodes (TOLEDs). To improve the barrier property of OLEDs, the number of dyads (Al2O3/polymer) and the thickness of polymer layer in the TFE structure are required to be increased. It is, however, demonstrated that a sharp dip appears in the transmittance of TFE films due to the interference of light caused by organic/inorganic multi-layered configuration, resulting in a dip in the top emission spectrum of TOLEDs. We have found that such a transmittance dip deepens when the number of dyads is large. What is worse, the number of transmittance dips and their sharpness are raised with increased thickness of the polymer layer. When the number of dyads is small, however, the effect of the polymer layer thickness on such a transmittance dip is weak. Therefore, we have addressed that the number of dyads needs to be reduced, but the thickness of the polymer layer should be increased to meet both optical and barrier properties of TOLEDs at the same time.

Directed emissive high efficient white transparent organic light emitting diodes with double layered capping layers

We report on white transparent organic light emitting diode (TOLED) with double layered capping layer (CL), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and ZnSe. By introducing the double layered CL, total efficiency was improved by 27% to have 90% of that of conventional bottom-emissive device. The achievement of highly improved efficiency was interpreted as the synergetic effect of constructive interference and maximized reflectance of CL. By adjusting the reflectance of the CL, it was possible to choose the emissions direction selectively and achieve spectral matching between top and bottom emissions as well as enhanced total efficiency in white TOLED.

The Optical Effects of Capping Layers on the Performance of Transparent Organic Light-Emitting Diodes

In transparent organic light-emitting diodes (TOLEDs), the asymmetry in the optical paths causes difference between the bottom and top emitting lights, both in emissions and spectral distributions. Capping layers (CLs) can be used as an optical functional to enhance the emissions and adjust the spectral distributions. Here, we report on the optical effects of an organic CL, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), on the characteristics of TOLEDs Both simulated and experimental results are presented. We demonstrate the possibility of improving the total emission and achieving spectral matching of bottom and top emissions by varying the CL thickness. The optical effects of CLs have been interpreted from interference perspectives. Finally, we have presented a guideline that is practically useful in designing high-performance TOLEDs with CLs.

Transparent organic light-emitting devices using a MoO3/Ag/MoO3 cathode

MoO3/Ag/MoO3 (MAM) stacks are investigated for utilization as transparent cathodes in organic light-emitting devices (OLEDs). The stacks can be fabricated using simple thermal evaporation at relatively low temperatures, and can be readily and safely utilized as top electrodes, without causing deposition damage to the organic layers. Results show that it is possible to achieve efficient electron injection from the MAM electrode into organic layers by means of incorporating a suitable electron injection layer (EIL) at the interface. Results also show that a MAM stack can exhibit high optical transmittance, amounting to about 65–80% in the 400–700 nm, and a low sheet resistance (9 Ω/□). A transparent OLED with a MAM cathode incorporated with a 10 nm Bphen: Cs2CO3 (10%) EIL is studied. By fine tuning the MAM structure, optimal OLED performance has been achieved: a total luminance of 1300 cd/m2 (representing ∼1000 cd/m2 and ∼300 cd/m2 from the bottom and the top, respectively) at 20 mA/cm2 and a corresponding driving voltage of 7.2 V. The OLED exhibits a peak transmittance of ∼90% in the 450–475 nm range, and a transmittance above 45% over the entire visible (i.e., 400–700 nm) range.

Realization of ultrathin silver layers in highly conductive and transparent zinc tin oxide/silver/zinc tin oxide multilayer electrodes deposited at room temperature for transparent organic devices

We report on transparent and highly conductive multilayer electrodes prepared at room temperature by RF sputtering of zinc tin oxide (ZTO) and thermal evaporation of ultrathin silver (Ag) as top contact for transparent organic light emitting diodes (TOLED). Specifically, we study the morphological, electrical and optical properties of the multilayer structure in particular of the thin Ag film. The tendency of Ag to form agglomerates over time on top of ZTO is shown by atomic force microscopy. From the optical constants derived from ellipsometric measurements we evidenced a bulk like behavior of an Ag film with a thickness of 8nm embedded in ZTO leading to a low sheet resistance of 9Ω/sq. Furthermore we verify the optical constants by simulation of an optimized ZTO/Ag/ZTO structure. As an application we present a highly efficient TOLED providing a device transmittance of >82% in the visible part of the spectrum. The TOLED shows no damage caused by sputtering on a lighting area of 80mm2 and exhibits efficiencies of 43cd/A and 36lm/W.

Transparent organic light-emitting diodes with LiF:Al composite cathodes

A transparent and electrical conductive layer comprising LiF and Al was designed and obtained. The composite layer was characterized using transparency spectroscopy and sheet resistance measurement. When the ratio of LiF:Al reached 4:4, the sheet resistance and transmittance of the composite layer were measured to be 573 Ω/□ and 55.5% (at 532 nm), respectively. Transparent organic light-emitting diodes (TOLEDs) employing LiF:Al composite layer as cathodes were fabricated and the electroluminescent properties of the TOLEDs were investigated. When the composite cathode had the equal contents of LiF and Al, the TOLED showed acceptable electroluminescent behavior of both top and bottom emission and the total maximum current efficiency (summed from both top and bottom emission) of the TOLED was calculated to be 2.34 cd/A.

Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects

We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical.

P‐169: Single Side Emitting Transparent OLED Lamp

AbstractTransparent OLEDs offer great potential for novel applications. Preferably, the light should be emitted from one side only. This can be done to some extent by modifying electrode thicknesses, but at the cost of reduced transparency. Here, we demonstrate a new approach to make single side emissive OLEDs while maintaining good transparency.