Showing papers in "IEEE\/OSA Journal of Display Technology in 2007"

Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

Abstract: Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

Polymer Light-Emitting Electrochemical Cells for High-Efficiency Low-Voltage Electroluminescent Devices

Abstract: Organic light-emitting devices exhibiting high power conversion efficiency and long operating lifetime may potentially be achieved with the polymer light-emitting electrochemical cell (LEC) configuration. An LEC device typically uses a thin layer of conjugated polymer sandwiched between two contact electrodes. The polymer layer contains an ionically conductive species that are essential in the formation of a light-emitting p-i-n junction. LEC devices are characterized with balanced electron and hole injections, high current density at relatively low bias voltages (2-4 V), and high electroluminescent power efficiency. We will describe the working mechanism of the LECs and review the recent developments in LEC materials, device fabrication and performance. Among the important developments are planar (surface-typed) LECs, bilayer LECs that emit different colors at forward and reverse biases, frozen p-i-n junction LECs that functions like diodes, and phosphorescent LECs. Extensive efforts have been made to improve the LEC performance by controlling the blend morphology, including the use of bipolar surfactant additives and new electrolytes, the synthesis of conjugated polymers with ion-transporting main chain segments or side groups and polyelectrolyte. Degradation mechanisms that limit the lifetime of the LECs will also be discussed

Optimization of Light-Diffracting Photonic-Crystals for High Extraction Efficiency LEDs

Abstract: Photonic-crystal (PhC)-assisted light extraction is a promising method for ultrahigh efficiency, planar light-emitting diodes (LEDs). However, modeling of such structures is challenging due to the variety of their parameters and the heavy computational burden they represent. We present a thorough theoretical discussion of the optimization of PhC LEDs, which relies both on approximate treatments and on rigorous 3-D calculations. Two material systems (GaAs and GaN) are investigated, leading to quite different optimal regimes. Notably, it appears that besides the properties of the 2-D PhC itself, design of the vertical structure plays a major role in optimization

Fast Switching Liquid Crystals for Color-Sequential LCDs

Abstract: High birefringence (0.3 < Deltan < 0.4) and relatively low-viscosity liquid crystal mixtures containing isothiocyanato tolane and isothiocyanato terphenyl liquid crystals are developed. A twisted-nematic (TN) 1.6-mum thin cell for color-sequential liquid crystal display with ~1-ms response time is demonstrated.

AMOLED Pixel Circuit With Electronic Compensation of Luminance Degradation

Abstract: A new voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diodes (AMOLEDs) is presented. In addition to compensating for the shift in threshold voltage of TFTs, the circuit is capable of compensating for OLED luminance degradation by employing the shift in OLED voltage as a feedback of OLED degradation

Performance of High Power Light Emitting Diodes in Display Illumination Applications

Abstract: The performance of liquid crystal displays and micro display projection systems is for a large part determined by the lamps and illumination optics they are using. Traditionally, gas discharge lamps are used, but light-emitting diode lamps are gradually taking over because of their attractive characteristics. We will discuss how the transfer to light-emitting diodes in the various display architectures changes the key performance parameters, remaining technical challenges, and prospects for future improvement

ELiXIR-Solid-State Luminaire With Enhanced Light Extraction by Internal Reflection

Abstract: A phosphor-converted light-emitting diode (pcLED) luminaire featuring enhanced light extraction by internal reflection (ELiXIR) with efficacy of 60 lm/W producing 18 lumens of yellowish green light at 100 mA is presented. The luminaire consists of a commercial blue high power LED, a polymer hemispherical shell lens with interior phosphor coating, and planar aluminized reflector. High extraction efficiency of the phosphor-converted light is achieved by separating the phosphor from the LED and using internal reflection to steer the light away from lossy reflectors and the LED package and out of the device. At 10 and 500mA, the luminaire produces 2.1 and 66 lumens with efficacies of 80 and 37 lm/W, respectively. Technological improvements over existing commercial LEDs, such as more efficient pcLED packages or, alternatively, higher efficiency green or yellow for color mixing, will be essential to achieving 150-200 lm/W solid-state lighting. Advances in both areas are demonstrated

Random Projections Imaging With Extended Space-Bandwidth Product

Abstract: We propose a novel approach to imaging that is not based on traditional optical imaging architecture. With the new approach, the image is reconstructed and visualized from random projections of the input object. The random projections are implemented within a single exposure by using a random phase mask which can be placed on a lens. For objects that have sparse representation in some known domain (e.g., Fourier or wavelet), the novel imaging systems have larger effective space - bandwidth product than conventional imaging systems. This implies, for example, that more object pixels may be reconstructed and visualized than the number of pixels of the image sensor. We present simulation results on the utility of the new approach. The proposed approach can have broad applications in efficient imaging capture, visualization, and display given ever increasing demands for larger and higher resolution images, faster image communications, and multidimensional image communications such as 3-D TV and display.

