Journal of the
Edited by Aris Silzars
Optical characteristics of tandem and microcavity tandem organic light-emitting devices
National Taiwan University,Taiwan
Rapid advances in organic light-emitting devices (OLEDs) and their applications in displays and lighting impose substantial demands in OLED structures having enhanced brightness yet without increasing driving currents. Two techniques associated with such demands have been developed in parallel in recent years. Tandem OLEDs provide enhanced luminance and current efficiency (cd/A) by stacking multiple emitting units vertically in series. On the other hand, incorporating well-designed microcavity structures in OLEDs can provide up to a two-fold enhancement in luminance and cd/A efficiency through redistributing the radiation generated in the devices.
Ultra-low-power LTPS TFT-LCD technology using a multi-bit pixel memory circuit
Sony Corp., Japan
Abstract — Two types of low-temperature poly-Si TFT-LCDs, which integrate a multi-bit memory circuit and a liquid-crystal driver within a pixel, have been developed using two different TFT process technologies. Both a 1.3-in. 116-ppi LCD having a 2-bit pixel memory and a 1.5-in. 130-ppi LCD having a 5-bit pixel memory consume very little power, less than 100 μW, which indicates that this technology is promising for mobile displays.
Figure 2 shows the basic concept of pixel memory technology. When SRAMs and a liquid-crystal ac driver are integrated in a pixel area beneath the reflective pixel electrode, the LCD is driven by only the pixel circuit when displaying a still image. This means that no charging current to the data line, which has a large load capacitance, is required. And only a small charging current to the pixel capacitor is necessary. This results in an ultra-low-power operation.
FIGURE 2 — Basic concept of pixel memoy technology.
A novel portable LCD using a new AFFS technology for outdoor readability
Hyang Yul Kim
Soon Ju Jang
Kyung Ha Lee
Jung Yeal Lee
BOE-Hydis Technology Co., Ltd., Korea
Abstract — The advanced fringe-field switching (AFFS) mode is suitable for portable display devices. It has high transmittance, wide viewing angle, low power consumption, high contrast ratio, and a ripple-free property resistant to pressure. Recently, a 10.4-in. XGA prototype which demonstrates excellent transmittance and very good outdoor readability has been developed. To obtain these properties, a new pixel structure has been designed which has a large aperture and a metal embossing pattern for inner reflectance without the use of a black matrix on the embossing metal pattern. Precise control of the liquid-crystal electrodynamics and surface reflectance using an AR polarizer was achieved. This new panel shows distinguished characteristics for outdoor readability with the backlight on under a bright outside light source.
The AFFS mode is a normally black mode with homogeneously aligned liquid crystal and crossed polarizers. By using an applied field, a fringe field is generated between the pixel and common electrode. This field rotates the liquid-crystal molecules above the entire area of the subpixel due to the dielectric torque and elastictorque. To maximize the transmittance property, the pixels and common electrodes are made of transparent metal. Therefore, unlike the IPS mode, the AFFS mode shows very high transmittance with a wide-viewing-angle characteristic.
FIGURE 1 — Comparison of display images on mobile displays in sunlight at various viewing angles.
Flexible AMOLED displays on stainless-steel foil
Samsung SDI, Korea
Abstract — The world's thinnest flexible full-color 5.6-in. active-matrix organic-light-emitting-diode (AMOLED) display with a top-emission mode on stainless-steel foil was demonstrated. The stress in the stainless-steel foil during the thermal process was investigated to minimize substrate bending. The p-channel poly-Si TFTs on stainless-steel foil exhibited a field-effect mobility of 71.2 cm2/V-sec, threshold voltage of –2.7 V, off current of 6.7 ´ 1013 A/μm, and a subthreshold slope of 0.63 V/dec. These TFT performances made it possible to integrate a scan driver circuit on the panel. A top-emission EL structure was used as the display element, and thin-film encapsulation was performed to realize a thin and flexible display. The full-color flexible AMOLED display on stainless-steel foil is promising for mobile applications because of its thin, light, rugged, and flexible properties.
