Development of a new blue CaMgSi2O6:Eu2+ phosphor for PDPs

Abstract— A blue‐light‐emitting Eu²⁺‐doped CaMgSi₂O₆ phosphor having a long lifetime for a plasma‐display panel (PDP) was developed. The CaMgSi₂O₆:Eu²⁺(CMS:Eu) phosphors show no luminance degradation during the baking process, and an equivalent photoluminescence peak intensity compared to that of the conventional blue‐phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) after baking. CMS: Eu shows a poor luminescent characteristic for the Xe excimer band excitation due to the lack of absorption. To introduce the absorption center for the Xe excimer band, we performed Gd‐codoping of CMS: Eu as a sensitizer and found a new excitation band around 172 nm, which originated from Gd³⁺. The test PDPs panels using synthesized CMS: Eu phosphor and CMS: Eu, Gd phosphor were examined to investigate the luminescent and aging characteristics of a Xe‐discharge excitation source. The CMS: Eu panel shows an emission peak intensity comparable to that of the BAM panel (i.e., a comparable stimuli L/CIEy, 93% of BAM), while the CMS: Eu, Gd panel shows poorer blue emission intensity compared to the BAM panel (up to 53% of total stimuli of BAM). The CMS: Eu panel and the CMS: Eu, Gd panel show less luminance degradation than the BAM panel under the aging test, and the panel retains 90% of its luminance after 300 hours of driving. It was found that CMS: Eu appears to be a candidate for a new blue PDP phosphor because of its longevity in a Xe‐discharge plasma environment.     

Solid‐state lasers for projection

Abstract— The unique advantage of projection displays is the ability to produce large images from small devices. The use of lasers as the projection light source will mean a further step in terms of compactness as well as efficiency for projection systems. However, the advent of laser projection is currently still limited by the availability of low‐cost green lasers. Blue‐diode‐pumped solid‐state lasers are one promising way to realize green as well as red lasers that are specifically suited for projection applications. An efficient solid‐state laser that is based on Pr³⁺:YLF as the laser material, pumped by a blue‐laser diode and emitting at 523 nm, is presented here. The laser reaches power‐conversion efficiencies of more than 7% and output powers of up to 378 mW at green wavelengths. By making only minor modifications to the laser resonator, a red laser emitting at 640 nm can be realized within the same setup. An output power of 166 mW at a power‐conversion efficiency of 6.9% is demonstrated in the red. By combining a red‐ and a green‐emitting blue‐diode‐pumped solid‐state laser with another blue diode, an integrated RGB projection light source can be realized that is based on a single‐diode technology.     

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.     

A new perspective for the thermal degradation mechanism of blue BAM

A new thermal degradation mechanism for blue BAM (BaMgAl₁₀O₁₇:Eu²⁺) under thermal treatment in the presence of water is proposed. The water can be originated either from the thermal treatment steps, such as organic binder baking steps, or from the atmosphere under which blue BAM is being heated. Under such thermal treatment conditions, water molecules can be easily intercalated into the conduction layers of blue BAM, resulting in not only the depreciation of luminance but also in the emission color change toward green. The intercalated water molecules in the conduction layer, where Eu²⁺ ions are located along with Ba²⁺ ions, are strongly associated with Eu²⁺ ions, creating different coordination environments around Eu²⁺ ions. In this paper, the details of changes in emission behaviors along with water content in the water intercalated blue BAMs are discussed.     

Optoelectronic properties of poly(fluorene) co‐polymer light‐emitting devices on a plastic substrate*

Abstract— The optoelectronic properties of red, green, and blue poly(fluorene) co‐polymer light‐emitting devices (PLEDs) on a plastic substrate having a multi‐layered structure with water vapor and oxygen transmission rates of less than 10^?5 g/cm²‐day‐atm and 10^?7 cc/cm²‐day‐atm, respectively, is reported. A semitransparent thin metal multi‐layer (i.e., Au/Ag/Au or Ag/Au/Ag) is placed between the plastic substrate and the ITO coating, achieving a low sheet resistance of 12–13 Ω/□ and an adequate optical transmission greater than 75%. A wider color gamut and a maximum emission efficiency of 0.7, 10, and 1.7 cd/A for red, green, and blue PLEDs, respectively, was obtained. Finally, a simple equivalent‐circuit model was used to simulate the current‐density—voltage characteristics of PLEDs.     

