Full‐color AMOLED with RGBW pixel pattern

Abstract— A full‐color AMOLED display with an RGBW color filter pattern has been fabricated. Displays with this format require about one‐half the power of analogous RGB displays. RGBW and RGB 2.16‐in.‐diagonal displays with average power consumptions of 180 and 340 mW, respectively, were characterized for a set of standard digital still camera images at a luminance of 100 cd/m². In both cases, a white‐emitting AMOLED was used as the light source, and standard LCD filters were used to provide the R, G, and B emission. The color gamuts of these displays were identical and the higher overall efficiency of the RGBW format results from two factors. First, a large fraction of a typical image is near neutral in color and can be reproduced using the white sub‐pixel. Second, the white sub‐pixel in an RGBW AMOLED display is highly efficient because of the absence of any color filter. The efficiency of these displays can be further enhanced by choosing a white emitter optimized to the target display white point (in this case D65). A two‐emission layer configuration based upon separate yellow and blue‐emitting regions is shown to be well suited for both the RGBW and RGB formats.     

High‐temperature thin‐film barriers for foldable AMOLED displays

We present a thin‐film dual‐layer bottom barrier on polyimide that is compatible with 350°C backplane processing for organic light‐emitting diode displays and that can facilitate foldable active‐matrix organic light‐emitting diode devices with a bending radius of <2 mm. We demonstrate organic light‐emitting diodes that survive bending over 0.5 mm radius for 10.000× based on the high‐temperature bottom barrier. Furthermore, we show compatibility of the bottom barrier with the backplane process by fabricating active‐matrix organic light‐emitting diode displays on GEN1‐sized substrates.     

Analysis of color volume of multi‐chromatic displays using gamut rings

There are claims that multi‐chromatic displays can achieve a wider color gamut by the use of additional highly saturated secondary color channels. However, there are other claims that these displays lose lightness and/or color saturation at brighter levels. These apparently divergent views have led to some controversy in the display industry and at standard setting organizations. This study examines the color gamut volume for a variety of simulated and measured multi‐chromatic (sometimes incorrectly referred to as “multi‐primary”) displays using combinations of white and/or secondary color channels, such as cyan, magenta, and yellow. Furthermore, a two‐dimensional gamut representation, referred to as “gamut rings,” is introduced to illustrate that the addition of nonprimary optical color channels to a trichromatic (RGB) display can result in a significant decrease in the chroma at higher lightness levels. The additional saturated color channels can increase the gamut volume only around their hues at darker levels. The results also confirm the validity of comparing the color light output and white light output for revealing the design trade‐offs between the high‐peak white and the color‐image brightness for multi‐chromatic displays.     

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.     

Thermal management of edge‐lit wide color gamut backlight for LCD using red laser diodes and cyan LEDs

The color gamut is one of the critical parameters that dictate the image quality of displays. The liquid crystal displays using white color light‐emitting diodes (LEDs) as the backlight, though having been widely employed recently, are not very satisfactory in terms of their color gamut because of the broad spectrum inherent to white LEDs. This prompted the authors to develop improved liquid crystal displays using an edge‐lit wide color gamut backlight that used red laser diodes and cyan LEDs. Generating laser beams with high color purity, the laser diodes are light sources with a significant effect on expanding the color gamut. However, laser diodes, red ones in particular, have unfavorable thermal characteristics. To cope with this shortcoming, the authors clearly defined the restrictive criteria for laying out two kinds of light source on the edge‐lit backlight and made a prototype 55‐type laser backlight for performance evaluation.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.     

Prospects and challenges of mini‐LED and micro‐LED displays

We review the emerging mini/micro–light‐emitting diode (LED) displays featuring high dynamic range and good sunlight readability. For mini‐LED backlit liquid crystal displays (LCDs), we quantitatively evaluate how the device contrast ratio, local dimming zone number, and local light profile affect the image quality. For the emissive mini/micro‐LED displays, the challenges of ambient contrast ratio and size‐dependent power efficiency are analyzed. Two figure‐of‐merits are proposed for optimizing the optical and electrical performances of mini/micro‐LED displays.     

A pixel structure for simultaneous programming and emission method for shutter‐glasses‐type stereoscopic 3‐D AMOLED displays

Abstract— A pixel structure for shutter‐glasses‐type stereoscopic 3‐D active‐matrix organic light‐emitting‐diode (AMOLED) displays is proposed. The proposed pixel programs data to the pixel during the light‐emission time of an OLED. Because the emission time of the proposed pixel is extended, it is expected that the proposed pixel not only decreases the peak current of the OLED during the emission period but also reduces flicker. Moreover, the aperture ratio of the proposed pixel is 58.69% for a 50‐in. full‐high‐definition (FHD) condition by minimizing the number of thin‐film transistors (TFTs), capacitors, and control signal lines as seven TFTs, two capacitors, two power lines, and four control lines per unit pixel. Simulation results show that the error in the emission current of the proposed pixel is from ?0.82% to +0.90% when the threshold‐voltage variation of the driving TFT is ±1.00 V, and the maximum variation of the emission current is ?1.35% when a voltage drop in the power line is ?0.50 V on a full‐white‐image display.     

