Quantum dot phosphors free from hazardous elements: Current status and prospects for established materials and new ZnTe‐based alloys

Television broadcasts are moving to a digital video format based on ultra‐high‐definition (UHD). In addition to high resolution, such UHD displays require a wide color gamut. Quantum dots (QDs) have narrow and color‐tunable emission making these unique light sources for achieving a wide color gamut. Most progress in QDs has been made based on cadmium‐containing materials, as represented by CdSe. These systems have excellent performance in terms of their narrow‐band emission and high emission quantum yield; however, the toxicity of cadmium represents a barrier to practical applications of QD displays. Over the last decade, considerable efforts have been made to develop QDs that do not contain cadmium. Some established alternative materials include InP, CuInS₂, and lead halide perovskites. In addition, ZnTe‐based alloy QDs have recently been proposed as promising green and red phosphors. Narrow‐band and green emission (30 nm of full‐width at half‐maximum at a wavelength of 535 nm) has also been reported for Zn (Te, Se) alloy QDs. In this review article, we give a brief overview of progress in established cadmium‐free QDs and describe the current status and future challenges of new cadmium‐free QDs, ZnTe‐based alloy QDs.     

Design considerations for highly efficient edge‐lit quantum dot displays

Quantum dots (QDs) are increasingly the technology of choice for wide color gamut displays. Two popular options to incorporate QDs into displays include on‐edge and on‐surface solutions. The opto‐mechanical design for an on‐edge QD solution including a LED light bar (“on‐edge QD light bar”) is more complex than the design for a standard white phosphor LED light bar. In this paper, we identify and investigate a range of design parameters for an on‐edge QD light bar, and we show that these parameters have significant influence on system efficiency and color uniformity. The effects of varying these parameters are explored through the use of a custom adjustable testbed and optical raytracing methods. Our testbed data demonstrate the inherent trade‐offs between efficiency and color uniformity and provide guidance for the design of high‐performing displays. The optical raytracing data demonstrate a good predictive capability and support the use of optical modeling methods for a detailed exploration of a wider range of design parameters.     

Color tuning of indium phosphide quantum dots for cadmium‐free quantum dot light‐emitting devices with high efficiency and color saturation

Semiconductor quantum dots (QDs) promise facile color tuning and high color saturation in quantum‐dot light‐emitting devices (QD‐LEDs) by controlling nanoparticle size and size distribution. Here, we demonstrate how this promise can be practically realized for the cadmium‐free InP/ZnSe/ZnS multishell quantum dots. We developed a set of synthesis conditions and core/shell compositions that result in QDs with green, yellow, and red emission color. The QD‐LEDs employing these QDs show efficient electroluminescence (EL) with luminance up to 1800 cd/m² and efficiency up to 5.1 cd/ A . The color coordinates calculated from the EL spectra clearly demonstrate the outstanding color saturation as an outcome of the narrow particle size distribution. These results prove that the performance gap between cadmium‐free and cadmium‐based QDs in QD‐LEDs is shrinking rapidly.     

Extensible multi‐primary control sequences

Abstract— In a display with more than three primaries (called a multi‐primary display), a color can be expressed as multiple combinations (called control sequences) of primaries. This paper presents an algorithm for assigning control sequences that preserves current assignments when further primaries are added. These control sequences are cal led extensible. It is shown that the gamut of any number of primaries is a zonohedron, which can be dissected into parallelepipeds. Control sequences are assigned within each parallelepiped. The current parallelepipeds remain when more primaries are added, so the current assignments are preserved. Multi‐primary displays can also cause unwanted metamerism and make continuous color scales appear discontinuous. The algorithm avoids these problems. When viewed through natural filters, such as yellowed ocular lenses, multi‐primary displays can sometimes make two different colors appear identical. If the primaries satisfy the Binet—Cauchy criterion, which is always the case when all primaries are monochromatic, then these spurious matches are avoided.     

Ten micrometer pixel,quantum dots color conversion layer for high resolution and full color active matrix micro‐LED display

A fine patternable quantum dots (QDs) color conversion layer (CCL) for high resolution and full color active matrix (AM) micro‐LED (μ‐LED) display is demonstrated. QDs CCL could be patterned until 10 μm using photolithography process. It is found that multicoatings with red and green QDs (R‐ and G‐QDs) CCLs on LED array can provide full color AM display.     

The present and future of electronic paper

Abstract— With the rise of electrophoretic‐display media from several sources, the world is opening up for new uses of electronic displays. Where “immersive reading” used to be a task strictly reserved for paper, displays can now fulfill that role. Many challenges still remain, such as full‐color photograph‐like performance and video speeds. However, in view of recent accomplishments showing near‐video‐speed switching and potential for full color, after electrophoretic displays obtain a slice of the reading market, application of these developments will take us a significant step towards full‐color animated paper‐like displays. The developments that have led to the presence of electronic paper in the market today will be described, and developments that are about to happen will be discussed.     