A Hybrid Compression Method for Integral Images Using Discrete Wavelet Transform and Discrete Cosine Transform

Abstract: Integral imaging (II) is a promising three-dimensional (3-D) imaging technique that uses an array of diffractive or refractive optical elements to record the 3-D information on a conventional digital sensor. With II, the object information is recorded in the form of an array of subimages, each representing a slightly different perspective of the object In order to obtain high-quality 3-D images, digital sensors with a large number of pixels are required. Consequently, high-quality II involves recording and processing large amounts of data. In this paper, we present a compression method developed for the particular characteristics of the digitally recorded integral image. The compression algorithm is based on a hybrid technique implementing a four-dimensional transform combining the discrete wavelet transform and the discrete cosine transform. The proposed algorithm outperforms the baseline JPEG compression scheme applied to II and a previous compression method developed for II based on MPEG II.

Pretilt Angle Effects on Liquid Crystal Response Time

Abstract: Pretilt angle effect on liquid crystal dynamics is analyzed theoretically. Analytical expressions are derived to describe liquid crystal response time under nonzero pretilt angle conditions. The theoretical analysis is confirmed experimentally using vertically aligned liquid crystal cells. This finding quantitatively correlates pretilt angles with liquid crystal response time. This study improves the understanding of the liquid crystal dynamic process which is helpful for optimizing liquid crystal response time.

Free View 3-D Visualization of Occluded Objects by Using Computational Synthetic Aperture Integral Imaging

Abstract: In this paper, we present computational methods for reconstructing a free view of a partially occluded object using integral imaging To obtain unobstructed images with high resolution, low focus error, and large depth of focus, synthetic aperture integral imaging utilizing a digital camera has been adopted Two novel algorithms are proposed: 1) an algorithm for reconstructing volumetric perspective images and 2) an algorithm for scaling reconstructed images at arbitrary distances

Recent Trends on Patterned Vertical Alignment (PVA) and Fringe-Field Switching (FFS) Liquid Crystal Displays for Liquid Crystal Television Applications

Abstract: This paper reviews the recent trends on super-patterned vertical alignment (S-PVA) and fringe-field switching (FFS) technologies for liquid crystal display (LCD)-television (TV) applications. For PVA mode, Samsung originally announced S-PVA technology in 2004 to enhance off-axis viewing quality of conventional PVA mode. S-PVA is a new technology which enables screen quality advantages over super-in plane switching (S-IPS) and multi-domain VA (MVA), including high transmittance, >2300:1 contrast ratio, and wide viewing angle with no off-axis image inversion. This paper explores and updates Samsung's latest developments toward its goal of ultimate LCD-TV performance. For FFS mode, the technology appeared in 1998 by HYDIS and now it has been commercialized in all kinds of display applications, implying its technical importance. This paper reviews recent developments and performance of LCD-TV using the FFS mode based on published papers and our knowledge.

Solution-Processed Organic Light-Emitting Diodes for Lighting

Abstract: In this paper, the vapor-deposited and solution-processed organic light-emitting diode (OLED) technology development paradigms are described and then compared with respect to their prospects for enabling general lighting applications. Two key development needs are improved device efficiency and lower cost fabrication methods. Progress in these areas for solution-processed OLEDs is illustrated by describing recent methods for attaining high efficiency blue emission and introducing novel low cost process methods for device fabrication which enable high performance devices without the need for any vacuum processing steps

Novel Color-Sequential Transflective Liquid Crystal Displays

Abstract: A novel transflective liquid crystal display architecture and its system driving schemes are proposed. In the reflective mode, the ambient light is used to readout the displayed images. While in the transmissive mode, a color-sequential light emitting diode backlight is used to eliminate the color filters. Under such device configuration, several advantages such as increased brightness and maximized color saturation for both transmissive and reflective modes can be achieved

In-Plane Switching Technology for Liquid Crystal Display Television

Abstract: In-plane switching (IPS) technology used for liquid crystal display (LCD) TV is reviewed. Requirements of HD TV application are listed, and the main performances are compared among various competing display technologies. IPS LCD mode shows superior performances, satisfying the requirement for HD TV application.