Microdisplay‐based industrial 3‐D and microstructure measurement systems

Abstract— Microdisplays, whether they are of the liquid‐crystal‐on‐silicon (LCOS) or organic light‐emitting diode (OLED) type, have been, up until now, mainly used in multimedia applications or head‐mounted displays. Due to their interesting possibilities, these displays open more and more alternative applications; for example, in optical metrology. Projection lenses for this application area need to be specially designed because the requirements on these systems differ completely from those for multimedia applications. The lenses must have very low geometrical image distortion and they have to be adapted to small objects and/or image distances. On the other hand, they often work with light sources with small spectral bandwidths; consequently, they do not need to be corrected for chromatic aberrations. In addition, the numerical aperture has to be large enough to collect and transfer as much light as possible, but also the size of the projection lens has to be as small as possible to ensure compact measurement systems. All these requirements lead to a compromise in optical lens design. Three optical system designs and realizations — one with an OLED microdisplay and two with an LCOS microdisplay — are presented.     

Highly efficient white OLEDs with single solution‐processed emitting layer fabricated by one‐step coating using self‐layered technique

We propose a self‐layered technique to form an emitting layer with a pseudo‐multilayered structure by one‐step coating and demonstrate the feasibility of its concept. We also fabricated a highly efficient white organic light‐emitting diode with the proposed technique. A maximum power efficiency of 70 lm/W was obtained by improving the effective radiation efficiency, carrier balance efficiency, and light‐extraction efficiency.     

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.     

Enhancement of blue‐light‐emission properties for OLED displays by using a polarized light‐recycling structure

Abstract— The blue‐light‐emission properties of organic light‐emitting‐diode (OLED) displays must be enhanced to meet the requirements for color purity and luminous efficiency because few blue‐light‐emitting materials meet these requirements. This is particularly true for polymeric and phosphorescent light‐emitting materials. To attain the required purity and efficiency, a polarized‐light‐recycling structure for blue light that is called a blue enhanced circular polarizer (BECP) has been developed. The principle of the structure and the fabricated prototype device is described and it is shown that the structure increases blue‐light intensity and color purity, improves efficiency, provides a wide color gamut, and limits ambient‐light reflection.     

An integrated poly‐Si TFT current data driver with a data‐line pre‐charge function

Abstract— We have developed an integrated poly‐Si TFT current data driver with a data‐line pre‐charge function for active‐matrix organic light‐emitting diode (AMOLED) displays. The current data driver is capable of outputting highly accurate (±0.8%) current determined by 6‐bit digital input data. A novel current‐programming approach employing a data‐line pre‐charge function helps achieve accurate current programming at low brightness. A 1.9‐in. 120 × 136‐pixel AMOLED display using these circuits was demonstrated.     

An area‐ratio gray‐scale method to achieve image uniformity in TFT‐LEPDs

Abstract— An area‐ratio gray‐scale method (ARG) has been developed for low‐temperature‐polysilicon thin‐film‐transistor‐driven light‐emitting‐polymer displays (LTPS TFT‐LEPDs). A pixel consists of plural sub‐pixels, which are controlled to be in either an on‐state or off‐state. The gray scale is acquired by selecting the number of the on‐state sub‐pixels, that is, the ratio of the light‐emitting area. One advantage of the ARG is to improve image uniformity. In the on‐state, since TFT resistance is negligible, the current is determined by the LEP diode resistance. Therefore, the TFT characteristic deviation has no effect on the current. Moreover, the dimensions of each sub‐pixel are the same, and the shapes of the sub‐pixel are circular in order to improve their uniformity. As a result, the image becomes uniform. Another advantage of the ARG is to achieve digital operation, which makes interfacing easy. A digital‐analog converter (DAC) automatically exists in the sub‐pixel and the naked eye.     

Single‐layered retardation films with negative wavelength dispersion birefringence made from liquid‐crystalline monomers

Organic light‐emitting diode displays have a cathode composed of a layer of highly reflective metal. Reflection of external light from this layer can be suppressed with a broadband quarter‐wave plate. Although various types of quarter‐wave plate retardation films have been prepared by copolymerizing, mixing, or laminating multiple polymers, ideal wavelength dispersion has not been achieved with thin retardation films because of their relatively low birefringence and the complexity of the procedure required manufacturing them. In this study, we developed (i) new liquid‐crystalline monomers with negative wavelength dispersion birefringence that we used to obtain thin, single‐layered retardation films suitable for coating and (ii) retardation films with different molecular alignments. We found that the monomers showed high solubility and high regularity of molecular alignment, with less damage to the substrate and alignment films. We also investigated the wavelength dispersion and thermal stability of the films. We succeeded in developing retardation films that had a homeotropic alignment or a hybrid alignment with negative wavelength dispersion. These alignments can be used to obtain antireflection films with an improved viewing angle for organic light‐emitting diode displays or next‐generation thin displays.     

