A preview of some of the most interesting papers appearing in the June 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
Industrial Technology Research Institute
Abstract — Color-sequential displays offer a better luminous efficiency, a higher spatial resolution, and a lower cost than conventional displays. However, a common problem is that visual effects cause color edge-blurring of a moving picture, a phenomenon called color breakup or rainbow effects. Most driving methods, such as increasing the frame rate and inserting a black/white frame or another color sub-frame to reduce the color breakup in color- sequential displays, has been presented in many papers, but every method has some limitations and problems. An innovative driving method and device to reduce the color-breakup phenomenon will be demonstrated in this paper, designed without increasing the driving frequency. Instead, the brightness is increased by one third at the very least. This method divides the driving frequency into four sub-frames (WRGB), which is operated at 180 Hz compared to 240 Hz for conventional driving. Our result shows that the image quality is improved. The color-breakup simulation based on "eye trace integration" and compensated white light will also be presented in this paper.
The gray level of the white sub-frames was obtained from subtracting the minimum value of the three sub-frames, as was indicated in Fig. 13. The best driving method to reduce the color breakup was found to be WRGB. The color breakup has been reduced by decreasing the backlight ratio, increasing the driving frequency, and inserting a black frame and a white frame. The WRGB driving method shows the weakest color-breakup phenomenon in the white test pattern and offers higher brightness compared to RGBKKK.
FIGURE 13 — Compared with different driving frequency and inserting color frame v = 12 ppf, BL ratio = 1. (a) RGB driving, (b) RGBK driving, (c) WRGB driving, and (d) RGBRGB driving.
The circuit configurations, planar photographs, and characteristics of the artificial retina are shown in Fig. 5. The retina pixel is based on an elementary current mirror, but some improvements are added by considering the device characteristics of the p–i–n TFPT and the poly-Si TFT and operation of the artificial retina. The part for the current mirror consists of two p-type poly-Si TFTs, and the part for the load resistance consists of two n-type poly-Si TFTs.
FIGURE 5 — Artificial retina.
Noel A. Clark
Feng Chia University
Abstract — A new operation mode of a bistable smectic A (SmA) display using two sets of electrodes, one without specific features to induce homeotropic orientation of the director and the other with an in-plane pattern to induce planar orientation of the director, has been demonstrated. Both statements of the director orientation are the stable states of SmA liquid crystals. Compared with the electrical addressing mode of a conventional SmA display, the SmA display mode presented in this study exhibits a high contrast ratio, excellent bistability, and reasonably fast switching under the employment of two crossed polarizers. Moreover, gray level can be achieved by regulating the frequency, owing to the formation of the focal-conic defect. This operation mode of a bistable SmA device demonstrated great potential for further application in flexible displays.
To exemplify the bitsability of a SmA LC cell, a striped electrode pattern on the bottom substrate is adopted. In these cells, the alignment layer favors homeotropic orientation, so the dark state appears initially, as can be seen in Fig. 9(a). By applying a horizontal electric field on the striped electrodes, the LC molecules are then switched from the homeotropic to planar texture between two striped electrodes in the S2 cell, as shown in Fig. 9(b). The bright state remains stable after turning the voltage off [Fig. 9(c)].
FIGURE 9 — Microscopic observation of an S2 striped cell with homeotropic alignment layers and a cell gap of 3.0 μm. (a) The dark state arises from the homeotropic alignment and crossed polarizers. (b) The bright state appears after applying an in-plane voltage of 100 V with a 1-kHz square waveform on the striped electrodes and (c) then turning the field off. The LC director now lies in the cell plane at 45° with respect to the transmission axis of either polarizer. (d) The planar texture is confirmed by rotating the cell to make the LC director parallel to the transmission axis. (e) The dark state appears after applying the normal-to-the-plane voltage of 100 V at 1 kHz and (f) then turning the field off.
K. Y. Ho
C. H. Cheng
C. C. Cheng
P. C. Chen
Y. H. Yeh
Abstract — Low-temperature deposited a-Si:H TFTs have been successfully fabricated on colorless polyimide (CPI) substrate for flexible-display applications. A serious degradation in threshold voltage was observed after applying external thermal stress. The threshold-voltage shift saturates after applying several thermal stress cycles. In addition, the TFTs show instability under long periods of thermal stress with fixed temperature. This phenomenon was composed of thermally induced traps and substrate-expansion-induced mechanical stress. Finally, the a-Si:H TFT backplane fabricated on a PI substrate at low temperature has been successfully demonstrated for flexible AMLCDs.
