Stable Chromaticity Research Articles

Luminescent properties of the Sr8−xBaxMgY(PO4)7:Eu2+ solid solution

Photoluminescence generated by substituting rare-earth ions on multiple crystallographically independent cation sites within a crystalline host material is one of the most common approaches for creating a full-spectrum phosphor-converted white light-emitting diode (pc-wLED). In this work, a series of complex orthophosphates with the composition (Sr8−xBax)0.99Eu0.08MgY(PO4)7 (x = 0, 1, 2, 3) were synthesized via high-temperature solid-state synthesis. These materials possess five potential substitution sites achieving broadband (5905 cm−1) yellow photoluminescence through multi-site substitution showing a violet-excited photoluminescent quantum yield of nearly 50%. Developing such an efficient broad emission spectrum is crucial for enhancing the color rendering ability of pc-wLEDs. Varying the Sr2+ to Ba2+ shifts the emission color from green-yellow to yellow under violet excitation (λex = 400 nm) while influencing the full-width-at-half-maximum (fwhm). Temperature-dependent photoluminescence measurements indicate that phosphor has good thermal stability with T50, the temperature at which the emission intensity is half of the low-temperature intensity, at 420 K and stable chromaticity coordinates, further supporting that the (Sr8−xBax)0.99Eu0.08MgY(PO4)7 series are promising candidates for LED white lighting. Constructing a prototype pc-wLEDs device using (Sr7Ba)0.99Eu0.08MgY(PO4)7, the highest quantum yield phosphor, produced a functional daylight bulb with a color rendering index (CRI) of 90.

The impacts of (Ba,Sr)3BP3O12:Eu2+ on the luminous flux of phosphor-converted-white light emitting diode packages

<span>The use of (Ba,Sr)<sub>3</sub>BP<sub>3</sub>O<sub>12</sub>:Eu<sup>2+</sup> in the remote phosphor structure has been proposed and analysed to offer significant improvement to the lighting performance of the phosphor-converted white light emitting diode (LED). The phosphor emits green and blue spectra centred at 520 nm and 465 nm, respectively. Thus, the phosphor can compensate the blue and green light energy components in the white-light spectral band, helping to enhance the luminous efficiency and colour uniformity of the dual-layer remote phosphor package. The increase in (Ba,Sr)<sub>3</sub>BP<sub>3</sub>O<sub>12</sub>:Eu<sup>2+</sup> however is not advantageous to the colour rendering index because of the lower red emission. The backscattered and back-reflected lights are degraded when the (Ba,Sr)<sub>3</sub>BP<sub>3</sub>O<sub>12</sub>:Eu<sup>2+</sup> phosphor layer appears in the structure. The stable chromaticity and luminous flux at good values are observed when 10% weight percentage of (Ba,Sr)<sub>3</sub>BP<sub>3</sub>O<sub>12</sub>:Eu<sup>2+</sup> is applied</span>.

Room-temperature Continuous-Wave Upconversion White Microlaser Using a Rare-earth-Doped Microcavity

White laser covering the visible color spectrum is critical in various applications including vivid display, holographic imaging, and light-based version of Wi-Fi, but it is still challenging to realize a white microlaser due to the rigorous requirement for the balance between the optical gain and feedback in a wide wavelength range. Here, we employ thulium, erbium, and ytterbium elements corporately for the upconversion white lasing by an individual ultrahigh-quality (Q) (up to 108) doped microcavity with microwatt thresholds for the red, green, and blue lasers. To the best of our knowledge, it is the first rare-earth-element-based room-temperature continuous-wave white microlaser. This microlaser also exhibits relatively stable chromaticity over 180 min, which makes it possible for practical applications.

Design and synthesis of D-A-D′ type compounds with AIE characteristics for hybrid WOLED with stable chromaticity

A series of donor–acceptor–donor’ (D-A-D′) type compounds with aggregation-induced emission (AIE) were designed and synthesized by using imidazole as an acceptor and triphenylamine and indolocarbazole as different donors. The non-coplanar structure could effectively eliminate the molecular aggregation-caused quenching (ACQ). Meanwhile, the electron push-pull character could be adjusted by introducing different substituents to achieve the high-efficiency fluorescence from its locally excited (LE) state. The electronic structures, electrochemical behavior, thermal stability, photophysical properties and electroluminescence (EL) performance of these compounds were systematically studied. Four new materials were evaluated as the blue emitters in non-doped OLEDs, and all devices present stable blue emissions with premium EL performance. Furthermore, a hybrid white organic light-emitting diode (WOLED) was prepared by using the compound 5b as the double blue emitting layer and the compound 5d as the host doping with phosphorescent dye. The maximum current efficiency and external quantum efficiency of the device were 67.6 cd/A and 24.3%, respectively. Especially, the WOLED showed ultra-stable CIE coordinates of (x = 0.337 ± 0.003, y = 0.341 ± 0.002) as the driving voltage increases from 5 V to 12 V. This work provides an idea for the design of AIE molecules and a practical strategy to achieve multifunctional light-emitting devices.