Enhanced PL and VUV absorption of BaZr(BO3)2:Eu3+ by Al3+ incorporation

Abstract— The photoluminescence (PL) and vacuum‐ultraviolet excitation (VUV) properties of BaZr(BO₃)₂ doped with the Eu³⁺ activator ion were studied as a new red phosphor for PDP applications. The excitation spectrum shows strong absorption in the VUV region with an absorption band edge at 200 nm. The charge‐transfer excitation band of Eu³⁺ was enhanced by co‐doping with an Al³⁺ ion into the BaZr(BO₃)₂ lattices. The PL spectrum shows the strongest emission at 615 nm, corresponding to the electric dipole ⁵D₀ → ⁷F₂ transition of Eu³⁺ in BaZr(BO₃)₂, which results in good red‐color purity.     

Red phosphor Li2Mg2(WO4)3: Eu3+ with lyonsite structure for near ultraviolet light-emitting diodes

A series of Eu³⁺-activated Li₂Mg₂(WO₄)₃ (LMW) materials were synthesized by high temperature solid state reactions. The phosphor can be effectively excited by 394 nm near ultraviolet light and emit intense red light with high color purity. Prepared phosphors can be indexed to LMW with particular lyonsite structure. The occupation of Eu³⁺ in LMW is selective. Most of Eu³⁺ comes into ¹A sites without inversion symmetry. The present research suggests that LMW is a suitable host for luminescence applications and Eu³⁺-activated LMW is a promising phosphor for phosphor-converted white light-emitting diodes.     

Red shift of the PSL spectra by Al3+ doping in BaFBr:Eu2+

Abstract— Photostimulated luminescent (PSL) materials are currently used for digital storage and display in radiography. The phosphor BaFBr:Eu²⁺ doped with Al³⁺ is shown to have a very high PSL intensity, and the peak of the PSL spectrum shifts to a longer wavelength. Compared with BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺, BaAlFBr:Eu²⁺ is better suited to the stimulated light wavelength emitted by a semiconductor laser. The red‐shift mechanism is considered to be a FAcenter for BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺ and a F^Z1 center for BaFBr:Eu²⁺ doped with Al³⁺.     

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.     

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.     

Study of the effect of water on thermal and operating degradation of BaMgAl10O17: Eu2+ (BAM) blue phosphor

Abstract— From the adsorption and desorption characteristics of water, we showed that water can intercalate into BaMgAl₁₀O₁₇: Eu²⁺ blue phosphor. ESR, XANES, and XPS analyses confirmed that oxidation by water causes thermal degradation of BAM. We also demonstrated that intercalated water accelerates luminance degradation under VUV irradiation and showed oxidation of Eu²⁺ during panel operation by means of μ‐XPS. We concluded that the cause of thermal and operating degradation of BAM is the oxidation of Eu²⁺ due to water.     

A study on the chromaticity shifts of blue phosphor for color plasma displays

The dependency of the chromaticity shifts on the concentration of Eu²⁺ doped in BaMgAl₁₀O₁₇ (BAM) was investigated under heat‐treatment and vacuum ultraviolet (VUV) irradiation. The Eu²⁺ ions in BAM show an asymmetrical broad emission band with a maximum at ～452 nm under excitation of VUV light at room temperature, showing that multiple crystalline cationic sites exist in the host. It was found that the chromaticity shifts greatly decrease with increasing heat‐treatment temperature. Regardless of the Eu²⁺ concentration, the chromaticity shifts caused by heat‐treatment are greater than that caused by VUV irradiation. Compared with conventional BAM, a solid solution of BAM with barium aluminate as a powder and film was also studied, and very few chromacity shifts were observed. It is suggested that the distribution of Eu²⁺ ions in different sites in a BAM lattice results in different chromaticity coordinates. By increasing the Eu²⁺ concentration in BAM, or under heat‐treatment and VUV irradiation, the emission band shifts towards longer wavelengths.     

