High‐efficiency p‐i‐n organic light‐emitting diodes with long lifetime

Abstract— High‐performance organic light‐emitting diodes (OLEDs) are promoting future applications of solid‐state lighting and flat‐panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p‐doped hole‐transport layer/intrinsically conductive emission layer/n‐doped electron‐transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very‐high‐efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double‐emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)₃], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m² are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top‐emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive‐matrix substrates as well as active‐matrix substrates on low‐temperature polysilicon (LTPS) or, in particular, amorphous silicon (a‐Si).     

Solution‐processable thermally activated delayed fluorescence emitters for application in organic light emitting diodes

Solution‐processed organic light emitting diodes (OLEDs) have been fabricated using the thermally activated delayed fluorescence (TADF) emitter, DACT‐II, and its soluble derivative, tBu‐DACT‐II, as emitting dopants. DACT‐II reportedly exhibits very high external quantum efficiencies (EQEs) in vacuum‐processed OLEDs. The solution‐processed DACT‐II‐based and tBu‐DACT‐II‐based OLEDs exhibited external quantum efficiency exceeding the theoretical upper limit of classical fluorescent OLEDs.     

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.     

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.     

Printable phosphorescent organic light‐emitting devices

Abstract— A new approach to full‐color printable phosphorescent organic light‐emitting devices (P²OLEDs) is reported. Unlike conventional solution‐processed OLEDs that contain conjugated polymers in the emissive layer, the P²OLED's emissive layer consists of small‐molecule materials. A red P²OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m², a green P²OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m², a light‐blue P²OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m², and a blue P²OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m² is presented.     

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.     

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.     

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.     

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.     

Composite silver electrode for double‐sided‐emitting stacked organic light‐emitting diode

Abstract— A reflective composite silver electrode is proposed and characterized as the middle electrode of a stacked organic light‐emitting diode (OLED) with double‐sided light emission. The proposed electrode is composed of a thermally evaporated stack of LiF (1 nm)/Al (3 nm)/Ag (70 nm) layers. The LiF/Al and the plasma‐treated Ag of the electrode function well as the respective cathode and anode of the bottom‐ and top‐emitting stacked OLEDs, with both being of the non‐inverted type. Power efficiencies of 10.3 and 12.1 lm/W at 100 cd/m² have been measured for bottom‐ and top‐emitting OLEDs, respectively, using dye doping. The stacked OLED having this bipolar middle electrode can be constructed as a two‐terminal‐only device, allowing for simpler driving schemes in double‐side‐emitting passive‐/active‐matrix OLED displays.     

Transflective device with a transparent organic light‐emitting diode and a reflective liquid‐crystal device

Abstract— A high‐transmittance transflective device based on a hybrid structure consisting of a transparent organic light‐emitting diode (OLED) stacked on top of a reflective liquid‐crystal device (RLCD) was conceptually demonstrated. By placing the transparent OLED on top of a vertically aligned LCD operated under normally black mode, a transmittance as high as 75.7% was obtained due to the asymmetric emission characteristics of a transparent OLED. To further improve the performance in the transmissive mode, a polarizer‐free LCD was used, which yielded an ultra‐high transmittance (82.2% overall).     

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.     

Improved performances in organic and polymer light‐emitting diodes using solution‐processed vanadium pentoxide as a hole injection layer

We report that a solution‐processed vanadium pentoxide (V₂O₅) layer can be utilized as an effective and stable hole injection layer in organic light‐emitting diodes and polymer light‐emitting diodes instead of polyethylene dioxythiophene : polystyrenesulfonate (PEDOT : PSS). The organic light‐emitting diode and polymer light‐emitting diode with the V₂O₅ layer have driving voltages that are 2.2 and 0.3 V lower for 1000 cd/m², respectively, than the devices with PEDOT : PSS. In addition, the devices with the V₂O₅ layer show improved operational stability compared with the devices with PEDOT : PSS. Therefore, a solution‐processed V₂O₅ layer can be utilized as an effective and stable hole injection layer instead of PEDOT : PSS.     

