A preview of some of the most interesting papers appearing in the January 2008 issue of the Journal of the SID. To obtain access to these articles on-line, please go to www.sid.org
Edited by Aris Silzars
John W. Hamer
Andrew D. Arnold
Michael L. Boroson
Tukaram K. Hatwar
Margaret J. Helber
Charles I. Levey
John E. Ludwicki
David C. Scheirer
Jeffrey P. Spindler
Steven A. Van Slyke
Eastman Kodak Co.
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.
With respect to lifetime, steady advances in OLED materials and OLED formulations have resulted in lifetime characteristics that are thought to be acceptable for many AMOLED display applications, especially imaging-centric applications. The key unresolved issue is whether AMOLED displays can be designed and fabricated that will have a unit manufacturing cost (UMC) lower than other flat-panel-display (FPD) technologies. In order to achieve this goal, we describe in this paper a design that uses TFT backplanes made with amorphous Si (a-Si) combined with an advanced white-emitting structure and display configuration that provides superior performance to existing FPD technologies in the 32–52-in. range.
Abstract — Organic thin-film-transistor (OTFT) technologies have been developed to achieve a flexible backplane for driving full-color organic light-emitting diodes (OLEDs) with a resolution of 80 ppi. The full-color pixel structure can be attained by using a combination of top-emission OLEDs and fine-patterned OTFTs. The fine-patterned OTFTs are integrated by utilizing an organic semiconductor (OSC) separator, which is an insulating wall structure made of an organic insulator. Organic insulators are actively used for the OTFT integration, as well as for the separator, in order to enhance the mechanical flexibility of the OTFT backplane. By using these technologies, active-matrix OLED (AMOLED) displays can be driven by the developed OTFT backplane even when they are mechanically flexed.
An effective way to achieve a full-color pixel structure is to employ a top-emission structure. In the top-emission structures, OLEDs and the pixel circuit are arranged in a tandem configuration. Such a configuration enables smaller pixel sizes than those achievable using a conventional bottom-emission structure, in which OLEDs and the pixel circuit are arranged in a side-by-side configuration. Unlike bottom-emission structures, a top-emission OLED does not require a transparent substrate, allowing a wider selection of plastic substrates to be used. Top-emission structures are thus advantageous for producing flexible OTFT-OLED displays.
FIGURE 7 — Photograph of the flexible AMOLED display driven by pentacene TFTs.
Abstract — A paper-thin QVGA, flexible 2.1-in. active-matrix electrophoretic display (AMEPD) that features a thickness of 100 μm and a 192-ppi resolution has been developed. An LTPS-TFT backplane with integrated peripheral driver circuits was first fabricated on a glass substrate and then transferred to a very thin (30-μm) plastic film by employing surface-free technology by laser ablation/annealing (SUFTLA®). A micro-encapsulated electro-phoretic imaging sheet was laminated on the backplane. A supporting substrate was used to support the LTPS-TFT backplane. Fine images were successfully displayed on the rollable AMEPD. The integrated driver circuits dramatically reduce the number of external connection terminals, thus easily boosting the reliability of electrical connections even on such a thin plastic film.
FIGURE 5 — Sectional view of the very thin AMEPD.
FIGURE 6 — Photograph of the rollable AMEPD.
J. William Doane
Abstract — This paper describes the first substrate-free cholesteric liquid-crystal displays. The encapsulated cholesteric displays are ultra-thin (with a total thickness around 20 μm) and ultra-light-weight (0.002 g/cm2). The displays exhibit unprecedented conformability, flexibility, and drapability while maintaining electro-optical performance and mechanical integrity. All functional display layers are sequentially coated on a preparation substrate and then lifted-off from the preparation substrate to form a free-standing display. The display fabrication process, electro-optical performance, and display flexibility are discussed.
The release of the display from the preparation substrate does not seem to reduce its optical performance. The display is rugged, can be folded, and can sustain some reversible stretching (1–2%) without loss of the display integrity and electrical addressability. All materials in the display, including polymers for the carrier film, the binder for the liquid-crystal droplets, conductive electrodes, and clear coat are elastic and durable enough to allow for a rather tough handling despite the very thin display thickness. Figure 5(a) demonstrates a passively driven 13 x 13 monochrome cholesteric display on the preparation substrate. Figurre 5(b) shows the same display lifted from the preparation substrate. The total thickness of this substrate-free display is about 20 μm.
Abstract — A novel reflective color LCD without polarizers has been developed using a PDLC film and a retro-reflector. Bright color images including moving images are achievable with ambient light. This novel LCD will enable the new application area of electronic paper.
