A preview of some of the most interesting papers appearing in the
February 2009 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
Sang Soo Kim (SID Member)
Bong Hyun You (SID Member)
Jung Hwan Cho
Dong Gyu Kim
Brian H. Berkeley (SID Senior Member)
Nam Deog Kim (SID Member)
Samsung Electronics Co., Ltd.
Abstract — An ultra-definition (UD or 3840 x 2160) resolution 82-in. product with 120-Hz high-frame-rate driving has been developed for LCD-TV applications. The resolution increase from full HD to UD greatly reduces the available charging time. This problem has been overcome by employing a half-gate two-data-line design (hG-2D) for Super PVA pixels. Additionally, cost-effective single-bank driving has been achieved by adopting a vertical-quarter-partitioned (VQP) driving scheme. A viewing angle of 180°, contrast ratio of 2200:1, and brightness of 550 nits have been achieved while maintaining all of the other advantages of the Super-PVA structure.
The hG-2D pixel's operation is shown in Fig. 11. Sub-pixel B almost stays black at a data input below 25% of full white, but grows brighter with increasing gray values. For the best viewing-angle performance, these two subpixels are optimized at an area ratio of 2:1. Looking at Fig. 11(c), which shows the CS S-PVA pixel displaying a full white pattern, it is not easy to identify a brightness difference between subpixels A and B. However, the brightness of sub-pixel B is measured to be 30% lower than that of subpixel A. This is due to the loss of charge when TFT3 is switched on.
FIGURE 11 — Pixel operation of CS super-PVA. (a) 25% gray, (b) 50% gray, and (c) full white.
Eiji Kanda (SID Member)
Tsukasa Eguchi, Yasunori Hiyoshi
Taketo Chino, Yasushi Tsuchiya
Tokuro Ozawa (SID Member)
Tomotaka Matsumoto (SID Member)
Abstract — An active-matrix capacitive sensor for use in AMLCDs as an in-cell touch screen has been developed. Pixel sensor circuits are embedded in each pixel by using low-temperature polycrystalline-silicon (LTPS) TFT technology. It detects a change in the liquid-crystal capacitance when it is touched. It is thin, light weight, highly sensitive, and detects three or more touch events simultaneously.
Figure 2 shows a partial cross section of the LCD panel. The manufacturing process was designed to be the same process as that for a standard LCD because the sensing components consist of the same components as a standard LCD array. The fabrication does not require additional process steps and does not cause an increase in cost. Furthermore, a fringe-field switching (FFS) mode structure was used because the sensing circuit capacitance is easy to optimize. In the FFS structure, the common electrode is on the TFT substrate.
FIGURE 2 — Cross section of the manufactured LCD. The manufacturing process was designed to be the same process as for a standard LCD. The thickness of both substrates is 0.5 mm.
Jae Kyeong Jeong (SID Member)
Jong Han Jeong
Hui Won Yang
Tae Kyung Ahn
Kwang Suk Kim
Bon Seog Gu
Hye Dong Kim
Ho Kyoon Chung
Samsung SDI Co., Ltd.
Abstract — A full-color 12.1-in. WXGA active-matrix organic-light-emitting-diode (AMOLED) display was, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) as an active-matrix backplane. It was found that the fabricated AMOLED display did not suffer from the well-known pixel non-uniformity in luminance, even though the simple structure consisting of two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductors. The n-channel a-IGZO TFTs exhibited a field-effect mobility of 17 cm2/V-sec, threshold voltage of 1.1 V, on/off ratio > 109, and subthreshold gate swing of 0.28 V/dec. The AMOLED display with a-IGZO TFT array is promising for large-sized applications such as notebook PCs and HDTVs because the a-IGZO semiconductor can be deposited on large glass substrates (larger than Gen 7) using the conventional sputtering system.