Printable phosphorescent organic light‐emitting devices

Abstract— A new approach to full‐color printable phosphorescent organic light‐emitting devices (P²OLEDs) is reported. Unlike conventional solution‐processed OLEDs that contain conjugated polymers in the emissive layer, the P²OLED's emissive layer consists of small‐molecule materials. A red P²OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m², a green P²OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m², a light‐blue P²OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m², and a blue P²OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m² is presented.     

PIN OLEDs — Improved structures and materials to enhance device lifetime

Abstract— Currently, three issues are identified that decide upon the commercial success of organic light‐emitting diodes (OLEDs), both in display and lighting applications: power efficiency, lifetime, and price competitiveness. PIN OLEDs are widely seen as the preferred way to maximize power efficiency. Here, it is reported that this concept also delivers the world longest lifetimes. For a highly efficient deep‐red PIN OLED, a half‐lifetime of 25,000 hours for a starting brightness of 10,000 cd/m² and a minimal voltage increase over lifetime is reported. This value corresponds to more than 1 × 10⁶ hours at 1000 cd/m² using an exponent of n = 1.7, which was measured by driving the OLEDs at different starting luminances. Because there is no initial luminance drop, these PIN OLEDs also exhibit a very high 80% lifetime (>300,000 hours at 1000 cd/m²). New record lifetime values for blue and green will be reported as well. Additionally, further topics that have impact on the production yield and cost such as the newly developed air‐stable organic n‐doping material NDN‐26 and top‐emitting structures will be discussed.     

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.     

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.     

Improved performances in organic and polymer light‐emitting diodes using solution‐processed vanadium pentoxide as a hole injection layer

We report that a solution‐processed vanadium pentoxide (V₂O₅) layer can be utilized as an effective and stable hole injection layer in organic light‐emitting diodes and polymer light‐emitting diodes instead of polyethylene dioxythiophene : polystyrenesulfonate (PEDOT : PSS). The organic light‐emitting diode and polymer light‐emitting diode with the V₂O₅ layer have driving voltages that are 2.2 and 0.3 V lower for 1000 cd/m², respectively, than the devices with PEDOT : PSS. In addition, the devices with the V₂O₅ layer show improved operational stability compared with the devices with PEDOT : PSS. Therefore, a solution‐processed V₂O₅ layer can be utilized as an effective and stable hole injection layer instead of PEDOT : PSS.     

Eu2+‐doped AlN—SiC solid‐solution phosphors: Synthesis and cathodoluminescence properties

Abstract— Eu and Si co‐doped AlN was reported to be an interesting blue phosphor for field‐emission displays (FEDs). In this paper, SiC instead of Si³N⁴ was used as the Si source. Eu²⁺‐doped AlN—SiC phosphors were prepared by firing the powder mixtures of AlN, SiC, and Eu²O³ at 2050°C for 2 hours under 1‐MPa N². Solid solutions between AlN and SiC were formed in a wide range, promoting the solution of Eu²⁺ in AlN. The phosphors showed intense blue emissions under electron‐beam excitation, indicative of potential phosphors for FEDs.     

Design and characterization of new lanthanide fluorides and their optical properties

Three kinds of lanthanide phosphors (La_xLu_1−xF₃: Eu³⁺, LaF₃–CaF₂:Eu³⁺ and LaF₃: Eu³⁺) have been successfully synthesized based on three different ways such as molten salts, co-precipitation, supersonic and microwave irradiations. The as-prepared powder materials all exhibited red luminescence. Their crystal structures or morphologies were studied by means of X-ray powder diffraction and scanning electronic microscope. Eu³⁺-doped LaF₃–CaF₂ phosphor can be emissive under excitation at longer wavelengths (466 and 533 nm) excitations. Supersonic and microwave irradiations have shortened the reaction time of LaF₃: Eu³⁺ crystals in 40 min under very low temperature (50 °C).     

Realization of RGB colors from top‐emitting white OLED by electron beam patterning

We demonstrate the realization of red, green, and blue colors from top‐emitting white organic light‐emitting diode (OLED) for display applications. In our approach, red, green, and blue colors are realized by microcavity‐based mode selection from the spectrum of a white OLED. For the tuning of individual microcavities, the OLED hole transport layer is patterned by an electron beam process.