The power consumption has been simulated for a QVGA (320 x 240) resolution panel. For the color panel that displays 256 gray scales with 60-Hz driving, where both source lines and common electrode are driven by 5 Vp–p, the power consumption is 32 mW. For a b/w display, the power consumption decreases to 1/3 because the number of pixels to be driven decreases to 1/3. By applying 6-Hz driving instead of 60-Hz driving, the power consumption decreases to 1/10, which is quite low. By decreasing the gray-scale displays from 256 to 2 steps, the power consumption decreases to 43%.
University of Central Florida
Abstract — An electrically controllable blueshift of the reflection band is observed in a cholesteric liquid crystal with either positive or negative dielectric anisotropy. The change in optical properties is a result of a two-dimensional periodic undulation of the cholesterictexture, known as Helfrich deformation. This blueshift mechanism was used to demonstrate area-color reflective displays in a cholesteric cell and a rollable polymeric film.
The electrically induced color change in cholesteric reactive-mesogen cells can be permanently recorded through UV curing when the voltage is applied. An area-color pattern can be recorded into a single cell by masked curing each part when different voltages are applied. The film thickness is controlled by the cell gap, which is 8 μm in our experiment. The glass substrates can be peeled off and a flexible film with an area-color pattern is fabricated.
Sony Deutschland GmbH
Abstract — A polymer-dispersed liquid-crystal (PDLC) matrix template embedded with nano/microparticles can be backfilled/infiltrated with a dye-doped liquid crystal for a paper-like reflective display. In this way, a desirable degree of diffusion can be realized to reduce the viewing-angle dependency of a gain reflector and metallic glare without changing other electro-optical properties.
FIGURE 1 — Schematic drawing of nanoparticle-embedded D-PDLC.
J. Cliff Jones
ZBD Displays, Ltd.
Abstract — The first commercial use of the Zenithal Bistable Display (ZBD™) is for electronic point-of-purchase (epop™) signage in the retail sector. As a reflective bistable display, this novel LCD technology only consumes power if new information is required and the image is updated. This allows complex images to be shown constantly for several years from the energy of a single low-cost battery, when the display is updated up to ten times each day – ideal for signage applications. Excellent performance characteristics are achieved in a TN-like STN-LCD in which one of the alignment surfaces is a relief grating. Correct design of the grating shape and surface properties not only imparts the bistability, but allows control of the optical performance, the latching voltages, and the temperature range. Being addressed using a simple passive-matrix approach, without the need for a thin-film-transistor backplane, large amounts of information may be displayed by STN drivers. A low-cost fabrication method has been devised that is compatible with conventional TN and STN manufacture, and with negligible equipment outlay. The device operating principles, manufacturing method, and performance of ZBDs are reviewed.
There are several potential device geometries depending on the opposing surface to the ZBD grating. The simplest structure is a simple TN-type geometry shown schematically in Fig. 1. The grating is used opposite the conventionally rubbed polymer alignment layer to give a 90° twist in the low-tilt state at the grating surface or a hybrid aligned (HAN) state without twist when in the high-tilt state at the grating surface. Optical contrast results from polarizers attached on either side of the panel and crossed with respect to each other. The device is usually operated in reflective mode using a reflective rear polarizer.
Abstract — A thin and flexible LSI driver with a thickness of less than 35 μm for a passive-matrix-driven Quick-Response Liquid-Powder Display (QR-LPD™) was successfully mounted onto the flexible printed circuit (FPC) and the back substrates of a flexible QR-LPD™. A mounted LSI driver on a plastic substrate shows no significant degradation in the driving performances and maintains physical flexibility without any connection failures. This technology can realize a fully flexible electronic paper in combination with a plastic-substrate QR-LPD™ fabricated by a roll-to-roll process.
In this study, we tried to mount a flexible driver on a flexible printed circuit (FPC) film in order to confirm the possibility of a fully flexible electronic-paper display. Figure 5 shows a photograph of a FPC with a flexible LSI driver. One can see that it has mechanical flexibility. In this case, the minimum radius of curvature without mechanical destruction is 20 mm. Some difficulty is expected in the bonding process because of the chip flexibility.