Figure 12 shows the relationship between the bending direction and Vth shift. The Vth shift has great directional property under mechanical stress. A larger Vth shift is observed when the bending direction is perpendicular to the channel length. Thermal-induced substrate expansion will also induce mechanical stress on a-Si:H TFTs. This biaxial expansion will induce stress in both directions perpendicular and parallel to the channel direction. The expansion in the channel width, just like the bandwidth in the perpendicular direction, results in a very large Vth shift. This explains why Vth degrades more in larger channel widths under long periods of thermal stress.
FIGURE 12 — Relationship between bending direction and threshold voltage shift.
Abstract — The temperature-dependent photoluminescence features of polycarbonate thin films doped with blue-phosphorescent molecules, either bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium (FIrpic) or bis(2-phenylpyridinato-N,C2′) (acetylacetonate) rhodium [(ppy)2Rh(acac)], which have an equivalent triplet energy of 2.64 eV, have been studied. The photoluminescence intensity of the FIrpic-doped polycarbonate thin film did not show any dependence on temperature. On the other hand, as for the (ppy)2Rh(acac)-doped polycarbonate thin film, decreasing photoluminescence intensity with increasing temperature (especially above 100K) was clearly visible. These results reflect that the internal heavy-atom effect of (ppy)2Rh(acac) is weaker than that of FIrpic. Furthermore, the steady-state and time-resolved photoluminescence spectra of tris(8-hydroxyquinoline) aluminum (Alq3) thin films heavily doped with FIrpic or (ppy)2Rh(acac) (50 wt.%) at 8K was studied. It was found that the enhanced phosphorescence from Alq3 is mainly due not to the external heavy-atom effect by doping with the phosphorescent molecule but to the exothermic triplet energy transfer from the phosphorescent molecule to Alq3.
The energy-transfer and light-emission mechanisms are discussed hereafter. Figure 6 shows the schematic energy-level alignment of the lowest singlet-excited (S1) states, the T1 states, and the singlet-ground (S0) states in Alq3, FIrpic, and (ppy)2Rh(acac). After photoexcitation, both the S1 states in Alq3 and the S1 states in FIrpic or (ppy)2Rh(acac) are generated. The prompt fluorescence emission from Alq3 consequently occurs. Moreover, delayed fluorescence with a longer lifetime should be considered. For FIrpic and (ppy)2Rh(acac), the rapid ISC from the S1 states to the T1 states might occur because of strong spin-or-bit coupling.
FIGURE 6 — Schematic energy-level alignment of singlet-excited state (S1), triplet-excited states (T1), and singlet-ground states (S0) in Alq3 and FIrpic, and (ppy)2Rh(acac). The energy-transfer and light-emission processes are is shown by the arrows.
National Taiwan University
Abstract — It is reported that by integrating OLEDs with solar cells, ambient-light reflection as low as 1.4% (even superior to that achieved with polarizers) can be achieved without compromising the EL efficiency for high-contrast display applications. Furthermore, in such a configuration, the photon energies of both the incident ambient light and the portion of OLED emission not getting outside of the device can be recycled into useful electrical power via the photovoltaic action, instead of being wasted as in other reported contrast-enhancement techniques. These features shall make this present technique attractive for high-contrast display applications and portable/mobile electronics that are highly power-aware.
Photos in Fig. 3 show the appearance of a low-reflection [OLED plus solar cell] stack (with one OLED on and others off) and a highly reflective bottom-emitting OLED under strong ambient illumination. For the present [OLED plus solar cell] stack, the high contrast between the off pixel (which is almost completely black) and the on pixel is clearly seen without using any contrast-enhancement films.
FIGURE 3 — Color photos of the [OLED plus solar cell] stack and the conventional bottom-emitting OLED.
Joshua B. Dinaburg
Hughes Associates, Inc.
Abstract — As use of handheld thermal-imaging cameras (TICs) becomes more prevalent in the first-responder community, it is important that standard test metrics be available to characterize imaging performance. A key performance consideration is the quality of the image presented on the TIC display. This paper focuses on TICs that use liquid-crystal displays to render an image for the user. Current research on TIC performance for first-responder applications makes use of trained observers and/or composite-video-output-signal measurements. Trained observer tests are subjective and composite video output tests do not evaluate the performance of the complete imaging system. A non-destructive objective method was developed that tests the performance of the entire thermal-imaging system, from the infrared sensor to the display. A thermal target was used to correlate the measured thermal imager composite video output signal with the luminance of the display. A well-characterized charge-coupled-device (CCD) camera and digital recording device were used to measure the display luminance. An electro-optical transfer function was determined that directly relates the composite video output signal to the luminance of the display, providing a realistic characterization of system performance.