Figure 1 shows a schematic cross section of the IGZO TFTs, which have an inverted-staggered bottom-gate architecture with an ESL. For an a-IGZO TFT without an ESL, severe degradation of the subthreshold gate swing and the uniformity of threshold voltage were observed. This is why an ESL-type structure rather than a back-channel etch structure was chosen, which is generally adopted for LCDs.
FIGURE 1 — The schematic cross section of an a-IGZO TFT with an inverted-staggered architecture.
Bong Hyun You (SID Member)
Byoung Jun Lee
Sang Youn Han
Brian H. Berkeley (SID Senior Member)
Nam Deog Kim (SID Member)
Sang Soo Kim (SID Member)
Abstract — A touch-screen-panel (TSP) embedded 12.1-in. LCD employing a standard existing a-Si:H TFT-LCD process has been successfully developed. Compared with conventional external touch-screen panels, which use additional components to detect touch events, the new internal TSP exhibits a clearer image and improved touch feeling, as well as increased sensing speed using discrete sensing lines to enable higher-speed sensing functions including handwriting. The new internal digital switching TSP can be fabricated with low cost because it does not require any additional process steps compared to a standard a-Si:H TFT-LCD.
Figure 4 illustrates the schematic cross-sectional view of the basic structure of the new internal TSP. In this structure, the conductive CS formed on the color-filter glass acts as a sensor switch which connects the common electrode voltage (VCOM) of the upper color-filter glass to the ITO sensor block on the lower TFT glass. When a touch event occurs, the gap between the conductive CS and contact area (sensorblock) is reduced by way of mechanical force so as to make mechanical contact. The conductive CS contact areas are located in each blue pixel.
FIGURE 4 — Schematic view of internal TSP structure. (a) Cross-sectional side view. (b) Top view.
Chang-Wook Han (SID Member)
Yoon- Heung Tak
In Byeong Kang
Byung-Chul Ahn (SID Member)
In Jae Chung (SID Member)
Abstract — An improved AMOLED with an a-Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual-plate OLED display (DOD). While a top-emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large-sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large-sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.
The DOD structure consists of two plates where TFTs are on the bottom plate and OLEDs are on the top plate, connected by a contact spacer as illustrated in Fig. 1. Both plates are separately made and encapsulated afterwards. This method provides a yield higher than that for the conventional method by separately screening bad plates method.
FIGURE 1 — Cross section of DOD (dual-plate OLED display) after encapsulation. The DOD structure consists of two plates which consist of the TFT on the bottom plate and the OLED on top plate, connected by a contact spacer.
Koji Hashimoto (SID Member)
Abstract — A new driving method for an advanced-CEL-structure panel has been developed. Picture qualities have been upgraded. Discharge time lags are drastically shortened by priming electron emission from magnesium oxide (MgO) single-crystal powder, refered to as a crystal emissive layer (CEL). The advanced-CEL-structure panel has CEL material on the surface of not only the surface-discharge-electrode side but also on the address-electrode side. This panel structure enables a stable opposed discharge when the address electrode functions as a cathode. By utilizing the opposed discharges in the reset and LSB-SF sustain periods, the dark-room contrast ratio has been drastically increased to over 20,000:1, which is higher than five times that of the conventional method, and the luminance of the least-significant-bit sub-field (LSB-SF) is as low as 0.1 cd/m2, which is one-fourth that of the conventional method. The high-picture-quality PDP TVs refered to as "KURO" that employs these technologies have been introduced into the marketplace.
The CEL is a single-crystal powder of MgO, which can emit exo-electrons effectively. And the amount of priming particles in the discharge cells with CEL attenuates more slowly than that without CEL, when the voltage pulse is not applied after discharge. Figure 7 shows the advanced CEL structure. CEL material is formed not only on the front-plate side but also on the rear-plate side. Thus, the discharge, in which A-electrodes can work as cathodes, can be stably generated.
FIGURE 7 — Advanced CEL cell structure.