A p-channel poly-Si TFT with a self-aligned coplanar structure was fabricated on 140-μm-thick stainless-steel substrates (304). The as-received stainless-steel foil was mechanically cut into a square shape of 185 ´ 200 mm2. Because the as-received stainless substrates were so rough (rms » 81.4 nm; scan area, 30 μm ´ 30 μm), the planarization of the substrates was performed by using the CMP technique. Hence, the CMP-treated substrates have an improved surface morphology (rms = 28 Å).The polished side of the substrate was coated with 1-μm-thick SiO2 film by PECVD at 330°C. Hydrogenated a-Si (a-Si:H) film with a thickness of 50 nm was grown on SiO2/stainless steel by using low-pressure chemical vapor deposition (LPCVD) at 450°C.
FIGURE 7 — The display images of the 5.6-in. top-emission AMOLED display on stainless-steel foil (ELVdd = +7 V, ELVss = –8 V, Vdata= 0–4 V, Vpos = 5V, Vneg = –7 V).
Adaptive scanning, 1-D dimming, and boosting backlight for LCD-TV systems
Pierre de Greef
Hendriek Groot Hulze
Hans van Mourik
NXP Semiconductors, The Netherlands
Abstract — Because TV applications require proper motion portrayal, overdrive and impulse driving are both required. Advanced lamp control enables Scanning Backlight Technology to drive the LCD panel with a pulsed exposure. Adaptive Dual-Pulse Technology can be applied to eliminate the induced image flicker. Light leaking through LCD panels driven to black can be described as a poor black level, limiting the contrast ratio. Adaptive Dimming Technology can be applied to dim the backlight, while maintaining image quality and saving power. The limited transmittance of LCD panels can be described as weak brightness. The transmission of the LCD panel should be maximized and Adaptive Boosting Technology can be used to boost the backlight (>100%). When combining these technologies, motion portrayal is enhanced, local brightness may double, and the contrast may be increased up to a factor of 20, still saving power.
Moving objects depicted on LCD-TV displays may look blurred. A combination of overdrive and impulse drive improves the image quality. Impulse-driven display systems can be realized using a scanning backlight. The LC cells are exposed by the backlight when the LC material has settled to the desired transmission level. Potential flicker artifacts can be reduced by adaptively splitting the scan pulse into two parts.
Moving image: uncorrected (left), overdrive corrected (middle), overdrive corrected and scanning (right).
Chemical and electrical stabilities of organic thin-film transistors for display application
Kyung Hee University, Korea
Abstract — The chemical and electrical stabilities of pentacene organic thin-film transistors (OTFTs) fabricated on plastic by a self-organized process was studied. The degradation in on-current, threshold voltage, and field-effect mobility of the OTFT under air exposure can be expressed in exponential form in time and can be reduced by using multilayer passivation on the organic semiconductor, which reduces the penetration of H2O and O2 into the pentacene. The threshold voltage degrades during negative gate bias stress, which can be reduced significantly by optimizing the organic gate insulator used for the OTFT. A stable OTFT can be fabricated by using the proper organic gate insulator.
Recent studies on the environmental stability of OTFTs indicate that some specific substance such as H2O and O2 affect the TFT performance significantly. For example, the performance of the OTFT without encapsulation degrades significantly in ambient air by the incorporation of H2O. It can diffuse into the grain boundaries and crystal lattice, and charges generated in the channel are captured because of its polarity. This results in a decrease in the on-current and field-effect mobility.
FIGURE 1 — A cross-sectional view of an OTFT.
Fabrication of a highly bendable LCD with an elastomer substrate by using a replica-molding method
Seoul National University, Korea
Abstract — A highly bendable liquid-crystal display was fabricated by using a multi-functional elastomer substrate of self-aligning LC molecules without any surface treatment. One of the two substrates is a plastic substrate while the other is a multi-functional elastomer substrate produced by a replica-molding technique. The multi-functional elastomer substrate has pixel-encapsulating walls that serve as spacers and provide mechanical stability and reproducibility against bending deformations. The highly bendable LCD demonstrates great flexibility, durability, and excellent electro-optic performances in a highly bent state.
