Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink‐jet‐printed organic light‐emitting‐diode devices is discussed. Unlike previously reported solution‐processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink‐jet‐printed P²OLED (IJ‐P²OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small‐molecule solution. Ink‐jet‐printed green P²OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m² and is based on phosphorescent emission. The latest solution‐processed phosphorescent OLED performance by spin‐coating is disclosed. The red P²OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m². The green P²OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m². Also discussed is a newly developed sky‐blue P²OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m².     

A novel electron‐transport material for organic light‐emitting devices

Abstract— A nanocrystalline electron‐transport material [ET68] was introduced into organic light‐emitting devices (OLEDs). By integrating a p‐doped transport system and phosphorescent emitters, a very bright and stable device could be obtained. Furthermore, 40% saving in power consumption can be achieved when the efficient pixels with ET68 were applied to AMOLEDs.     

Highly efficient deep‐blue organic light‐emitting devices

Abstract— A highly efficient deep‐blue organic light‐emitting device (OLED) incorporating a novel composite hole‐transport layer (c‐HTL) and an emitter based on the new non‐symmetrical mono(styryl)amine fluorescent dopant in the stable host MADN, which achieved a luminance efficiency of 5.4 cd/A with a Commission Internationale d'Eclairage (CIEx,y) of (0.14, 0.13) and an external quantum efficiency of 5.1% at 20 mA/cm² and 6.8 V, is reported. The increased device efficiency is attributed to an improved balance between hole and electron currents in the recombination zone.     

Passive‐matrix polymer light‐emitting displays

Abstract— Organic light‐emitting device research focuses on the use of small‐molecule and polymer materials to make organic electroluminescent displays, with both passive‐ and active‐matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full‐color passive‐matrix displays. The stability of polymer OLEDs, and the use of ink‐jet printing for direct high‐resolution patterning of the light‐emitting polymers will also be discussed. It will be shown that the performance of light‐emitting polymers is at the brink of being acceptable for practical applications.     

Electrical and optical properties of white organic light‐emitting devices using blue and orange phosphorescent materials

Abstract— Highly efficient white organic light‐emitting devices have been fabricated by doping phosphorescent orange and blue emitters into the separate layers of a single host. The efficiency and electroluminescence spectrum were strongly affected by the sequence of doped layers. The phosphorescent white devices exhibiting high efficiency and reasonable white balances are obtained when the recombination region is overlapped by the blue doped region. By using this principle, a simple structured phosphorescent white device with a peak power efficiency of 40.7 lm/W and Commission International de L'Eclairage coordinates of (0.43, 0.42) have been demonstrated.     

Performance enhancement of top‐ and bottom‐emitting organic light‐emitting devices using microcavity structures

Abstract— In order to improve the efficiency of top‐ and bottom‐emitting devices, metallic electrodes have been used to create microcavity effects within the OLED structure. Semi‐transparent Ag is used as the anode in bottom‐emitting microcavity structures, whereas various reflective opaque metallic anodes are used for the top emitters. The cathode used in both configurations is MgAg — thick and opaque in the case of the bottom emitter and thin and semi‐transparent in the case of the top emitter. Modeling and experiments show that for the top‐emitting structures, the device efficiency is roughly proportional to the reflectivity of the anode in the low reflectivity range and increases significantly more than predicted by reflectivity alone in the high‐reflectivity range. An ultrathin CF_x or MoO_x hole‐injecting layer allows for the use of many metals as anodes and is an important feature of the device structure. With an Ag anode, both the top‐ and bottom‐emitting microcavity devices are about twice as efficient (on axis) as the analogous nonmicrocavity bottom‐emitting device. Microcavity devices employing a C545T‐doped Alq emitter exhibit efficiencies of 21 cd/A at 6.4 V and 20 mA/cm², with operational stability equivalent to conventional bottom‐emitting structures.     

Energy‐recycling high‐contrast organic light‐emitting devices

Abstract— It is reported that by integrating OLEDs with solar cells, ambient‐light reflection as low as 1.4% (even superior to that achieved with polarizers) can be achieved without compromising the EL efficiency for high‐contrast display applications. Furthermore, in such a configuration, the photon energies of both the incident ambient light and the portion of OLED emission not getting outside of the device can be recycled into useful electrical power via the photovoltaic action, instead of being wasted as in other reported contrast‐enhancement techniques. These features, we believe, shall make this present technique attractive for high‐contrast display applications and portable/mobile electronics that are highly power‐aware.     