Development of 8‐in. oxide‐TFT‐driven flexible AMOLED display using high‐performance red phosphorescent OLED

An 8‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display driven by oxide thin‐film transistors (TFTs) has been developed. In‐Ga‐Zn‐O (IGZO)‐TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200 °C. To form a SiO_x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution‐processable photo‐sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high‐performance phosphorescent OLED was developed as a red‐light‐emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.     

AMOLED displays with transfer‐printed integrated circuits

Abstract— Small integrated circuits of crystalline silicon (chiplets) transfer‐printed onto a flat‐panel‐display substrate provide greatly improved electrical performance and uniformity in active‐matrix organic light‐emitting‐diode (OLED) displays. The integrated circuits are formed in high‐performance crystalline silicon using conventional photolithographic processes and then transfer‐printed onto a substrate using a stamp that transfers hundreds or thousands of chiplets at once. The chiplets are connected to an external controller and to pixel elements using conventional photolithographic substrate processing methods. Active‐matrix OLED (AMOLED) displays using transfer‐printed chiplets have good yields, excellent uniformity, and electrical performance and are thermally robust.     

System design for a wide‐color‐gamut TV‐sized AMOLED display

Abstract— By using current technology, it is possible to design and fabricate performance‐competitive TV‐sized AMOLED displays. In this paper, the system design considerations are described that lead to the selection of the device architecture (including a stacked white OLED‐emitting unit), the backplane technology [an amorphous Si (a‐Si) backplane with compensation for TFT degradation], and module design (for long life and low cost). The resulting AMOLED displays will meet performance and lifetime requirements, and will be manufacturing cost‐competitive for TV applications. A high‐performance 14‐in. AMOLED display was fabricated by using an in‐line OLED deposition machine to demonstrate some of these approaches. The chosen OLED technologies are scalable to larger glass substrate sizes compatible with existing a‐Si backplane fabs.     

Energy‐efficient liquid‐crystal displays (e2‐LCDs) using a photonic‐crystal backlight system

Abstract— Cholesteric liquid crystals automatically form one‐dimensional photonic crystals. For a photonic crystal in which light‐emitting moieties are embedded, unique properties such as microcavity effects and simultaneous light emission and light reflection can be expected. Three primary‐color photonic‐crystal films were prepared based on cholesteric liquid crystal in which fluorescent dye is incorporated. Microcavity effects, i.e., emission enhancement and spectrum narrowing, were observed. Two types of demonstration liquid‐crystal displays (LCDs) were fabricated using the prepared photonic‐crystal films in a backlight system. One is an area‐color LCD in which a single photonic‐crystal layer is used for each color pixel and the other is a full‐color TFT‐LCD in which three stacked photonic‐crystal layers are used as light‐conversion layers. The area‐color LCD was excited by using 365‐nm UV light, and the full‐color TFT‐LCD was excited by using 470‐nm blue LED light. Because of the photonic crystal's unique features that allow it to work as light‐emitting and light‐reflecting layers simultaneously, both LCDs demonstrate clear readable images even under strong ambient light, such as direct‐sunlight conditions, under which conventional displays including LCDs and OLED displays cannot demonstrate clear images. In particular, an area‐color LCD, which eliminated color filters, gives clear images under bright ambient light conditions even without backlight illumination. This fact suggests that a backlight system using novel photonic‐crystal layers will be suitable for energy‐efficient LCDs (e²‐LCDs), especially for displays designed for outdoor usage.     

Pulse‐width modulation with current uniformization for AM‐OLED micro‐displays on Si LSI chips

We have developed pulse‐width modulation (PWM) with current uniformization for active‐matrix organic light‐emitting diode (AM‐OLED) micro‐displays on Si large‐scale integration (LSI) chips. This driving method can simultaneously solve luminance unevenness and image sticking due to characteristic deviations and degradations of driving transistors and OLEDs. With the use of circuit simulation, it is verified that the PWM with current uniformization (PWM‐CU) can simultaneously achieve precise gray scale and exceedingly improve luminance uniformity. Moreover, an actual panel is designed and fabricated, where the OLEDs are layered on the Si LSI chip. It is found that the luminance uniformity can be improved within 2% to 3%. It is meaningful that the correct images can be displayed using the PWM‐CU for the first time for AM‐OLED micro‐displays on Si LSI chips.