Passive‐matrix polymer light‐emitting displays

Abstract— Organic light‐emitting device research focuses on the use of small‐molecule and polymer materials to make organic electroluminescent displays, with both passive‐ and active‐matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full‐color passive‐matrix displays. The stability of polymer OLEDs, and the use of ink‐jet printing for direct high‐resolution patterning of the light‐emitting polymers will also be discussed. It will be shown that the performance of light‐emitting polymers is at the brink of being acceptable for practical applications.     

Flexible active‐matrix OLED displays: Challenges and progress

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.     

Perspectives of passive‐matrix‐LCD improvements

Abstract— The current status of STN‐LCDs is described. The accomplishments and the main problems (low contrast, pure color, and slow response) are discussed. Ways to make improvements (driving methods, retardation film, and memory displays) are considered. The use of memory displays appears to be the most appropriate.     

High efficiency and ultra‐wide color gamut quantum dot LEDs for next generation displays

Colloidal quantum dot‐based hybrid light‐emitting diodes (QLEDs) have been demonstrated that exhibit quantum efficiencies (EQEs) >10% for all three fundamental colors red, green, and blue (21% EQE, 82 cd/A for green). This is the first report of a green QLED with EQE >20% and current efficiency >80 cd/A. The devices have the longest lifetimes reported in the literature (280k hrs) and extremely well‐tuned color fidelity. The narrow QLED emission spectra (full width at half maximum < 30 nm) and well‐controlled peak wavelengths generate a color gamut covering >170% of the National Television System Committee (NTSC) 1987 color space and ~90% of the Rec. 2020 color space. This color gamut is larger than that of OLED televisions in mass production and is the largest of all QLEDs reported. Additionally, these devices are completely fabricated using solution‐processing techniques. The extremely desirable properties of high efficiency, color tunability/fidelity, long lifetime, and low cost processing from solutions make QLED technology disruptive and will lead to next generation displays.     

A novel driving method and device to reduce color breakup in color‐sequential displays

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. Our 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.     

Energy‐efficient liquid‐crystal displays (e2‐LCDs) using a photonic‐crystal backlight system

Abstract— Cholesteric liquid crystals automatically form one‐dimensional photonic crystals. For a photonic crystal in which light‐emitting moieties are embedded, unique properties such as microcavity effects and simultaneous light emission and light reflection can be expected. Three primary‐color photonic‐crystal films were prepared based on cholesteric liquid crystal in which fluorescent dye is incorporated. Microcavity effects, i.e., emission enhancement and spectrum narrowing, were observed. Two types of demonstration liquid‐crystal displays (LCDs) were fabricated using the prepared photonic‐crystal films in a backlight system. One is an area‐color LCD in which a single photonic‐crystal layer is used for each color pixel and the other is a full‐color TFT‐LCD in which three stacked photonic‐crystal layers are used as light‐conversion layers. The area‐color LCD was excited by using 365‐nm UV light, and the full‐color TFT‐LCD was excited by using 470‐nm blue LED light. Because of the photonic crystal's unique features that allow it to work as light‐emitting and light‐reflecting layers simultaneously, both LCDs demonstrate clear readable images even under strong ambient light, such as direct‐sunlight conditions, under which conventional displays including LCDs and OLED displays cannot demonstrate clear images. In particular, an area‐color LCD, which eliminated color filters, gives clear images under bright ambient light conditions even without backlight illumination. This fact suggests that a backlight system using novel photonic‐crystal layers will be suitable for energy‐efficient LCDs (e²‐LCDs), especially for displays designed for outdoor usage.     

Room‐temperature deposition of thin‐film indium tin oxide on micro‐fabricated color filters and its application to flat‐panel displays

Abstract— Direct deposition of indium tin oxide (ITO) thin film on color filters is of practical use in the fabrication of state‐of‐the‐art flat‐panel displays. Room‐temperature dc magnetron sputtering of thin‐film ITO and issues related to the integration of ITO‐on‐glass panels containing micro‐fabricated color filters and other functional materials have been investigated. The resulting polycrystalline ITO exhibited good adhesion to the underlying color filters, as well as good optical transparency and high electrical conductivity. Application of this ITO deposition technology to color liquid‐crystal and organic light‐emitting diode displays will also presented.     

Analysis of color volume of multi‐chromatic displays using gamut rings

There are claims that multi‐chromatic displays can achieve a wider color gamut by the use of additional highly saturated secondary color channels. However, there are other claims that these displays lose lightness and/or color saturation at brighter levels. These apparently divergent views have led to some controversy in the display industry and at standard setting organizations. This study examines the color gamut volume for a variety of simulated and measured multi‐chromatic (sometimes incorrectly referred to as “multi‐primary”) displays using combinations of white and/or secondary color channels, such as cyan, magenta, and yellow. Furthermore, a two‐dimensional gamut representation, referred to as “gamut rings,” is introduced to illustrate that the addition of nonprimary optical color channels to a trichromatic (RGB) display can result in a significant decrease in the chroma at higher lightness levels. The additional saturated color channels can increase the gamut volume only around their hues at darker levels. The results also confirm the validity of comparing the color light output and white light output for revealing the design trade‐offs between the high‐peak white and the color‐image brightness for multi‐chromatic displays.     