Electrical simulations of doped multilayer organic light‐emitting diodes (OLEDs) under temperature stress for high current densities

Abstract— This work aims at explaining and predicting the influence of the thickness of organic materials, dye doping, and space‐charge effects on charge‐carrier transport at different operating temperatures for high current densities (50 ≤ J ≤ 7000 mA/cm²). For the purpose of determining these influences, current‐voltage characteristics for typical electrically doped multilayer organic light‐emitting diodes (OLEDs) have been simulated. The results of the simulations concur with experimental data.     

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.     

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.     

Highly efficient white organic light‐emitting diodes with over 100 lm/W for next‐generation solid‐state lighting

High‐performance two‐unit all‐phosphorescent white devices on a built‐up light extraction substrate that comprised high‐index materials were studied. As a result of suitable optical and electrical design, the device showed an extremely high efficacy of 114 lm/W at 1000 cd/m². The device also showed 102 lm/W with long lifetime (LT70) of over 10,000 h at 3000 cd/m². Outstanding external quantum efficiency of almost 50% was also achieved in a flat panel with an emissive area of 25 cm². Color coordinates of the panel met the Energy Star ® criteria of solid‐state lighting with CIE (Commission Internationale de l'Éclairage) 1931 (x, y) = (0.477, 0.423), and the color rendering index was 81.     

Preparation and characterization of highly efficient blue TPBI:FIrpic organic light‐emitting devices

Abstract— Blue phosphorescent organic light‐emitting devices (PhOLEDs) using 1,3,5‐tris(N‐phenyl‐ benzimiazole‐2‐yl)benzene [TPBI] as the host and bis((4,6‐difluorophenyl)‐pyridinate‐N,C2′)picolinate [FIrpic] as the dopant in the emitter were fabricated with different treatments of the hole‐transport layers and doping levels. Among the experimental devices, the best electroluminescent characteristics were obtained in the device with the combined hole‐transport layer of N,N′‐diphenyl‐N,N′‐bis‐[4‐ (phenyl‐m‐tolylamino)‐phenyl]‐biphenyl‐4,4′‐diamine [DNTPD]/1, 1‐bis‐(di‐4‐polyaminophen yl)‐ cyclo‐hexane [TAPC] and a doping level of 10‐vol.% FIrpic. The device with a structure of DNTPD/TAPC/TPBI:Firpic (10%) showed a luminance of 1300 cd/m² at an applied voltage of 10 V, a maximum current efficiency of 18 cd/A, and color coordinates of (0.17, 0.43) on the Commission Internationale de I'Eclairage (CIE) chart.     

Flexible high‐resolution full‐color top‐emitting active‐matrix organic light‐emitting diode display

We developed a high‐performance 3.4‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display with remarkably high resolution using an oxide semiconductor in a backplane, by applying our transfer technology that utilizes metal separation layers. Using this panel, we also fabricated a prototype of a side‐roll display for mobile uses. In these AMOLED displays, a white OLED combined with a color filter was used in order to achieve remarkably high resolution. For the white OLED, a tandem structure in which a phosphorescent emission unit and a fluorescent emission unit are serially connected with an intermediate layer sandwiched between the emission units was employed. Furthermore, revolutionary technologies that enable a reduction in power consumption in both the phosphorescent and fluorescent emission units were introduced to the white tandem OLED.     

Thin‐film barriers using transparent conducting oxides for organic light‐emitting diodes

Abstract— This study covers thin‐film barriers using inorganic barriers of transparent conducting oxides (TCOs) such as zinc oxide (ZnO) and indium tin oxide (ITO). The TCOs were fabricated using a sputtering method with a process gas of pure argon at room temperature. ITO showed better properties as a barrier than the ZnO and exhibited the electronic performance necessary to perform additional functions. The ITO has superior barrier performance because it has a lower crack density due to its partial amorphous phase. For organic/inorganic multilayer barriers, the organic underlayer decreased the water‐vapor transmission rate (WVTR) more than the organic upper layer, indicating that the planarization effect was important in reducing the WVTRs. The results of this organic/ITO multilayer barrier study are expected to be useful in finding a practical solution to OLED encapsulation.