Flexible hybrid substrates of roll‐to‐roll manufacturing for flexible display application

A flexible hybrid substrate was developed and demonstrated for roll‐to‐roll (R2R) manufacturing. Layer‐by‐layer misalignment can be well controlled within 5 µm. Top‐gate amorphous InGaZnO thin‐film transistor was fabricated on the flexible hybrid substrate by R2R process for the first time. A 4.3‐in. segment‐type reflective Electro‐Phoretic Display (EPD) display was also demonstrated to show the R2R capability of flexible substrates.     

Development of a high‐resolution RGBW flexible display using a white organic light‐emitting diode with microcavity structure and that of a side‐roll touch panel

In this study, white organic electroluminescent devices with microcavity structures were developed. A flexible high‐resolution active‐matrix organic light‐emitting diode display with low power consumption using red, green, blue, and white sub‐pixels formed by a color‐filter method was fabricated. In addition, a side‐roll touch display was developed in combination with a capacitive flexible touch screen.     

Development of a color‐tunable polychromatic organic‐light‐emitting‐diode device for roll‐to‐roll manufacturing

A flexible vertically stacked flexible polychromatic color‐tunable OLED has been developed by means of low resistive intermediate electrode technology. The polychromatic OLED has a capability to show 16 million colors with 105% National Television Committee Standard (NTSC) color reproduction. The device can produce arbitrary shape with arbitrary colors, suitable for artistic expressions, just as many as those used in information displays. Independently controlled red, green, and blue light‐emitting layers are stacked vertically. With conventional indium tin oxide technology, because of the temperature restriction, it was quite difficult to achieve low resistance on plastic substrate. The reported numbers were all more than 80 Ω/□. According to the surface mobility control using Fick's law analysis, low sheet resistance 7.34 Ω/□ on plastic film was developed. At first, flexible 7.17 cm² transparent OLED was fabricated for the performance confirmation of transparent electrode. And then polychromatic color‐tunable OLED with the same size were successfully fabricated on plastic. With optical length optimization for each color stack of polychromatic OLED, more than 100% color reproduction in National Television Committee Standard was achieved by stack design. The polychromatic device can be used for colored illumination, as well as for organic‐light‐emitting display pixels for three times emission than conventional pixel design. The device is fabricated on plastic substrate so that the polychromatic organic‐light‐emitting‐diode device is manufacturable with roll‐to‐roll production line.     

Direct imaging exposure equipment with high overlay accuracy for flexible substrate in roll‐to‐roll method

In order to fabricate thin‐film transistors on a flexible substrate, it is required to overlay patterns with high alignment accuracy. A polymer film can be deformed greatly and easily through thin‐film transistor fabrication processes because the glass transition point of a polymer film is below the maximum temperature of the processes. Furthermore, low stiffness of a polymer film can cause deformation when the polymer film is loaded on a working holder. These matters make it difficult to make patterns directly on a polymer film with high alignment accuracy. We proposed three concepts that solve three technical issues in order to directly make patterns with high overlay accuracy on a flexible film without a support substrate. We developed roll‐to‐roll exposure equipment with high overlay accuracy for a flexible substrate. Using this equipment, we succeeded to directly make patterns on a PET film, achieving a resolution of less than 6‐μm line and space pattern and an overlay accuracy of less than ±5 μm.     

Roll‐to‐roll manufacturing of electronics on flexible substrates using self‐aligned imprint lithography (SAIL)

Abstract— The manufacture of large‐area arrays of thin‐film transistors on polymer substrates using roll‐to‐roll (R2R) processes exclusively is being developed. Self‐aligned imprint lithography (SAIL) enables the patterning and alignment of submicron‐sized features on meter‐scaled flexible substrates in the R2R environment. SAIL solves the problem of precision interlayer registry on a moving web by encoding all the geometry information required for the entire patterning steps into a monolithic three‐dimensional imprint with discrete thickness modulation. The pre‐aligned multiple‐step mask structure maintains its alignment regardless of subsequent substrate distortion. Challenges are encountered in relation to the novel nature of using flexible substrates and building toolsets for the R2R processing. In this paper, methods of the SAIL process, the resulting active‐matrix backplanes, the trajectory of SAIL process development, and the remaining issues for production are presented.     

Optical design and roll‐to‐roll fabrication process of microstructure film for wide viewing LCDs

We have developed a new microstructure film for wide viewing liquid crystal displays (LCDs). By attaching it to the surface of a conventional LCD, the viewing angle characteristics of LCD has drastically improved without causing a blur of the frontal image and a decrease in the contrast ratio under bright ambient light conditions. This film can be applied to various LC modes including twisted nematic and multidomain vertical alignment by changing its internal micrometer‐size 3D structure. Further, this film can be mass‐produced efficiently by self alignment roll‐to‐roll process.     