Review Paper: The Helmholtz‐Kohlrausch effect

Abstract— The Helmholtz‐Kohlrausch (H‐K) effect is the influence of color purity on the perceived brightness of a color object (or source). In addition to a review of the effect, a survey of color and brightness‐perception studies from 1825 to the present (including our own studies on disabling glare) is presented. Disabling glare is the blinding experience which results from a bright source in our field of view. There has been a great deal of work on the H‐K effect, and this paper is our personal view of the subject. The H‐K effect has lead to new color‐appearance models which help to describe the brightness/luminance discrepancies, and we will only touch on this subject. It will be shown how the H‐K effect affects many of the displays, mobile devices, and phones. Additionally, how the H‐K effect is involved with our perception at mesopic light levels encountered during night driving will be discussed.     

Red light emission from hybrid organic/inorganic quantum dot AC light emitting displays

An alternating current electroluminescent display has been direct written onto a flexible plastic substrate. A hybrid layer of poly(2-methoxy,5(2-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) and CdSe quantum dots (QDs) were used to convert the light emitted by electroluminescent ZnS phosphor into red light. The emission wavelength of the display is found to be directly related to the emission of CdSe quantum dots. The integration of QDs into thin film electroluminescent (TFEL) displays has the potential to enhance its color spectrum.     

LED‐backlight feedback control system with integrated amorphous‐silicon color sensor on an LCD panel

Abstract— Thin‐film‐transistor liquid‐crystal displays (TFT‐LCDs) have the largest market share of all digital flat‐panel displays. An LCD backlighting system employing a three‐color red‐green‐blue light‐emitting diode (RGB‐LED) array is very attractive, considering its wide color gamut, tunable white point, high dimming ratio, long lifetime, and environmental compatibility. But the high‐intensity LED has problems with thermal stability and degradation of brightness over time. Color and white luminance levels are not stable over a wide range of temperature due to inherent long‐term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is the key technology for any LED‐backlight system. In this paper, the feasibility of an optical color‐sensing feedback system for an LED backlight by integrating the amorphous‐silicon (a‐Si) color sensor onto the LCD panel will be presented. To minimize the photoconductivity degradation of a‐Si, a new laser exposure treatment has been applied. The integrated color‐sensor optical‐feedback‐controlled LED‐backlight system minimized the color variation to less than 0.008 Δu'v' (CIE1976) compared to 0.025 for an open‐loop system over the temperature range of 42–76°C.     

Suppression of color breakup in color‐sequential multi‐primary projection displays

Abstract— Field‐sequential projection displays exhibit a phenomenon of color breakup (Rainbow effect). This is considered to be a disturbing artifact with negative marketing impact. The results of a psychophysical experiment comparing the visibility of the phenomenon in RGB and multi‐primary displays is described. Surprisingly, it is found that color breakup in color‐sequential projection displays with five primaries is equally (for 75 Hz) or less (for 105 Hz) visible than in similar displays with three primaries (at 180 Hz), despite the lower refresh rates.     

A common approach to characterizing autostereoscopic and polarization‐based 3‐D displays

Abstract— Autostereoscopic and polarization‐based stereoscopic 3‐D displays recreate 3‐D images by providing different images in the two eyes of an observer. This aim is achieved differently for these two families of 3‐D displays. It is shown that viewing‐angle measurements can be applied to characterize both types of displays. Viewing‐angle luminance measurements are made at different locations on the display surface for each view emitted by the display. For autostereoscopic displays, a Fourier‐optics instrument with an ultra‐high‐angular‐resolution VCMaster3D is used. For polarization‐based displays, a standard Fourier‐optics instrument with additional glass filters is used. Then, what will be seen by an observer in front of the display is computed. Monocular and binocular quality criteria (left‐ and right‐eye contrast, 3‐D contrast) was used to quantify the ability to perceive depth for any observer position. Qualified monocular and binocular viewing spaces (QMVS and QBVS) are deduced. Precise 3‐D characteristics are derived such as maximum 3‐D contrast, optical viewing freedom in each direction, color shifts, and standard contrast. A quantitative comparison between displays of all types becomes possible.     

Color reproduction with a various number of sub‐pixels

Monochrome reflective‐type displays are widely used for portable reading applications such as electric papers because this type does not need a back light unit and can be used outdoors for a long time. Color reflective‐type displays without back light units are desired to expand the market further. The current color reproduction is based on three sub‐pixel red, green and blue (RGB) methods, and when used in reflective type, its luminance is reduced to a third of that of monochrome type. Adding a white sub‐pixel to the current method can improve the luminance, making the sub‐pixel number four. However, in the case of a high resolution display with a four‐sub‐pixel method, the pixel structure is complex, and the luminance improvement may be limited. Instead of increasing the sub‐pixel number, two sub‐pixel methods are investigated. These methods can improve luminance with limited color gamut. The performances are compared with those of other methods quantitatively.