High Brightness GaN-Based Light-Emitting Diodes

Abstract: This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces

LED-Based Projection Systems

Abstract: A novel design methodology for LED-array-based projection displays has been developed. By combining etendue limitation, system intensity, and efficiency requirements, a novel parameter space is proposed. Using this parameter space, LED lens-array and compound parabolic concentrator (CPC)-array illumination systems have been designed. A 1000-lm LED light source is built. Based on these lens-array and CPC-array illuminators, several LED-based liquid crystal on silicon (LCOS) projection systems are suggested. Among them, a one-panel LCOS projection system is proposed and tested. The method discussed here should be useful in the design of LED-array illumination systems for projectors in general.

Charge Transport and Injection to Phenylamine-Based Hole Transporters for OLEDs Applications

Abstract: We studied the hole injection and transporting properties of four phenylamine-based hole-transporters useful for organic light-emitting diodes. The hole-transporting properties were examined in details by time-of-flight technique whereas their hole-injecting properties by current-voltage measurements and dark-current space-charge-limited-current technique (DI-SCLC). All materials were found to exhibit essentially trap-free hole transports. We discovered that a conducting polymeric anode, PEDOT:PSS, can act effectively as an Ohmic contact for hole injection into the PA compounds for DI-SCLC experiments

Efficient and Rigorous Modeling of Light Emission in Planar Multilayer Organic Light-Emitting Diodes

Abstract: In this paper, we propose a technique to enable efficient and rigorous modeling of light emission in planar organic light-emitting diodes (OLEDs) composed of an arbitrary number of layers with different permittivity (including metals of a complex permittivity). The effects of the change of exciton radiative decay rate are explicitly included in the simulation. The numerical implementation of the technique is comprehensively discussed through an illustrative example. By using the proposed method, a bottom emitting OLED with a thick glass substrate is rigorously analyzed. The calculated results show a good agreement with the experimental results. The proposed method is efficient and well suited for optimizing OLEDs with complicated device structures. As a demonstration, we optimize the two-unit tandem top-emitting OLEDs with three types of charge generating layer. The results and design guidelines are given and discussed in detail

Integrated a-Si:H Source Drivers for 4 $^{\prime\prime}$ QVGA Electrophoretic Display on Flexible Stainless Steel Substrate

Abstract: Integrated source drivers for a 4'' Quarter Video Graphics Array (QVGA) electrophoretic display have been developed using amorphous silicon TFTs on flexible stainless steel substrate. The current design reduces the number of column interconnects to the display by more than 3times compared to a display with external drivers. This integration reduces cost and improves reliability

The World of Liquid-Crystal Display TVs—Past, Present, and Future

Abstract: The full-fledge development for liquid-crystal display (LCD) technologies began aiming to achieve a ldquowall-mounting TVrdquo in the 1960 s. Since then, it has been more than 30 years from the birth of LCD product, the market of LCDs has grown above 10 billion dollars, and in the future, it is expected to further expand at a fast pace. The driving force in the current marketplace will be the original goal, ldquoLCD TVs,rdquo which contribute to drastically improving display performance of LCD and the evolution of LCDs. This paper, therefore, will focus on the LCD-TVs, and describe their history, the state-of-the-art of the technologies, as well as the prospect of their potential in the future.

Mixed Color Sequential Technique for Reducing Color Breakup and Motion Blur Effects

Abstract: This paper proposes a mixed color sequential (MCS) algorithm with high contrast enhancement technique in RGB light-emitting diode (LED) backlight display. Owing to synchronous control of liquid crystal display (LCD) and LED panels, high quality image with suppressed color breakup (CBU) and motion blur effects is achieved by our novel color sequential technique. Importantly, MCS algorithm is useful for color filter-less optical compensated bend (OCB) panel display for alleviating CBU and motion blur effects. Furthermore, high contrast image is also presented on LCD panel because of mixed red-green-blue (RGB) and cyan-magenta-yellow (CMY) backlights with optimum power consumption. In other words, MCS algorithm with high contrast enhancement technique can have the better performance compared with other field sequential color techniques. Experimental results demonstrate by an actual RGB backlight module for 32-in 1366*768 LCD panel the improvement of CBU and motion blur effects.