Taro Naoi (SID Member)
Kimio Amemiya (SID Member)
Abstract — Pioneer Corporation introduced plasma-display-panel (PDP) TVs in 2005, which achieved the highest dark-room contrast ratio of 4000:1 at the time. These PDPs had a novel discharge cell structure consisting of a crystal emissive layer (CEL) on a MgO protective thin film. This cell structure is refered to as a CEL structure. Magnesium-oxide single-crystal particles, which have a unique luminance peak around 230–250 nm and a good exo-electron-emission property, were found to be an excellent material for CEL and were utilized in CEL panels. In 2007, newly developed PDP TVs in which CEL was formed on a phosphor layer, in addition to the previous CEL structure, were introduced, and this discharge cell structure is refered to as advanced CEL structure. By using the new cell structure, the opposed discharge characteristics have been drastically improved, and a stable reset discharge has been realized with only a weak opposed discharge. As a result, black luminance has been drastically reduced, and a dark-room contrast ratio of over 20,000:1, the highest ever reported, has been achieved.
This CEL material is fabricated by using a vapor-oxidation process that utilizes magnesium metal. CEL material consists of high-purity crystal particles of MgO that do not have doping impurities. Each particle is mainly a cubic single crystal. The distribution of the grain size is broad: from submicron to micron sized. The scanning-electron-microscopy (SEM) image of CEL material is shown in Fig. 1.
FIGURE 1— SEM image of CEL material.
Toshiyuki Akiyama (SID Member)
Takashi Yamada (SID Member)
Tsutae Shinoda (SID Fellow)
Advanced PDP Development Center Corp.
Abstract — The discharge mechanism concerning the width of the display electrodes in high-Xe-content gas mixtures to improve the luminous efficacy of PDPs has been researched. It was found that a luminous efficacy of 5 lm/W was realized for a high-Xe-content gas mixture and narrower display electrodes. For a high-Xe-content gas mixture, the luminous efficacy increases as the display electrode becomes narrower. This phenomenon was analyzed by observing the emission from a discharge cell. The observation data indicate that a high electron heating efficiency contributes to increased luminous efficacy along with narrow electrodes for a high-Xe-content gas mixture as well as high excitation efficiency.
Figure 2 shows the luminous efficacy as a function of the sustaining pulse voltage and width of the display electrode for a Ne + Xe20% gas mixture. A pulse cycle of 50 μsec is used in Fig. 2(a). All of the luminous-efficacy curves for a pulse cycle of 50 μsec have a peak with respect to the voltage. The voltages showing a peak efficacy increase as the width of the electrode gets narrower, and the peak efficacy simultaneously becomes higher. Among the pulse cycles, these characteristics shift to lower voltages with shorter pulse cycles.
FIGURE 2 — The luminous efficacy as a function of the sustaining pulse voltage for a pulse cycle of 50 μsec.
Sang-Hoon Yoon (SID Member)
Jae Jun Ko
Yong-Seog Kim (SID Member)
Abstract — Exo-electron emission from MgO thin film was measured by attaching a high-precision current sensor to the address electrode of the rear plate of an ACPDP test panel. The measured results revealed that the exo-electron emission currents can vary very sensitively with the type of doping elements used in MgO film and the measuring temperature. The activation energy of the exo-electron emission estimated from the emission curves indicated that the trap levels are between 0.05 and 0.32 eV below the bottom of its conduction band. This suggests that shallow electron-trap levels within MgO film are mainly responsible for the exo-electron emission.
In ACPDPs, various forms of discharge energies, including UV and visible photon energy, kinetic and potential energies of electrons and ions irradiate the MgO surface. A fraction of these energies may be stored in MgO thin film in the form of trapped electrons and holes at the donor and acceptor levels, respectively. Therefore, the emission of exo-electrons from the MgO layer may occur through the relaxation process of those trapped charges. There is only little quantitative data available for the exo-electron emission from the MgO layer and its influence on the discharge characteristics of ACPDPs.
FIGURE 3 — Effect of bias voltage between the sustain and address electrodes on exo-electron current density.
