Near‐infrared‐emitting ZnS: Er and ZnS(Se): Cr TFEL devices

Abstract— Electroluminescent near‐infrared (NIR) emitters are of interest in creating special displays for applications such as communications, chemical sensoring, friend‐and‐foe identification, aligning and checking systems that detect NIR radiation, medicine, etc. By performing this research, known NIR emitters (thermal sources, semiconductor diodes and lasers, and solid‐state lasers) were supplemented by NIR‐emitting TFEL devices based on ZnS: Er(F) and ZnS(Se): Cr electron‐beam‐evaporated films. Some characteristics of these devices as NIR emitters are given. There are two narrow intensive bands at ^～980 and ～1530 nm in the emission spectrum of ZnS: Er(F) TFEL devices. The broadband emission is the range of from 1.8 μm up to 2.7 μm takes place in the ZnS(Se): Cr devices. The wavelength of the peak emission can be varied owing to interference. The mechanism of electroluminescence in the ZnS: Er(F) and ZnS(Se): Cr devices is the direct impact excitation of Er³⁺ and Cr²⁺ ions, respectively.     

Uniform carbon‐nanotube emitter for field‐emission displays

Abstract— The synthesis of carbon‐nanotube (CNT) field emitters for FEDs by thermal chemical vapor deposition (CVD) and their structural and emission characterization are described. Multi‐walled nanotubes (MWNTs) were grown on patterned metal‐base electrodes by thermal CVD, and the grown CNTs formed a network structured layer covering the surfaces of the metal electrode uniformly, which realized uniform distribution of electron emission. A technique for growing narrow MWNTs was also developed in order to reduce the driving voltage. The diameter of MWNT depends on the growth temperature, and it has changed from 40 nm at the low temperature (675°C) to 10–15 nm at the high temperature (900–1000°C). Moreover, narrower MWNTs were grown by using the metal‐base electrode covered with a thin alumina layer and a metal catalyst layer. Double‐walled nanotubes (DWNTs) were also observed among narrow MWNTs. The emission from the narrow CNTs showed a low turn‐on electric field of 1.5 V/μm at the as‐grown layer.     

AlGaN multiple quantum well deep-ultraviolet micro-light-emitting diodes for high color conversion efficiency quantum dots display

Displays based on inorganic micro-light-emitting diodes (micro-LEDs) are highly anticipated for next-generation technologies. AlGaN-based deep-ultraviolet (deep-UV) micro-LED as the excitation source for quantum dots display with high efficiency was reported in this paper. To achieve optimized electro-optical performance, deep-UV micro-LEDs with different electrodes were fabricated and analyzed in sizes from 200 × 200 to 10 × 10 μm². At the same forward bias, the devices with Ti/Al-based electrodes achieved 10 times injection current and three times electroluminescence intensity than those with Cr/Al-based electrodes. The blueshift phenomenon of deep-UV light was observed from 292 nm at 2 A/cm² to 287 nm at 200 A/cm² with the increasing current density. By the excitation of deep-UV micro-LED, quantum dot film achieved high light conversion efficiency and optimized color rendering, as the converted color emission peak was separate from the pumping source. The high energy of deep-UV photons and the narrow emission bandwidth of QDs resulted in prominent color purity. The forward voltage and electroluminescence intensity uniformity of a 250 × 250 micro-LED array with each pixel size of 30 × 30 μm² were further discussed. The optical microscope images of green QD film pumped by a deep-UV micro-LED demonstrated its competitive application in the color-converted display.     

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.     

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.     

Role of triplet‐triplet annihilation in highly efficient fluorescent devices

Abstract— A study of delayed electroluminescence in model highly efficient OLEDs based on anthracene derivatives indicate that triplet‐triplet annihilation (TTA) contributes significantly to overall efficiency. Highly efficient devices (6–9% external quantum efficiencies) based on 9,10‐bis(2‐naphthyl)‐2‐phenylanthracene show that the TTA contribution depends primarily on operating current density, reaching as much as 20–30% of the overall emission intensity at moderate current densities (>5 mA/cm²). Revision of the classical estimates of maximum external quantum efficiency of fluorescent OLEDs to 8% and maximum internal quantum efficiency to 40% is recommended to account for TTA contribution (even further revision may be necessary to account for a better‐than‐20% optical outcoupling).     

Tetrahedrally bonded amorphous carbon for field‐emission displays

Abstract— Field emission from a series of tetrahedrally bonded amorphous‐carbon (ta‐C) films, deposited in a filtered cathodic vacuum arc, has been measured. The threshold field for emission and current densities achievable have been investigated as a function of sp³/sp² bonding ratio and nitrogen content. Typical as‐grown undoped ta‐C films have threshold fields of the order 10–15 V/μm and optimally nitrogen doped films exhibit fields as low as 5 V/μm. In order to gain further understanding of the mechanism of field emission, the films were also subjected to H₂, Ar, and O₂ plasma treatments and were also deposited onto substrates of different work function. The threshold field, emission current, and emission site densities were all significantly improved by the plasma treatment, but little dependence of these properties on work function of the substrate was observed. This suggests that the main barrier to emission in these films is at the front surface.     