Efficient and bright blue‐emitting phosphorescent materials

Abstract— A new type of ancillary ligand for blue‐emitting heteroleptic iridium complexes has been successfully developed. New ligands, 3‐(trifluoromethyl)‐5‐(pyridin‐2‐yl)‐1,2,4‐triazolate and 5‐(pyridin‐2‐yl)‐tetrazolate, show stronger blue‐shifting power than that of the picolate of FIrpic [iridium (III) bis(4,6‐difluorophenylpyridinato)picolate]. Organic light‐emitting diodes (OLEDs) fabricated with a new complex, FIrtaz [iridium (III) bis(4,6‐difluorophenylpyridinato)(5‐(pyridine‐2‐yl)‐1,2,4‐triazolate) or FIrN4 [(iridium (III) bis(4,6‐difluorophenylpyridinato)(5‐(pyridin‐2‐yl)‐tetrazolate], as the blue dopant in the host of mCP [1,3‐ bis(9‐carbazolyl)benzene], exhibit near‐saturated blue electrophosphorescence with Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14, 0.18) and (0.15, 0.24), respectively.     

Influence of the atmosphere on organic–organic interfacial layers and deterioration in organic light‐emitting diodes

The influence of organic–organic interfacial contaminants and deterioration in organic light‐emitting diodes (OLEDs) was investigated. There was deterioration of the device characteristics when atmospheric contamination was introduced to the emission site. We simultaneously observed a decrease of the maximum capacitance, C_max, of the OLEDs, implying that there was charge accumulation at the interface. Our study demonstrates that maintaining the interface adjacent to emission site free from contaminants is crucial to protect the device from deterioration.     

Model for assessing organic light‐emitting diode display lifetime across applications

A model for assessing organic light‐emitting diode (OLED) display lifetime is developed and discussed for estimating OLED display lifetime in various applications. The lifetime model extends existing stretched exponential models of luminance decay for OLED devices to permit this decay to be estimated as a function of time and current density. This extended model is illustrated within an application to assess the power consumption and luminance decay of diodes within an OLED display. Various metrics of display lifetime are discussed with the aim of developing methods to assess the perceived lifetime of an OLED display to global and local luminance decay mechanisms. Finally, these metrics are applied to illustrate the performance of the model for assessing the impact of an image processing algorithm on OLED display lifetime.     

Highly efficient solution‐processible phosphorescent dendrimers for organic light‐emitting diodes

Abstract— Currently, most research into organic light‐emitting diodes (OLEDs) has focused on two main classes of materials: small organic molecules and conjugated polymers. An alternative approach is to use conjugated dendrimers. We show that conjugated dendrimers are a promising new class of solution‐processible materials for use as the active layer in highly efficient organic LEDs. By optimizing the choice of device structure, host material, and electron transport layer, we can obtain efficiencies of 55 cd/A and power efficiencies of 40 lm/W. This is an excellent result for a spin‐coated emissive layer.     

Towards large‐area full‐color active‐matrix printed polymer OLED television

Abstract— We have developed a new multi‐head polymer OLED ink‐jet‐printing technology to make large‐screen OLED television displays. This printer is used to make a 13‐in.‐diagonal 16:9‐format polymer‐OLED prototype driven by an LTPS active matrix with a pixel circuit which compensates for TFT threshold‐voltage variations. A novel scrolling‐bar addressing scheme is used to reduce motion artifacts and to make sparkling images with a high local peak brightness. The scalability of the polymer‐OLED technology to larger sizes for television applications is discussed.     

Analyzing and tuning emission characteristics of top‐emitting organic light‐emitting devices

Abstract— Top‐emitting organic light‐emitting devices (OLEDs) have several technical merits for application in active‐matrix OLED displays. Generally, stronger microcavity effects inherent with top‐emitting OLEDs, however, complicate the optimization of device efficiency and other viewing characteristics, such as color and viewing‐angle characteristics. In this paper, using the rigorous classical electromagnetic model based on oscillating electric dipoles embedded in layered structures, the emission characteristics of top‐emitting OLEDs as a function of device structures will be analyzed. From comprehensive analysis, trends in the dependence of ewmission characteristics on device structures were extracted, and, accordingly, a general methodology for optimizing viewing characteristics of top‐emitting OLEDs for display applications will be suggested. The effectiveness of the analysis and the methodology was confirmed by experimental results.     

Displacement‐current analysis of organic light‐emitting diodes

Abstract— Organic light‐emitting diodes (OLEDs) having multiple organic layers were fabricated to analyze the physical phenomena occurring in an OLED according to the amplitude of the applied voltage. The staircase voltage with both an increasing period and a constant period was designed and applied to an OLED. The displacement current began to change at a voltage where the conduction current began to change, and partly originated from the formation of space charge due to the low mobility of the majority carrier. The displacement current was shown to be constant at low voltage and decreased after showing a maximum value as the applied voltage increased. The exact voltage for the injection of two types of carriers and light emission could be obtained from the variation in the displacement current.     