The measured display performance of three TICs are shown in Fig. 5. Th eerror bars represent the standard deviation from many identical experiments. It was clear that the three TICs subjected to this test do not exhibit similar display characteristics. Therefore, any test measuring only the NTSC output signal will not accurately describe the quality of the TICs relative to each other, highlighting the necessity of display testing. The vast differences in the character of the visual display of each TIC indicated that determining performance solely from data captured from the video output port may significantly mislead users about the actual TIC imaging quality.
FIGURE 5 — Relative luminance as a function of 8-bit recorded NTSC output signal for three different TICs. The 16-bit relative luminance is measured in counts and the NTSC output signal is measured as an 8-bit pixel gray level.
A preview of some of the most interesting papers appearing in the July 2008 issue of the Journal of the SID. To obtain access to these articles on-line, please go to www.sid.org
Kochi University of Technology
Abstract — Ga-doped ZnO (GZO) films with thicknesses of 30–560 nm were prepared on glass substrates at 200°C by ion plating with direct-current arc discharge. The dependences of the characteristics of GZO films on thickness were investigated. All the polycrystalline GZO films, which showed high transmittance in the visible region, were ZnO crystallites with a wurtzite structure highly oriented along the (0002) plane. The resistivity, ρ, of GZO films decreases with increasing film thickness. The highest ρ achieved is 4.4 x 10–4 Ω-cm with a carrier concentration, n, of 7.6 x 1020 cm–3 and a Hall mobility, μ, of 18.5 cm2/V-sec, determined by Hall effect measurement for the GZO films with a thickness of 30 nm, and the lowest ρ is 1.8 x 10–4 Ω-cm with n = 1.1 x 1021 cm–3 and μ = 31.7 m2/V-sec for the GZO film with a thickness of 560 nm. In addition, highly transparent GZO films with thicknesses of 12–300 nm were fabricated on unheated polymethyl methacrylate (PMMA). The ρ of these transparent GZO films decreased from 20 to 4 x 10–4 Ω-cm with film thickness.
Zinc oxide (ZnO) with a wurtzite structure is a versatile material with a wide band gap of 3.37 eV at room temperature. n-type ZnO thin films have, in recent years, been rediscovered as a subject of considerable research interest due to their unique physical properties [low resistivity of 2 x 10–4 Ω-cm, high visible transmittance (90%) and high infrared (IR) reflectance and absorbtance in the microwave region] and their wide range of possible electronic and optical applications. Special attention has been directed toward the blue-to-UV-wavelength LED because of the wide band gap of ZnO and the highly efficient ultraviolet photoluminescence.
FIGURE 1 — Schematic diagram of ion plating by direct-current arc discharge with a traveling substrate.
Jun Hyuk Cheon
G. P. Kennedy
Jung Ho Bae
Kyung Hee University
Abstract — The channel-length-dependent transfer characteristics of TFTs using poly-Si by metal-induced crystallization through a cap (MICC) of a-Si to evaluate the parasitic and channel resistances have been studied. The MICC p-channel TFTs studied in the present work showed a maximum field-effect mobility, threshold voltage, and gate swing of 53 cm2/V-sec, –4.4 V, and 0.8 V/dec forW/L = 12 μm/6 μm, 71 cm2/V-sec, –5.3 V, and 0.9 V/dec for W/L = 12 μm/12 μm, and 113 cm2/V-sec, –7 V, and 1 V/dec for W/L = 12 μm/24 μm, respectively. It is found that the parasitic resistance is higher than the channel resistance, and both decrease with increasing temperature.
304 stainless-steel foil, 150-μm-thick with a composition ratio of Fe/Cr/Ni: 72/18/10 wt.%, was used to fabricate the devices used here. The metal foil was polished by chemical mechanical polishing (CMP) before TFT fabrication. Then, a 1-μm-thick SiO2 buffer layer was deposited on the front and back sides of the metal foil. A buffer layer improves the surface roughness of the metal foil and prevents the con-tamination from the metal foil during the heating and annealing processes. The poly-Si TFT was fabricated as a self-aligned coplanar structure on metal foil.
FIGURE 5 — (a) Channel and (b) parasitic resistance variation as a function of MICC TFT gate voltage and temperature.