OLEDs based on an europium(IIIrpar; complex: {Tris(thenoyltrifluoroacetonate)[1,2,5]‐thiadiazolo[3,4‐f] [1,10]phenanthroline}europium(III)

Abstract— The contribution of radiative and non‐radiative processes to the electroluminescence emission of OLEDs based on Eu‐complex, {tris(thenoyltrifluoroacetone)[1,2,5]thiadiazolo[3,4‐f][1,10]phenanthroline} europium(III), [Eu(TTA)³TDZP], which acts as transporting and emitting layers, is investigated. The Eu‐complex presented an intense photoluminescence with high color purity in the red region, characteristic of the Eu(III) ⁵D⁰ → ⁷F² narrow line transition. However, when used in a double‐layered OLED its electroluminescence showed additional undesired broad bands, which can be attributed to the possible electrophosphorescence of the ligand and to an inefficient energy transfer from the organic ligand to the Eu(III). The characteristic narrow lines could be achieved using a co‐deposited active layer with the Eu‐complex acting as a dopant in a matrix comprised of 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP).     

Composite silver electrode for double‐sided‐emitting stacked organic light‐emitting diode

Abstract— A reflective composite silver electrode is proposed and characterized as the middle electrode of a stacked organic light‐emitting diode (OLED) with double‐sided light emission. The proposed electrode is composed of a thermally evaporated stack of LiF (1 nm)/Al (3 nm)/Ag (70 nm) layers. The LiF/Al and the plasma‐treated Ag of the electrode function well as the respective cathode and anode of the bottom‐ and top‐emitting stacked OLEDs, with both being of the non‐inverted type. Power efficiencies of 10.3 and 12.1 lm/W at 100 cd/m² have been measured for bottom‐ and top‐emitting OLEDs, respectively, using dye doping. The stacked OLED having this bipolar middle electrode can be constructed as a two‐terminal‐only device, allowing for simpler driving schemes in double‐side‐emitting passive‐/active‐matrix OLED displays.     

High‐efficiency quantum dot light‐emitting diodes with blue cadmium‐free quantum dots

We report outstanding electroluminescence properties of high‐efficiency blue cadmium‐free quantum dot light‐emitting diodes (QD‐LED). External quantum efficiency (EQE) of 14.7% was achieved for QD‐LED emitting at 428 nm. Furthermore, we developed high‐efficiency and narrow wavelength emission zinc selenide (ZnSe) nanocrystals emitting at 445 nm and achieved QD‐LED with an EQE of 10.7%. These new QDs have great potential to be used in next‐generation QD‐LED display with wide color gamut.     

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.     

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.     

Blue‐to‐red electroluminescence from organic light‐emitting field‐effect transistor using various organic semiconductor materials

Abstract— We succeeded in observing visible bright electroluminescence from blue to red in an organic field‐effect‐transistor structure. In particular, tetraphenylpyrene (TPPy) demonstrated a high photoluminescence efficiency of φ_PL ～ 70% and a maximum electroluminescence efficiency of η_EL ～ 10^?2%. The electroluminescence efficiency (ηEL) was enhanced by using a short source‐to‐drain channel length (L_SD < 1 μm). In addition, doping the TPPy layer with highly fluorescent rubrene molecules led to an #PL of ～100% and a maximum ηEL of 0.8%.     

Lithium doping and gate dielectric dependence study of solution‐processed zinc‐oxide thin‐film transistors

Abstract— The effects of lithium (Li) doping concentration and gate dielectrics on the performance of solution‐processed zinc‐oxide (ZnO) thin‐film transistors (TFTs) has been investigated. ZnO films with strong c‐axis orientation and lower background conductivity was obtained with 15 at.% of Li. Different crystallization behavior of ZnO was observed in the case of various dielectric surfaces. The 15‐at.% Li‐doped ZnO films (thickness ～20 nm) prepared on SiO² and SiN^x were found to be present in crystalline form, whereas the film prepared on aluminum titanium oxide (ATO) was found to be amorphous. A field‐effect mobility of 1.81 cm²/V‐sec and an Ion/Ioff ratio of 2 × 10⁶ were obtained for the 15‐at.% Li‐doped ZnO TFTs with a bilayer gate dielectric of SiO² and SiN^x. The comparison of dielectric studies showed that the performance of TFTs prepared on SiN^x and ATO are higher than that of the TFTs prepared on SiO².     

High‐efficiency p‐i‐n organic light‐emitting diodes with long lifetime

Abstract— High‐performance organic light‐emitting diodes (OLEDs) are promoting future applications of solid‐state lighting and flat‐panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p‐doped hole‐transport layer/intrinsically conductive emission layer/n‐doped electron‐transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very‐high‐efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double‐emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)₃], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m² are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top‐emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive‐matrix substrates as well as active‐matrix substrates on low‐temperature polysilicon (LTPS) or, in particular, amorphous silicon (a‐Si).     