FIGURE 2 — The schematic diagram of our bendable LC cell with a multi-functional elastomer substrate: l =300 μm, b =30 μm, d = 4.5 μm, and h = 40 μm.
Brightness preservation for LCD backlight dimming
Sharp Laboratories of America, U.S.A.
Abstract — Backlight dimming is a dominant method for power reduction in LCDs. Image processing with a simple boost and clip can compensate for such dimming-restoring image brightness. We propose a low complexity approach which replaces hard clipping with smooth roll-off to reduce clipping artifacts. An additional high-quality approach uses a two-channel spatial-frequency decomposition to preserve highlight detail attenuated by the roll-off. Image quality is improved and power savings can be increased by more-aggressive backlight dimming.
FIGURE 6 — Reduced backlight compensation. (a) Original @ 100% backlight. (b) Original @ 70% backlight (simulated). (c) BP @ 70% backlight (simulated).
Quantitative evaluation of display characteristics of AMOLED displays
Toshiba Matsushita Display Technology Co., Japan
Abstract — Understanding the display characteristics of OLEDs is not only of general interest but also of technological importance for expanding the application of OLEDs. The display characteristics of AMOLEDs were quantitatively evaluated and compared with LCD or CRT performance. The fast response time and high contrast ratio, which are attractive characteristics of OLEDs, were also retained under low temperature and bright ambient, respectively. Moreover, the luminance and color barely changed with viewing angle at any gray-scale level. The optical design of OLED diodes is important for the emission characteristics, luminance, and color reproducibility.
Organic light-emitting-diode (OLED) displays are one of the most promising next-generation flat-panel displays because of several of their advantages, such as high contrast ratio, very fast response time, very wide viewing angle, and simple structure. Recently, active-matrix OLED (AMOLED) displays, having a thin-film-transistor (TFT) backplane, have been developed to pursue higher resolution and lower power consumption. Although AMOLED performance has often been explained quasi-quantitatively, there are few reports in the literature on actual quantitative evaluation.
FIGURE 3 — Dependence of response time on temperature. Triangles and squares indicate the response times of the AMOLED and the conventional LCD, respectively.
How color break-up occurs in the human-visual system: The mechanism of the color break-upphenomenon
Waseda University, Japan
Abstract — In the present set of experiments, the mechanisms underlying color break-up (CBU), a phenomenon observed when images produced with a color-sequential projector are viewed, were examined. The perceived position of CBU was measured during fast eye movement, saccade with static and briefly flashed stimuli. Results showed that CBU did not simply correspond to the locus of the stimulus on the retina during saccades because the width of the CBU perception was narrower than the distance of the eye movements. This effect was thought to be related to visual stability, which allows objects to be perceived as stationary even when the eyes move and the retinal image changes position. Visual stability is assumed to operate by compensating for the change in retinal image position using eye-position signals; however, this compensation is imperfect during saccades. Thus, incomplete compensation results in a CBU perception that is of a narrower width than the amplitude of the saccade. In conclusion, CBU cannot be simulated with moving video cameras because it results largely from the mechanisms of visual perception.
When we see a pattern that is projected by a field-sequential projector/display (such as a one-chip digital-light-projection system) and move our eyes rapidly, a vivid colored blur is sometimes perceived. This phenomenon, which occurs during saccadic eye movements, is known as color break-up (CBU). CBU is most readily perceived when a bright pattern is viewed surrounded by a dark background.
FIGURE 2 — Changes in retinal image with simulated and actual eye movements. In both cases, subjects have the same retinal images; however, a different perception is expected. (a) The simulation situation when a moving stimulus or a moving camera is used. (b) The actual retinal image when the subject moves their eyes, with the orange arrow indicating eye movement.