Development of new high‐luminance green EL devices based on Eu‐doped calcium thioaluminate materials system

Abstract— A new green EL phosphor with excellent brightness, efficiency, and color saturation was developed. This phosphor is based on the europium‐activated calcium thioaluminate materials system. The performance of this phosphor is greatly enhanced by the addition of Gd as a co‐dopant. Similarly, the addition of ZnS layers on the top and bottom of the phosphor film improved the device luminance considerably. The luminance of the EL devices can be as high as 1700 cd/m² with CIE color coordinates (0.190, 0.645) which were tested by using 32‐μsec‐wide square pulses with a frequency of 120 Hz. The device efficiency was over 2 lm/W.     

Inorganic EL devices with high‐performance blue phosphor and application to 34‐in. flat‐panel televisions

Abstract— A high‐performance inorganic electroluminescence (EL) device has been successfully developed by using an EL structure with a thick dielectric layer (TDEL) and sputtered BaAl₂S₄:Eu blue phosphor. The luminance and efficacy were higher than 2300 cd/m² and 2.5 lm/W at L⁶⁰, 120 Hz, respectively. Furthermore, the luminance at L⁶⁰, 1.2 kHz was more than 23,000 cd/m². The phosphor layer has a single‐phase and a highly oriented crystalline structure. The phosphor also shows high stability in air. A 34‐in. high‐definition television (HDTV) has been developed by combining a TDEL structure and color‐conversion materials. The panels with an optimized color filter demonstrated a peak luminance of 350 cd/m², a color gamut of more than 100% NTSC, and a wide viewing angle similar to that of plasma‐display panels. The high reproducibility of the 34‐in. panels using our pilot line has been confirmed.     

White LEDs using the sharp β‐sialon: Eu phosphor and Mn‐doped red phosphor for wide‐color gamut display applications

The sharp β‐sialon (Si_6‐zAl_zO_zN_8‐z : 0 < z < 0.1):Eu green phosphor, combining with a blue LED and CaAlSiN₃:Eu red phosphor, is suitable for the wide‐color gamut white LEDs backlighting system, because of its sharp and asymmetric emission spectrum shape. However, the color gamut and the brightness of the aforementioned display is restricted because of the wide emission band of the CaAlSiN₃:Eu red phosphor. In this work, we used K₂SiF₆:Mn as an alternate red phosphor, which has a sharp emission spectrum. The display with the white LED using sharp β‐sialon:Eu and K₂SiF₆:Mn shows a wide‐color gamut, which covers the hole NTSC triangle. The use of K₂SiF₆:Mn enables to realize not only a wider color gamut but also a higher brightness of displays, compared with the use of CaAlSiN₃:Eu. Furthermore, it is confirmed that the white LED using sharp β‐sialon:Eu and K₂SiF₆:Mn is stable against temperature and also durable under the accelerated drive conditions.     

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.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.     

Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink‐jet‐printed organic light‐emitting‐diode devices is discussed. Unlike previously reported solution‐processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink‐jet‐printed P²OLED (IJ‐P²OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small‐molecule solution. Ink‐jet‐printed green P²OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m² and is based on phosphorescent emission. The latest solution‐processed phosphorescent OLED performance by spin‐coating is disclosed. The red P²OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m². The green P²OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m². Also discussed is a newly developed sky‐blue P²OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m².     

Eu2+‐doped barium thioaluminate EL devices prepared by two‐target pulsed‐electron‐beam evaporation

Abstract— New blue‐emitting thin‐film‐electroluminescent (TFEL) devices that satisfy the requirements for full‐color TFEL displays were developed. Eu²⁺‐doped BaAl²S⁴ thin films were used for the emission layer. BaAl²S⁴:Eu thin films were prepared by two‐target pulsed‐electron‐beam evaporation suitable for the deposition of multinary compounds that have difficulty in obtaining stoichiometoric thin films. The EL spectrum only had a peak at around 470 nm. The Commission Interantionale de l'Eclairge (CIE) color coordinates were x = 0.12 and y = 0.10. The luminance level from a 50‐Hz pulses voltage was 65 cd/m².     