Masahiro Kawasaki, Shuji Imazeki,
Shoichi Hirota, Tadashi Arai,
Takeo Shiba, Masahiko Ando,
Yutaka Natsume, Takashi Minakata,
Sei Uemura, Toshihide Kamata
Abstract — A solution-processed organic thin-film-transistor array to drive a 5-in.-diagonal liquid-crystal display has been fabricated, where semiconductor films, a gate dielectric film, and passivation films have all been formed using solution processes. A field-effect mobility of 1.6 cm2/V-sec, which is among the highest for solution-processed organic thin-film transistors ever reported, was obtained. This result is due to semiconductor material with large-grain-sized pentacene crystals formed from a solution and adoption of three-layered passivation films that minimize the performance degradation of organic thin-film transistors.
A planar view of the solution-processed OTFT array with patterned and passivated pentacene films is shown in Fig. 10. The purple pentacene island covered with passivation films can be clearly observed. The TFT performance is comparable with that of amorphous silicon, so the TFT size can be reduced such that the channel width and length are 50 and 8 μm, respectively. Therefore, the aperture ratio of a dot with a 318 x 106-μm size is designed to be 60%, which is the same level as that of commercial TFT-driven LCDs.
Julie J. Brown
Universal Display Corp.
Abstract — Organic light-emitting-device (OLED) devices are very promising candidates for flexible-display applications because of their organic thin-film configuration and excellent optical and video performance. Recent progress of flexible-OLED technologies for high-performance full-color active-matrix OLED (AMOLED) displays will be presented and future challenges will be discussed. Specific focus is placed on technology components, including high-efficiency phosphorescent OLED technology, substrates and backplanes for flexible displays, transparent compound cathode technology, conformal packaging, and the flexibility testing of these devices. Finally, the latest prototype in collaboration with LG. Phillips LCD, a flexible 4-in. QVGA full-color AMOLED built on amorphous-silicon backplane, will be described.
A flexible full-color AMOLED–on–metal-foil prototype, combining LG.Philips LCD's innovative a-Si backplane with our team's high-efficiency PHOLED and FOLED flexible technologies has been demonstrated. The prototype is a portrait-configured 4-in. QVGA 100-ppi top-emitting OLED display, as shown in Fig. 7. The razor-thin display was built on 76-μm-thick metal foil and offers 256 gray-scale levels per color (8 bit). The display can portray a variety of images, including full-motion video.
Hyoung Sik Nam
Brian H. Berkeley
Sang Soo Kim
Abstract — Super-PVA (S-PVA) technology developed by Samsung has demonstrated excellent viewing-angle performance. However, S-PVA panels can place extra demands on charging time due to the time-multiplexed driving scheme required to separately address two subpixels. Specifically, a 2G-1D pixel structure theoretically requires subpixel charging in one-half of the time available for a conventional panel. In this paper, a new LCD driving scheme, super impulsive technology (SIT), is proposed to improve motion-blur reduction by driving an S-PVA LCD panel at 120 Hz. The proposed scheme allows a 120-Hz 2G-1D panel to be driven with an adequate charging-time margin while providing an impulsive driving effect for motion-blur reduction. Considering that the cost of a 2G-1D S-PVA panel is comparable to that of a conventional 60-Hz panel, this method achieves good performance at a reasonable price. The detailed algorithm and implementation method are explored and the performance improvements are verified.
The basic concept of the proposed algorithm in this paper is shown in Fig. 4, and we call it super impulsive technology (SIT). To eliminate gamma switching during the active charging time, only one gamma voltage set is used to drive the pixels within any given frame. In SIT driving, each frame is duplicated first at double speed so that the panel's frame frequency becomes 120 Hz, and even frames are just simple copies of the odd frames.
FIGURE 4 — Frame sequence of proposed driving scheme.
Alex Z. Kattamis
James C. Sturm
Abstract — The direct voltage programming of active-matrix organic light-emitting-diode (AMOLED) pixels with n-channel amorphous-Si (a-Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new "inverted" integration technique which makes the direct programming possible by connecting the driver n-channel a-Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn-on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active-matrix OLED displays with a-Si technology and conventional bottom-anode OLEDs.
The schematic cross section of an a-Si AMOLED pixel fabricated with the inverted integration process is shown in Fig. 3. The a-Si TFT backplane is fabricated at temperatures up to 300°C on glass. The apparent (i.e., not corrected for contact resistance) saturation mobility and threshold voltage of the driving TFTs (L = 5 μm) are 0.65 ± 0.04 cm2/V-sec and 1.7 ± 0.2 V, respectively. After processing the TFT backplane (including ITO as the OLED anode), insulating "separators" are formed by patterning a layer of positive photoresist using conventional photolithography. As shown in Fig. 3(a), the organic layers are then evaporated at an angle in such a way that an interconnect extension connected to the driving TFT is not coated with the organic layers, taking advantage of the separator's shadowing effect.