Development of flexible organic light‐emitting diode on barrier film and roll‐to‐roll manufacturing

To come out with a successful organic light‐emitting diode (OLED) lighting business, it is very important to have clear differentiation of OLED from LEDs. Flexible OLED has merits, such as capability to be mounted on the curved wall, which is not easy for LEDs to achieve the feature. There are several approaches to make flexible OLEDs especially among those plastic barrier films that can bring high level of flexibility, which could not be achieved by any conventional lighting method. In this paper, barrier films with various water vapor transmission rate values, including 10^{? 6} order, are applied, and the conditions to have almost no dark spot growth under 85 °C and 85% high temperature/humidity test are shown. Flexible OLED panels are manufactured with the world's first roll‐to‐roll equipment using plastic barrier film.     

A new wettability‐control technique for fabricating color OLED panels by an ink‐jet‐printing method

Abstract— The fabrication technique for color OLED panels by means of wettability‐controllable hole‐injection material (HIM) and a photocatalytic lithography method achieves both precise ink‐jet printing and long‐lifetime devices. The technique enables us to selectively change the non‐wetting surface of a hole‐injection layer (HIL) of metal‐oxide nanoparticles (MONPs) into a wetting surface without damage to the device performance. Wetting patterns formed by this method with photocatalyst‐coated photomasks made it possible to print emission material with patterns of precisely 98‐μm widths on the hole‐injection layer. A fluorescent green‐emitting device fabricated with an HIM of MONPs by the photocatalytic treatment exhibited a long lifetime of 365 hours at30,000 cd/m², which can be extrapolated to a lifetime of more than 110,000 hours at 1000 cd/m², assuming an acceleration coefficient of 1.7. A two‐color device and a monochrome passive‐matrix panel were also successfully fabricated. The two‐color device emitted light without the mixing of colors. The monochrome panel displayed alphabetical characters with good uniformity and no flaws.     

Detailed analysis of exciton decay time change in organic light‐emitting devices caused by optical effects

Abstract— The exciton decay time in organic light‐emitting devices (OLEDs) depends on the optical environment, i.e., the thicknesses and refractive indices of all layers in a device. The decay of an exciton can occur through a radiative or a non‐radiative channel. Each of these channels has a probability, which is expressed by, respectively, the radiative and the non‐radiative decay rate. The radiative decay rate is influenced by the optical environment, i.e., the OLED's thin‐film layer structure. In this paper, a model for estimating the change of the exciton decay time (inverse of the decay rate) is presented. In addition, the decay time change in both top‐ and bottom‐emitting OLEDs as a function of the charge‐transport layer thicknesses has been investigated. Furthermore, the most important mechanism responsible for the exciton decay time change is outlined.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Thermally cross‐linkable copolymer and its evaluation as a hole transport layer in organic light‐emitting diode devices

This study reports on the synthesis of new thermally cross‐linkable copolymers containing a reactive cross‐linking comonomer. Synthesized polymers showed narrow molecular weight distribution (polydispersity) between 1.18 to 1.22 and 54 to 67% monomer conversion and incorporation of 2 to 7 mol% vinylbenzylcyclobutene comonomer. The polymer was soluble in nonpolar organic solvents such as chloroform, dichloromethane, toluene, and chlorobenzenes, and when cross‐linked, showed resistance to solubility in the previously listed solvents. The cross‐linked films exhibited uniform surface roughness below 1 nm. A polymer containing ~3.6 mol% vinylbenzylcyclobutene was thermally cross‐linked and evaluated as a hole‐transporting layer in green organic light‐emitting diode devices. The devices showed a maximum current efficiency of 39.5 cd/A at a current density of 2.7 mA/cm² and a brightness of 1000 cd/m² with an International Commission on Illumination coordinate (0.33, 0.62). The device performances are found comparable with the ones with the conventional hole‐transporting layer material, NPD.     

Optical characteristics of tandem and microcavity tandem organic light‐emitting devices

Abstract— In pursuit of the further enhancement of the luminance and efficiency of organic light‐emitting devices (OLEDs), it is worthy of exploring what benefits could be obtained by combining two luminance‐enhancement techniques, i.e., microcavity and tandem OLEDs. Furthermore, a deeper understanding of the optics in tandem OLEDs will be useful for the design and optimization of tandem OLEDs. In this paper, the optical characteristics of noncavity and microcavity tandem OLEDs are theoretically and experimentally investigated. By the use of rigorous electromagnetic modeling of OLEDs, the radiation characteristics of tandem OLEDs as a function of device structures are analyzed and correspondingly, the guidelines for optimizing the performance of tandem devices are suggested. By making use of the analytical results, it is shown that with well‐designed microcavity conditions and device structures, a five‐fold enhancement in luminance in the normal direction can be achieved with cavity‐tandem devices having only two emitting units. A very high efficiency of 200 cd/A for a rather broad brightness range of 100–4000 nits is demonstrated with a phosphorescent cavity two‐unit device.