Organic wavelength‐converting‐film‐based hybrid planar white light‐emitting diodes

Abstract— In this study, organic wavelength‐converting films (WCFs) applied to InGaN blue LED‐based hybrid planar WLED has been fabricated. The organic dye layer in the WCF was formed between the upper and bottom polymer sheets by using a simple roll‐laminating technique. Subsequently, the hybrid planar WLEDs have been fabricated based upon these films. The luminous efficiency of green WCF‐based hybrid planar WLEDs with a single blue LED chip was 34.6 lm/W and that of red‐WCF‐assisted green WCF‐based hybrid planar WLEDs was 27.3 lm/W under 20 mA. The use of WCF to fabricate hybrid planar WLEDs showed better stability than that of directly coating organic color‐convergence materials (CCMs) on the LED chips. It only decreased to about 10% of the initial wavelength‐converting intensity after 1 hour of continual operation at 20 mA.     

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.     

Material and device properties of highly birefringent nematic glasses and polymer networks for organic electroluminescence

Abstract— Light‐emitting nematic liquid crystals are promising materials for organic light‐emitting devices because their orientational anisotropy allows polarized electroluminescence and improved carrier transport. Two classes of nematics, i.e., room‐temperature glasses and crosslinked polymer networks are discussed. The latter class has an additional advantage in that photolithography can be used to pixelate a full‐color display. We show that the order parameter and birefringence of a new light‐emitting nematic liquid crystal with an extended aromatic core both have values greater than 0.9. The performance of green light‐emitting devices incorporating liquid crystals of different conjugation lengths is discussed. Efficacies up to 11.1 cd/A at 1160 cd/m² at an operating voltage of 7 V were obtained. A spatially graded, color organic light‐emitting device obtained by overlapping pixels of blue‐, green‐, and red‐emitting liquid crystals were demonstrated. Some regions of the red pixel were only partially photopolymerized in order to obtain different hues in the overlapping region with green. We also show that the photolithographic process has micron‐scale resolution.     

The study of the low‐field behavior of optically generated charges in thin‐film electroluminescent devices

The measurement of photo‐stimulated currents (PSC) or thermally stimulated currents (TSC) in ACTFEL devices is sometimes used as a method to identify energy levels of trap centers in the phosphor layer. These methods are based on the fact that optically or thermally generated charges are able to move in the polarization field in the phosphor layer of an EL device, which remains after charging the device with a voltage pulse. Similar information about space charge can be obtained by the measurement of loss factor and capacitance of uncharged EL devices under illumination during a low ac‐voltage excitation. This new method avoids one of the major disadvantages of the other methods, namely, the continuous change of the field situation throughout the measurement. The measurement of low‐voltage photo‐stimulated currents (LVPSC) creates an identical and reproducible field situation at each wavelength, which makes interpretation more reliable. Moreover, this new method allows the extraction of information about the low‐field conduction behavior of EL thin films.     

Fabrication and performance characteristics of black‐dielectric electroluminescent 160 × 80‐pixel displays

Abstract— We report on recent technological progress in black‐dielectric electroluminescent (BDEL) displays. Fabrication of the first monochrome BDEL 160 × 80‐pixel 4‐in. displays driven with commercial low‐power (<5 W) drive circuitry is presented. Preliminary results on blue‐dielectric EL full‐color displays are also reported. Improvements in both BDEL display performance and display manufacturability underscore the recent development path.     

Enhancement of blue‐light‐emission properties for OLED displays by using a polarized light‐recycling structure

Abstract— The blue‐light‐emission properties of organic light‐emitting‐diode (OLED) displays must be enhanced to meet the requirements for color purity and luminous efficiency because few blue‐light‐emitting materials meet these requirements. This is particularly true for polymeric and phosphorescent light‐emitting materials. To attain the required purity and efficiency, a polarized‐light‐recycling structure for blue light that is called a blue enhanced circular polarizer (BECP) has been developed. The principle of the structure and the fabricated prototype device is described and it is shown that the structure increases blue‐light intensity and color purity, improves efficiency, provides a wide color gamut, and limits ambient‐light reflection.     