A New Pixel Circuit Compensating for Brightness Variation in Large Size and High Resolution AMOLED Displays

Abstract: A new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) display using low-temperature polycrystalline silicon thin-film transistor (LTPS-TFT) is proposed. The new circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using HSPICE software. The simulated error rate of the output current is also discussed in this paper. The novel pixel design has great potential for use in large size and high resolution AMOLED displays.

Efficient White OLEDs Employing Phosphorescent Sensitization

Abstract: We have investigated white-emitting organic light-emitting devices (WOLEDs) making use of both blue-phosphor-sensitized orange-red fluorescence and the residual blue phosphorescence. By carefully adjusting the concentrations the phosphor and the fluorophore in the emitting layer and choosing the carrier-transport layers in the device structure, WOLEDs containing a single phosphor-sensitized emitting layer (type-I devices) can give colors close to the equal-energy white (0.33, 0.33), CRI up to 75, and efficiencies up to (10%, 23 cd/A, 13.4lm/W). Furthermore, by doping a green phosphor into the poorly emitting electron-transport layer (type-II devices) to recycle excitons formed there, the EL efficiencies can be further enhanced up to (12.1%, 35.3 cd/A, 23.9lm/W). In both types of devices, the phosphor sensitization reduces population of triplet excitons in the emitting region and substantially mitigates the efficiency roll-off with the driving current or brightness that is often observed in all-phosphor OLEDs. At the brightness of 1000 cd/m2, both types of devices retain quantum and cadmium per ampere (cd/A) efficiencies similar to their peak values

Design Considerations Between Color Gamut and Brightness for Multi-Primary Color Displays

Abstract: Color displays show a vivid colorful image by combination of three or above primary colors on every individual pixel. Brightness of color displays, however, strongly restricts the color gamut of displays. From the color mixing theory, the area of color gamut on the color coordination would shrink smaller when brightness grows up. At the maximal brightness, displays can show one system white point only. The difficulties how to obtain the maximal brightness under an assigned color point or color gamut have been a key issue for display manufacturers. The paper proposes a theory to analyze the relation between brightness and color gamut based on the multi-primary color display. Simulations estimate the boundary of color gamut of multi-primary color displays under required brightness which had been proved by experimental results of tri-primary color display. The theory can be applied on the color temperature (CT) design which experimental results show the fact that a display apparatus with higher color temperature could sacrifice brightness less compared to one with lower color temperature when color temperature of the display image needs to change. The theory provides a design guideline for optimization between color gamut, color temperature and brightness on multi-primary color displays

Nitride-Based Green Light-Emitting Diodes With Various p-Type Layers

Abstract: The performance characteristics of green light-emitting diodes (LEDs) grown by metal-organic chemical-vapor deposition were investigated to study the dependence of the device performance on the materials and the growth conditions of p-type layer grown after the InGaN multiple-quantum-well active region. The electrical and structural qualities of Mg-doped p-In0.04Ga0.96N and p-GaN layers grown under different growth conditions were studied to optimize the growth conditions of p-type hole injection layers of green LEDs. A free-hole concentration of p=1.6times1018 cm-3 of with a resistivity of 0.33Omegamiddotcm was achieved for p-GaN:Mg layers grown at 1040degC. Lower hole concentrations and mobilities and rough surfaces were obtained when the growth temperature was decreased to 930degC in H2 ambient. In the case of p-In0.04Ga0.96N grown at 840degC in N2, a significant improvement of the hole concentration was achieved due to the reduced ionization activation energy of Mg acceptors in InGaN. Also we observed that as-grown p-GaN layers grown in N2 ambient showed p-type properties without Mg dopant activation. The electrical and optical properties of In0.25Ga0.75 N/GaN multiple-quantum-well green LEDs with such different p-layers were investigated. The electroluminescence intensity was improved for the LEDs with p-In0.04Ga0.96N layers grown at 840degC as compared to the LEDs with p-GaN layers grown at higher temperatures due to the reduced thermal damage to the active region, high hole injection, and low piezoelectric field induced in the active region. p-InGaN layers are very attractive candidates for the p-layer in green LED structures. The low temperature and N2 ambient used during the growth of InGaN layers are beneficial to protect the InGaN active region containing high-indium composition quantum-well layers in addition to the advantage of providing a higher hole concentration. However, the LEDs with p-In0.04Ga0.96 N layer showed a slightly higher turn on voltage which could originate from the potential barrier for hole transport at the interface of the p-InGaN layer and the last GaN quantum-well barrier. to reduce this problem, we designed and characterized an LED structure having a graded indium composition in the p-In0.04Ga0.96N layer in order to improve hole transport into the active region. Optimized LEDs with p-InGaN layers grown in a N2 ambient showed much brighter electroluminescence due to low damage to the active region during p-InGaN layer growth