Dimensioning a full color LED microdisplay for augmented reality headset in a very bright environment

This paper focuses on the dimensioning of a very bright full color 10 μm‐pitch light‐emitting device (LED) microdisplay for avionics application. Starting from the specifications of head‐mounted display to be used in an augmented reality optical system, a theoretical approach is proposed that enables predicting the specifications of the main technology building blocks entering into the microdisplay manufacturing process flow. By taking into account various material and technological parameters, kept as realistic as possible, it is possible to assess the feasibility of a very bright LED microdisplay (1 Mcd/m² full white) and to point out the main limitations. The theoretical specifications are then compared with the technical results obtained so far in the framework of the H2020 Clean Sky “HILICO” project. It shows that 350 000 cd/m² of white emission may be accessible with the present gallium nitride (GaN)‐micro‐LED technology provided a color conversion solution with stable external quantum efficiency of 30% is available. Beyond such level of luminance, the inherent limitations of driving circuit (4 V, 15 μA per pixel) commands working with materials enabling higher external quantum efficiency (EQE). In particular, 10‐μm‐pitch micro‐LEDs with electroluminescence EQE of 15% and color conversion EQE approaching 60% are needed, opening the way to future challenging material and technology research developments.     

Fluorescent‐based tandem white OLEDs designed for display and solid‐state‐lighting applications

Abstract— Highly efficient tandem white OLEDs based on fluorescent materials were developed for display and solid‐state‐lighting (SSL) applications. In both cases, the white OLED must have high power efficiency and long lifetime, but there are a number of attributes unique to each application that also must be considered. Tandem OLED technology has been demonstrated as an effective approach to increase luminance, extend OLED lifetime, and allow for use of different emitters in the individual stacks for tuning the emission spectrum to achieve desired performance. Here, examples of bottom‐emission tandem white OLEDs based on small‐molecule fluorescent emitters designed for displays and for SSL applications are reported. A two‐stack tandem white OLED designed for display applications achieved 36.5‐cd/A luminance efficiency, 8500K color temperature, and lifetime estimated to exceed 50,000 hours at 1000 cd/m². This performance is expected to meet the specifications for large AMOLED displays. A two‐stack tandem white OLED designed for SSL applications achieved 20‐lm/W power efficiency, 38‐cd/A luminance efficiency, 3500K color temperature, and lifetime estimated to exceed 140,000 hours at 1000 cd/m². With the use of proven light‐extraction techniques, it is estimated that this tandem device will exceed 40 lm/W with more than 500,000‐hour lifetime, performance that should be sufficient for first‐generation lighting products.     

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.     

Doping effect of Ir(ppy)3 on white-light electrophosphorescent devices based on platinum(II) [1,3-difluoro-4,6-di(2-pyridinyl)benzene] chloride

Phosphorescent white organic light-emitting diodes (WOLEDs) based on single doped platinum(II) [1,3-difluoro-4,6-di(2-pyridinyl)benzene] chloride (Pt-4) emission layers were investigated in this paper. The devices exhibited electroluminescence spectra composed of bluish (λ_max = 480 nm) and reddish (λ_max = 660 nm) emission bands, which corresponding to monomer and excimer emission originated from Pt-4 dopants. With optimized device structures, a maximum current efficiency of 11.5 cd/A was obtained and remained above 10 cd/A even the brightness was over 6000 cd/m². Furthermore, by integrating the fac-tris(2-phenylpyridine) iridium(III) as a complementary emitter and an additional 2,2′,2″-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) space layer, the device efficiency was further improved, which exhibited a maximum current efficiency of 20.4 cd/A at the luminance of 100 cd/m², and maintained the mild efficiency roll-off that similar to its single Pt-4 doped counterpart.     

Direct observations of vacuum ultraviolet rays for surface‐discharge ac plasma displays by using an ultra‐high‐speed electronic camera

Abstract— Vacuum ultraviolet (VUV) rays emitted from Xe during the operation of surface‐discharge ac plasma‐display panels (PDPs) were observed directly by using a recently developed ultra‐high‐speed electronic camera. It is confirmed that 147‐ and 173‐nm VUV rays are emitted from both the cathode and the anode simultaneously. The direct observation shows that the emitting area for 147‐ and 173‐nm emissions above the cathode and the anode extends outward from the edge of the gap. These emission extensions are considered to be caused by a lowering of the electric field above the area due to the accumulation of wall charges. The intensity of the 147‐ and 173‐nm emissions above the anode decays faster than those above the cathode. It is clarified that the difference in the decay characteristics of VUV rays above the cathode and the anode is caused by the difference in the wall‐charge‐accumulation rates above the cathode and the anode. The major reactions concerning the generation of Xe(1s⁴), a xenon resonant state, which is related to 147‐nm emission, and that of Xe^2Y*, a xenon molecule state, which is related to 173‐nm emission, are discussed.