Manufacture of organic transistors on large‐area flexible substrate using direct imaging exposure system by wet process suitable for roll‐to‐roll production

Organic thin‐film transistors (OTFTs) can be fabricated via a wet process and have exceptionally high flexibility. Therefore, production using the roll‐to‐roll (RtoR) method is expected. We succeeded in developing a new OTFT wet fabrication process adaptable to the RtoR process. Utilizing the electroless plating method for wiring formation, all materials can be formed in a wet process and can be patterned using the photolithography process. In addition, we succeeded in fabricating OTFT on an A4‐type flexible substrate using RtoR direct imaging exposure system.     

Fabrication of flexible organic light‐emitting‐diode display with a flexible color filter driven by In—Ga—Zn—Oxide TFTs

Abstract— A novel flexible active‐matrix organic light‐emitting‐diode (OLED) display fabricated on planarized stainless—used‐steel substrates with a resolution of 85 dpi in a 4.7‐in. active area has been demonstrated. Amorphous indium—gallium—zinc—oxide thin‐film transistors were used as the backplane for the OLED display with high device performance, high electrical stability, and long lifetime. A full‐color moving image at a frame frequency of 60 Hz was also realized by using a flexible color filter directly patterned on a plastic substrate with a white OLED as the light source.     

Properties of column spacers on a flexible color filter using a roll‐to‐roll process

Abstract— Column spacers were manufactured by a roll‐to‐roll process for a flexible plastic substrate and an LCD prototype was demonstrated for the first time. The purpose of this paper is to obtain a stable cell gap for plastic LCDs by designing the column spacers. The optimum condition was derived for the properties of the column spacers parameters, such as the cross section, density, and hardness. As a result, we could determine the optimum elastic modulus of the column spacers needed in order to precisely maintain the cell gap of the LCD as well as that of conventional glass plates, resulting in a uniform display luminance.     

Development of rollable silicon thin‐film‐transistor backplanes utilizing a roll‐to‐roll continuous lamination process

Abstract— Rollable silicon thin‐film‐transistor (TFT) backplanes utilizing a roll‐to‐roll process have been developed. The roll‐to‐roll TFT‐backplane technology is characterized by a glass‐etching TFT transfer process and a roll‐to‐roll continuous lamination process. The transfer process includes high‐rate, uniform glass‐etching to transfer TFT arrays fabricated on a glass substrate to a flexible plastic film. In the roll‐to‐roll process, thinned TFT‐glass sheets (0.1 mm) and a base‐film roll are continuously laminated using a permanent adhesive. Choosing both an appropriate elastic modulus for the adhesive and an appropriate tension strength to be used in the process is the key to suppressing deformation of the TFT‐backplane rolls caused by thermal stress. TFT backplanes that can be wound, without any major physical damage such as cracking, on a roll whose core diameter is approximately 300 mm have been sucessfully obtained. Incorporating the TFT‐backplane rolls into other roll components, such as color‐filter rolls, will make it possible to produce TFT‐LCDs in a fully roll‐to‐roll manufacturing process.     

Microcup® displays: Electronic paper by roll‐to‐roll manufacturing processes

Abstract— Rolls of flexible displays or electronic paper have recently been prepared by a high‐speed roll‐to‐roll manufacturing process based on SiPix's novel Microcup® and top‐sealing technologies. Both Microcup® electrophoretic displays (EPDs) and LCDs have been demonstrated. The display rolls are format flexible and may be cut into desirable size and shape for a variety of applications. High‐performance flexible passive‐matrix Microcup® EPDs having a wide range of threshold voltages have also been demonstrated.     

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.     

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.     

Development of flexible displays using back‐channel‐etched In–Sn–Zn–O thin‐film transistors and air‐stable inverted organic light‐emitting diodes

We developed flexible displays using back‐channel‐etched In–Sn–Zn–O (ITZO) thin‐film transistors (TFTs) and air‐stable inverted organic light‐emitting diodes (iOLEDs). The TFTs fabricated on a polyimide film exhibited high mobility (32.9 cm²/Vs) and stability by utilization of a solution‐processed organic passivation layer. ITZO was also used as an electron injection layer (EIL) in the iOLEDs instead of conventional air‐sensitive materials. The iOLED with ITZO as an EIL exhibited higher efficiency and a lower driving voltage than that of conventional iOLEDs. Our approach of the simultaneous formation of ITZO film as both of a channel layer in TFTs and of an EIL in iOLEDs offers simple fabrication process.     

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.     

Development of a liquid crystal with fine‐pitch light‐source display (LFD) using an organic light‐emitting diode (OLED)

Abstract— A novel display system, refered to as an LFD (liquid crystal with fine‐pitch light‐source display) is proposed. In an LFD, an auxiliary light source patterned with a fine pitch is attached to a reflective liquid‐crystal display (LCD), and a light shield is formed on the observer's side of the light source. A vertical‐alignment LCD (VA‐LCD) is attached as the reflective LCD, and an organic light‐emitting diode (OLED) is attached as the fine‐pitch light source. An LFD can produce bright, high‐contrast images under any ambient light. A test sample was built and its display characteristics confirmed.     

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.     

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.