Denis Y. Kondakov (SID Member)
Eastman Kodak Co.
Abstract — A study of delayed electroluminescence in model highly efficient OLEDs based on anthracene derivatives indicate that triplet–triplet annihilation (TTA) contributes significantly to overall efficiency. Highly efficient devices (6–9% external quantum efficiencies) based on 9,10-bis(2-naphthyl)-2-phenylanthracene show that the TTA contribution depends primarily on operating current density, reaching as much as 20–30% of the overall emission intensity at moderate current densities (>5 mA/cm2). Revision of the classical estimates of maximum external quantum efficiency of fluorescent OLEDs to 8% and maximum internal quantum efficiency to 40% is recommended to account for TTA contribution (even further revision may be necessary to account for a better-than-20% optical outcoupling).
The TTA process is of particular interest for OLED devices that utilize anthracene derivatives as emissive layer hosts, which can lead to high efficiencies and low operational voltages and also permit fabrication of blue-emitting OLED devices. The reasons why some OLED devices of this type exhibit EQEs far exceeding the theoretical maximum are not well understood. Although it is hypothetically possible that TTA makes a sufficiently high contribution to EQE to explain the exceedingly high efficiencies, no experimental evidence pro or contra has been reported.
FIGURE 4 — Time-resolved electroluminescence of device A1 as a function of excitation pulse width. The inset enlarges the initial part of the decays corresponding to 40-, 10-, 5-, 3-, and 2-μsec excitation pulse lengths, top curve to bottom curve, respectively.
Sangyeol Kim (SID Member)
Mugyeom Kim (SID Member)
Jeongbae Song, Eokchae Hwang
Shinichiro Tamura (SID Member)
Sungkee Kang, Hyoseok Kim
Chiwoo Kim, Jinseok Lee
Jongmin Kim (SID Member)
Sungwoo Cho, Jaeyoung Cho
Min Chul Suh, Hyedong Kim
Abstract — A 3.0-in. 308-ppi WVGA top-emission AMOLED display with a white OLED and color filters, driven by LTPS TFTs demonstrating a color gamut of >90% and a Δ(u¢, v¢) of <0.02, is reported. A white-emission source with a unique device structure was developed using all fluorescent materials and yielded efficiencies of 8.45% and 16 cd/A at 4000 nits with CIE color coordinates of (0.30, 0.32).
Figure 8 shows an image from the prototype panel. The resolution of the prototype panel is 308 ppi, which is the highest resolution for a full-color AMOLED panels ever reported. In addition, all of the deposition processes only use an open mask, and the thickness of each layer from anode to cathode at each red, green, and blue subpixel is the same. Therefore, the thickness of the white-light-emitting organic layers corresponding to the RGB subpixels is the same, and this panel architecture is obviously favorable for mass production.
FIGURE 8 — 3.0-in. 308-ppi WVGA AMOLED prototype panel using a top-emission white OLED with color filters.
Baek-woon Lee (SID Member)
Young In Hwang
Chi Woo Kim
Jin Seok Lee
Jun Hyung Souk (SID Member)
Abstract — Two optical structures used for a bottom-emitting white organic light-emitting diode (OLED) is reported. An RGBW color system was employed because of its high efficiency. For red, green, and blue (RGB) subpixels, the cavity resonance was enhanced by the use of a dielectric mirror, and for the white (W) subpixel, the mirror was removed. The optical length of the cavities was controlled by two different ways: by the thickness of the dielectric filter on top of the mirror or by the angle of oblique emission. With both methods, active-matrix OLEDs (AMOLEDs) that reproduced a color gamut exceeding 100% of the NTSC (National Television System Committee) standard were fabricated. More importantly, the transmission of a white OLED through R/G/B color filters was significantly higher (up to 50%) than that of a conventional structure not employing a mirror, while at the same time as the color gamut increased from ~75 to ~100% NTSC.