Seoul National University
Abstract — A new a-Si:H pixel circuit to reduce the VTH degradation of driving a-Si:H thin-film transistors (TFTs) by data-reflected negative-bias annealing (DRNBA) is presented. The new pixel circuit compensates VTH variation induced by non-uniform degradation of each a-Si:H pixel due to various electrical stress. The proposed pixel circuit was verified by SPICE simulations. Although the VTHof the driving a-Si:H TFT varies from 2.5 to 3.0 and 3.5 V, the organic light-emitting diode (OLED) current changes by only 1.5 and 2.8% in the emission period, respectively. During the negative-bias annealing period, the negative VGS is applied to the driving TFT by using its own data signal. It is expected that the VTH shift of the driving TFT can be effectively reduced and the VTH shift can be compensated for in our new pixel circuit, which can contribute to a stable and uniform image from an a-Si:H TFT active-matrix OLED.
The proposed CDRNBA pixel circuit is composed of six a-Si:H TFTs and two capacitors as shown in Fig. 3. S1 is connected to the data line and the gate of the DTR and controlled by the Sn signal, which is the nth scan signal. S2, whose gate is connected to the Sn–1signal is the n–1-th scan signal and is connected to the VDD line and the gate of the driving transistor (DTR). S3, controlled by the A signal, is connected to S1 and the source of DTR. S4 is connected to the gate of the DTR and VSS and controlled by signal N. The OLED is connected to S5 and VSS. The period of the OLED's emission is determined by S5 which is controlled by the emission signal and connected to the source of the DTR and OLED.
FIGURE 3 —The proposed CDRNBA a-Si:H pixel circuit for an AMOLED and the timing diagram. The DTR is a driving TFT.
Chung Yuan Christian University
Abstract — Liquid crystals have been extensively employed in photonic devices, especially in current flat-panel displays. Demands on high-quality electro-optical performance of liquid-crystal displays have continued to impel delicate molecular designs, chemical syntheses, as well as advanced cell-manufacturing processes, leading to a reduced dc offset and faster intrinsic response in the devices. Here, a novel approach toward the reduction of the residual dc and response time is reported based on carbon-nanotube doping. It is demonstrated that a minute amount of carbon nanotubes as a dopant can suppress the unwanted ion effect, invariably lower the rotational viscosity, and modify other physical properties of the liquid crystals, giving the approach an opportunity in display applications.
FIGURE 9 — Dopant-concentration dependence of E7's rotational viscosity at 35°C.
TABLE 2 — Optical decay times τd in planar-aligned E7 cells with various doping levels of CNTs at 30°C.
Wing Kai Lee
De Monfort University
Abstract — The development of a multi-user stereoscopic display that does not require the use of special glasses (autostereoscopic), and enables a large degree of freedom of viewer movement and requires only the minimum amount of information (a stereo pair) for the displays described. The optics comprise an RGB holographic laser projector that is controlled by the output of a multi-target head-position head tracker, an optical assembly that converts the projector output into steerable exit pupils, and a screen assembly comprising a single liquid-crystal display (LCD) and image multiplexing screen. A stereo image pair is produced on the LCD by simultaneously displaying left and right images on alternate rows of pixels. Novel steering optics that replace the conventional backlight are used to direct viewing regions, referred to as exit pupils, to the appropriate viewers' eyes. The results obtained from the first version of the display, where the illumination source consists of several thousand white LEDs, are given and the current status of the latest prototype being constructed on the basis of these results is described. The work indicates that a laser-based head-tracking display can provide the basis for the next generation of 3-D display.
The prototype comprises a holographic projector, an array assembly, a screen assembly, and a multi-user head tracker. The array consists of two separate 49-element arrays, one for the left exit pupils and one for the right pupils. The prototype uses a large mirror that is constructed from surface-silvered acrylic sheet whose surface contour is formed by this being inserted into a pair of curved grooves. A mirror is used instead of a Fresnel lens in this application in order to prevent fringing effects between this lens and the Fresnel lenses located on the back surface of the array elements.
FIGURE 17 — MUTED prototype.
Chih-Hung James Chang
Minghshin University of Science and Technology
Abstract — A novel flat discharge fluorescent lamp used as the light source of backlight modules for LCDs and general lighting systems has been researched and developed. This new type of lamp is a less-mercury flat fluorescent lamp with two-dimensional emission and superior to conventional one-dimensional cold-cathode fluorescent lamps in terms of optics, energy-savings, production efficiency, reliability, and chromatic performances. Physical characterization of the optics, temperature, mechanical design, thermal shocking, reliability, and corresponding environments have verified that flat fluorescent lamps will be the next-generation light sources for backlight modules and general lighting systems.
Test results show a more even distribution without the use of diffusers or enhancers for external electrode flat fluorescent lamps (EE-FFLs). Although new technologies may help to reduce the use of brightness enhancers and diffusers, EE-FFLs can provide similar results for brightness and distribution as shown in Fig. 4. The comparison of the lighting distribution shows that the EE-FFL (approx. 750 x 440 mm) provides a larger lighting area covered by the fluorescent lamps (approx. 1220 x 600 mm).