High performance color‐converted micro‐LED displays

A full‐color micro‐LED display can be achieved by red, green, and blue (RGB) chips or by a blue/ultraviolet (UV) micro‐LED array to pump downconverters. The latter helps relieve the burden of epitaxial growth of tri‐color micro‐LED chips. However, such a color‐converted micro‐LED system usually suffers from color crosstalk and low efficiency due to limited optical density of color converters. With funnel‐tube array and reflective coating on its inner surface, the crosstalk is eliminated, and the optical efficiency can be improved by more than two times. In addition, the ambient contrast ratio is also improved because of higher light intensity. The color gamut of this device is approximately 92% of DCI‐P3 standard.     

White stacked OLED with 38 lm/W and 100,000‐hour lifetime at 1000 cd/m2 for display and lighting applications

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.     

Light‐emitting polymers for full‐color display applications

The latest developments in light‐emitting‐polymer (LEP) technology at CDT continue to show steady progress. Device performance for blue, green, and red systems as well as a high‐performance yellow system in terms of device efficiency and stability will be described. Some of the issues associated with the commercialization of LEP technology including the development of direct‐patterning techniques enabling full‐color passive‐ and active‐matrix display will be discussed.     

Full‐color photo‐addressable electronic paper using cholesteric liquid crystals and organic photoconductors

Abstract— Full‐color photo‐addressable electronic paper using cholesteric liquid crystals and organic photoconductors was developed. The electronic paper is comprised of two stacked photo‐addressable elements displaying blue/green and red images, respectively. Each photo‐addressable element was independently controlled by two different color‐addressing lights. Furthermore, blue and green images were selectively switched by one organic photoconductor using the threshold characteristics of cholesteric liquid crystals. A highly reflective polymer‐dispersed cholesteric liquid‐crystal (PDCLC) layer was obtained by a new formation process based on the sol‐gel transition behavior of a gelatin matrix and an agar overcoat layer. The PDCLC layer had a close‐packed honeycomb‐like monolayer structure with a flat surface. The A6‐sized prototype had paper‐like features and showed full‐color bistable images instantly written with a viewer‐type writing apparatus.     

Spatio‐temporal scanning backlight mode for field‐sequential‐color optically‐compensated‐bend liquid‐crystal display

Abstract— A spatially and temporally scanning backlight consisting of ten isolated micro‐structured light guides has been developed to be combined with a fast‐response optically‐compensated‐bend‐mode field‐sequential‐color LCD in which the liquid‐crystal cell does not contain color filters. The sequential fields of three primary colors are generated by illumination of the red‐, green‐, and blue‐light‐emitting diodes, each illuminating for one‐half of the field, resulting in a luminance of 200 cd/m² for the LCD. The effect of light leakage between the blocks in the scanning backlight in field‐sequential‐color applications was measured and will be described.     

Roll‐to‐roll UV embossing process applied for light bar‐based sub‐wavelength gratings for backlight

This study proposes a roll‐to‐roll process‐based sub‐wavelength grating, which is attached on a light bar to turn the side‐lit red/green/blue (620, 520, and 450 nm) incident rays into a uniformly and normally output white light with high illuminance from the light bar's surface. On the basis of the rigorous coupling wave analysis, the relationship between the first‐order transmission/reflection efficiency and the pitch of the gratings with different shapes was analyzed. The optimal design can effectively reduce the coupling length and enhance the white color balance for display applications.     

Development of a high‐resolution RGBW flexible display using a white organic light‐emitting diode with microcavity structure and that of a side‐roll touch panel

In this study, white organic electroluminescent devices with microcavity structures were developed. A flexible high‐resolution active‐matrix organic light‐emitting diode display with low power consumption using red, green, blue, and white sub‐pixels formed by a color‐filter method was fabricated. In addition, a side‐roll touch display was developed in combination with a capacitive flexible touch screen.