Next-Generation Lighting Initiative at the U.S. Department of Energy: Catalyzing Science Into the Marketplace

Abstract: Solid-state lighting (SSL) is a pivotal emerging technology that promises to fundamentally alter and improve lighting systems-and buildings-of the future, significantly lowering energy use and costs and enhancing the quality of our building environments. No other lighting technology offers our nation so much potential to conserve electricity, at a time when our nation needs bold solutions to achieve greater energy independence. Major research and market introduction challenges must be addressed before the full promise of SSL is realized. This paper discusses the role of the U.S. Department of Energy (DOE) as a catalyst in accelerating SSL technology advances, as it works in partnership with industry, research and academic organizations, and market transformation partners. Through a series of ongoing, interactive workshops, DOE and its partners have developed an extensive plan designed to ensure that DOE funds the appropriate research and development (R&D) topics and commercialization support strategies to help our nation's best and brightest lighting experts move SSL from the laboratory into the marketplace

Revolution of the TFT LCD Technology

Abstract: The introduction of flat panel displays that are fabricated with thin-film-transistor liquid-crystal displays (TFT LCDs) has changed human's lifestyle very significantly. Traditionally, the revolution of the TFT LCD technology has been presented by the timeline of product introduction. Namely, it first started with audio/video (AV) and notebook applications in the early 1990s, and then began to replace cathode-ray tubes (CRTs) for monitor and TV applications. Certainly, TFT LCDs will continue to expand in all areas of our daily life in the future. Here a new concept of the revolution of the TFT LCD technology is presented for the major TFT LCD makers. In this new concept, there are four waves of technology revolution with the following themes, respectively: 1) product introduction; 2) performance enrichment; 3) power and material utilization; and 4) functions for human interface. The role of the LCD-TV in the revolution is also discussed.

A Statistical Model for Simulating the Effect of LTPS TFT Device Variation for SOP Applications

Abstract: In this paper, the variation characteristics of low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) are investigated with a statistical approach. A special layout is proposed to investigate the device variation with respect to various devices distances. Two non-Gaussian equations are proposed to fit the device parameter distributions, whose the coefficients of determination (R2) are both near 0.9, reflecting the validity of the model. Two benchmark circuits are used to compare the difference between the proposed model and the conventional Gaussian distribution for the device parameter distribution. The output behaviors of the digital and analog circuits show that the variation in the short range would greatly affect the performance of the analog circuits and would instead be averaged in the digital circuits.

Single-Technology-Based Statistical Calibration for High-Performance Active-Matrix Organic LED Displays

Abstract: Active-matrix organic light-emitting-diode (AMOLED) displays based on amorphous hydrogenated silicon (a-Si:H) thin-film transistors (TFTs) are the state of the art in display technology, owing to the feasilibility of low-cost fabrication and accessability to well-established TFT-LCD fabrication. While the a-Si:H TFT offers excellent matching of device properties over large areas, it suffers from a gate-bias-dependent threshold voltage shift in time, leading to grayscale inaccuracies. In order to counter this problem, many compensation circuits have been designed. The purpose of the compensation circuit is to estimate the threshold voltage shift in driver TFTs and apply a correction so as to maintain a constant brightness. However, all of the compensation circuits designed to date suffer from low spatial and temporal resolution and reliability issues or high cost due to the use of custom-made CMOS technology. In this paper, we focus on building AMOLED display systems solely based on a-Si:H TFT technology along with the use of off-the-shelf CMOS components to lower costs. Furthermore, we achieve high spatial and temporal resolution and high yield with the use of a two-TFT voltage programmed pixel circuit along with a statistical based external calibration circuit.