FIGURE 4 — Comparison of light distribution of flat fluorescent lamp (left) to standard fluorescent lamp (right).
Abstract — Several options to interconnect driver chips to a flexible display are discussed and investigated. In the first option, bare test dies are flip-chip (FC) assembled onto polyethylene terephthalate (PET) display substrates. The second option involves test flexible polyimide (PI) substrates, imitating tape-carrier-packaged drivers (TCP), bonded onto the same PET substrates, whereas the third option uses actual TCPs on stainless-steel display substrates. Each option makes use of bonding technology with anisotropically conductive adhesive, supplied as film (ACF). The reason for using ACF is that drivers typically have high output counts, and therefore very fine pad features, 200-μm pitch and below. The technology has been adapted for each option, considering the requirements of the substrate. Every option includes an explanation of the bond test setup, the bonding process itself, and a discussion of the test results. The conclusion summarizes the achievements made in the research reported in this article.
The principle of assembly technology using ACF is shown in Fig. 1: the ACF is applied to the substrate, then the component is aligned and positioned and, finally, the assembly is cured under thermocompression. As the ACF, Hitachi AC8408Y was used. The conductive particles are gold-coated plastic spheres, approximately 5 μm in diameter. These are coated with a very thin insulation layer that has to be cracked open during the thermocompression step. The purpose of this insulator is to prevent lateral conduction due to clustering particles.
FIGURE 1 — Principle of assembly technology using ACF.
Ya-Chi n King
National Tsing Hua University
Abstract — A photodetector using a silicon-nanocrystal layer sandwiched between two electrodes is proposed and demonstrated on a glass substrate fabricated by low-temperature poly-silicon (LTPS) technology. Through post excimer-laser annealing (ELA) of silicon-rich oxide films, silicon nanocrystals formed between the bottom metal and top indium thin oxide (ITO) layers exhibit good uniformity, reliable optical response, and tunable absorption spectrum. Due to the quantum confinement effect leading to enhanced phonon-assisted excitation, these silicon nanocrystals, less than 10 nm in diameter, promote electron–hole-pair generation in the photo-sensing region as a result resembling a direct-gap transition. The desired optical absorption spectrum can be obtained by determining the thickness and silicon concentration of the deposited silicon-rich oxide films as well as the power of post laser annealing. In addition to obtaining a photosensitivity comparable to that of the p–i–n photodiode currently used in LTPS technology, the silicon-nanocrystal-based photosensor provides an effective backlight shielding by the bottom electrode made of molybdenum (Mo). Having a higher temperature tolerance for both the dark current and optical responsibility and maximizing the photosensing area in a pixel circuit by adopting a stack structure, this novel photosensor can be a promising candidate for realizing an optical touch function on a LTPS panel.
The proposed silicon-nanocrystal-based photodetector is formed by post excimer-laser annealing (ELA) of silicon-rich oxide (SRO) layer deposited between the bottom metal and top ITO electrodes, as shown in Fig. 1. The transparent top ITO electrode available in the LTPS process allows for ambient light to penetrate and reach the photo-sensing region while the bottom metal layer made of molybdenum (Mo) is designed to shield the photo-sensing region from direct backlight illumination.
FIGURE 1 — Process step for forming a silicon nanocrystal layer on top of a metal electrode. The silicon-rich oxide film is first deposited by PECVD and then post-annealed by ELA. Finally, the ITO electrode is deposited on the top of silicon-nanocrystal layer.
Han-Ping D. Shieh
National Chiao Tung University
Abstract — A time-multiplexing technique employing a pulse-width-modulation (PWM) charge-pump scheme for driving active-matrix organic light-emitting-diodes (AMOLEDs) is described. This scheme greatly reduces the number of control lines. The two- or four-phase PWM driving technique not only reduces costs through the simplification of manufacture but also improves the uniformity and lifetime of OLED panels. Experimental results show that the proposed circuit effectively and precisely controls the timing of OLED data-writing and light emission.
In order to implement the time-multiplexing technique in the OLED-display system, we propose a novel pixel circuit shown in Fig. 4. In Fig. 4, transistors MP1 and MP2 are used to alternatively control the upper and lower OLED panels by the time-multiplexing technique. In other words, writing and illuminating periods are exchanged between the upper and lower parts to obtain the following advantages. First, the synchronization of two individual PWM control signals is controlled by one global PWM control signal. Second, when one block is in the writing mode, the other block is in the illuminating mode.
FIGURE 4 — The pixel-driving-circuit design using the charge-pump technique.