The thicknesses of the layers are determined in the following order. First, the thicknesses of the bottom-most IZO/ITO and SiOx layers are set to a quarter-wave. A 600 Å of IZO was used. Then, the thickness of the anode (the top-most IZO/ITO layer) is determined so that the combined thickness of the anode and the bottom-most IZO/ITO layer yield the best performance for the W subpixels. The white emitter was optimized for the total IZO thickness at 900 Å. Therefore, the thickness of the anode was determined to be 300 Å. The optimal green condition is determined by the thickness of the common SiNx layer. Last, the simultaneous R and B condition is satisfied by the thickness of the intermediate "filter" IZO/ITO layer.
FIGURE 5 — A micro-cavity design of a RGBW bottom-emitting AMOLED (Design 1). OC stands for overcoat. The RGB subpixels have DBR (IZO or ITO and SiOx), a filter common for RGB (SiNx), and another filter (IZO or ITO) for R and B subpixels. The W subpixels do not have DBR in order to avoid the spectral modification and dependence on the viewing angle.
Sean Xia (SID Member)
Jason J. Brooks (SID Member)
Mark Rothman, Tan Ngo
Patrick Hett, Raymond C. Kwong
Mike Inbasekaran (SID Member)
Julie J. Brown (SID Member)
Takuya Sonoyama, Masaki Ito
Shunichi Seki, Satoru Miyashita
Universal Display Corp.
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.
Figure 10 shows the EL spectra and emission images of red and green P2OLED pixels fabricated by ink-jet printing. The spectra measuredfrom the ink-jet-printed devices were identical to that of the spin-coated devices, indicating that the emission was purely from the phosphorescent dopant. In addition, absence of bright spots and an irregular emission pattern indicated that there was no phase separation or recrystallization. Despite the promising initial performance, the ink-jet-printed P2OLEDs exhibited lower operational stability compared to that of spin-coated devices.
FIGURE 10 — Electroluminescence spectra and images of green and red P2OLED pixels by ink-jet printing.
Ansgar Werner (SID Member)
Jan Birnstock (SID Member)
Abstract — The three critical parameters in determining the commercial success of organic light-emitting diodes (OLEDs), both in display and lighting applications, are power efficiency, lifetime, and price competitiveness. PIN technology is widely considered as the preferred way to maximize power efficiency and lifetime. Here, a high-efficiency and long-lifetime white-light-emitting diode, which has been realized by stacking a blue-fluorescent emission unit together with green- and red-phosphorescent emission units, is reported. Proprietary materials have been used in transport layers of each emission unit, which significantly improves the power efficiency and stability. The power efficiency at 1000 cd/m2 is 38 lm/W with CIE color coordinates of (0.43, 0.44) and a color-rendering index (CRI) of 90. An extrapolated lifetime at an initial luminance of 1000 cd/m2 is above 100,000 hours, which fulfils the specifications for most applications. The emission color can also be easily tuned towards the equal-energy white for display applications by selecting emitting materials and varying the transport-layer cavities.
The basic layered structure of a standard bottom-emission PIN OLED consists of a transparent anode (ITO)/p-type-doped hole-transport layer (HTL)/interlayer at the hole side (EBL)/emission layer (e.g., matrix with phosphorescent or fluorescent emitters)/interlayer at the electron side (HBL)/n-type-doped electron-transport layer (ETL)/metallic cathode (Al) [see Fig. 3(a)]. Since each PIN OLED starts from a highly p-type electrically doped HTL and ends with a highly n-type-doped ETL, one PIN OLED was simply stacked on top of another PIN OLED without any intermediate layer. Figure 3(b) shows an example of stacked OLEDs with red, green, and blue emission units to generate white light. The n-type-doped ETL, in one emission unit, is in contact with the p-type-doped HTL in the subsequent emission unit to form a doped so-called organic p–n junction.
FIGURE 3 — Schematic architectures of (a) standard PIN OLED and (b) stacked white OLED.