Bright Stretchable White Alternating‐Current Electroluminescent Devices Enabled by Photoluminescent Phosphor

AbstractAlternating current‐driven electroluminescent (ACEL) devices have recently emerged as a potential candidate for next‐generation smart lighting, stretchable displays, and wearable electronics. Herein, a simple and efficient method is developed to fabricate high brightness, chromatic‐stable, and stretchable white ACEL devices through doping wide‐spectrum yellow photoluminescent phosphor into blue electroluminescent emissive layer. By tuning the ratio of electroluminescent to photoluminescent phosphors in the emissive layer, the optimized device achieved a maximum brightness of 3150 cd m−2 for white emission with Commison Internationale de L'Eclairage coordinates of (0.28, 0.34), and manifested a more stable chromaticity than traditional ACEL devices. Moreover, a digital display and stretchable white ACEL device (with a tensile strain of 150%) are demonstrated to show the generality of this approach. The high brightness white ACEL devices proposed in this work offer a simplified architecture with facile fabrication process and large‐area compatibility, which may find broad range of applications in wearable optoelectronics.

Improved performance of fluorescent/phosphorescent hybrid white organic light-emitting diodes with enhanced chromatic stability utilizing bipolar interlayer

Fluorescent/phosphorescent hybrid white organic light-emitting diodes (WOLEDs) employing a bipolar CBP interlayer with different thickness have been fabricated. The two-color hybrid WOLEDs at the optimal interlayer thickness exhibit a maximum efficiency of 11.47 cd A−1. Compared with the device without a CBP interlayer, the efficiency of the device is increased by 38.5% and the chromatic stability is also improved. This work found that appropriate thickness of the CBP interlayer with a higher triplet energy level can effectively block the Dexter energy transfer between the phosphorescent and fluorescent layers, which can reduce the energy of triplet in the exciton recombination zone and improve the efficiency of the device. Furthermore, the introduction of the bipolar interlayer with a suitable energy level adjusts the balance of exciton distribution, thereby achieving white light emission with good color stability. The present research could provide valuable strategy in developing hybrid WOLEDs with stable chromaticity.

Multiple emission mechanism based four-peak tuning strategy to achieve ultra-high color rendering index and chromatic-stable white organic light emitting diodes

Abstract We successfully fabricated white organic light-emitting diodes(WOLEDs) based on double light-emitting layers(EMLs) with ultra-high color rendering index (CRI)and stable chromaticity by multiple mechanism four peaks tuning. The phosphorescent blue monomer and red excimer emission from Platinum [1,3-difluoro-4,6-di (2-pyridinyl) benzene]chloride(Pt-4) together with the deep blue fluorescence and yellow phosphorescence emission from a co-emission light-emitting layer realized excellent four-peak tuning WOLEDs. Ultra-high CRI of 94 and extremely stable Commission Internationale deL'Eclairage (CIE) coordinates of (0.420 ± 0.003,0.360 ± 0.002) over a wide voltage range were presented. The high CRI is ascribed to broad and continuous four peaks emission covering the fully visible spectrum. The mixed bipolar interlayer effectively control the distribution of excitons in the two regions of emission, which makes the chromaticity of WOLEDs extremely stable. Also the stable emission mechanism of two EMLs contribute to chromatic stability. Significantly, the devices showed high efficiency under the premise of high CRI and stable chroma by effective strategic of structural design.

Mode selection and high-quality upconversion lasing from perovskite CsPb2Br5 microplates

In recent years, halide perovskite nanostructures have had great advances and have opened up a bright future for micro/nanolasers. However, upconversion lasing by two-photon excitation with mode selection and high quality factor in one device is still rarely reported. Herein, two lasing modes are demonstrated in the all-inorganic perovskite CsPb 2 Br 5 microplates with subwavelength thickness and uniform square shape. The net optical gain is quickly established in less than 1 ps and persists more than 30 ps, revealed by ultrafast transient absorption spectroscopy. The temperature-dependent low-threshold amplified spontaneous emission confirms the net gain for stimulated emission with a high characteristic temperature of 403 K, far surpassing the all-inorganic CsPbBr 3 semiconductor gain media. Remarkably, upconversion lasing based on two kinds of microcavity effects, Fabry–Pérot and whispering-gallery modes, from the microplates at room temperature is successfully achieved with a low threshold operating in multi- or single-mode, respectively. Surprisingly, the quality factor ( ∼ 3551 ) is among the best values obtained from perovskite micro/nanoplate upconversion lasers without an external cavity. Moreover, the highly stable chromaticity with color drift only less than 0.1 nm also outbalances the all-inorganic CsPbBr 3 ones. These superior performances of CsPb 2 Br 5 microplate lasing with a facile solution synthesis procedure will offer a feasible structure to fabricate specific functionalities for high-performance frequency upconversion micro/nanoscale photonic integrated devices.

Green emitting Ba1.5Lu1.5Al3.5Si1.5O12: Ce3+ phosphor with high thermal emission stability for warm WLEDs and FEDs

By hetero-valence substituting Ba2+-Si4+ for Lu3+-Al3+ pair in Lu3Al5O12: Ce3+, a new green phosphor Ba1.5Lu1.5Al3.5Si1.5O12: Ce3+ (BLAS: Ce3+) has been obtained. It crystallizes in garnet structure with space group Ia-3d (230). In the structure, Ba2+ ions are incorporated into both dodecahedral Lu3+ and octahedral Al3+ sites while Si4+ ions only occupy tetrahedral Al3+ sites. Under the blue light irradiation of 450 nm, an intense green light peaking at 520 nm was observed and the PL spectrum can be fitted in two Gaussian components, due to the crystal field splitting of Ce3+ 5d states under D2 symmetry constrains. The optical doping concentration of BLAS: Ce3+ is 6% mol, of which the IQE and EQE are 89.1% and 51.8%, respectively. Furthermore, this sample exhibits an extremely good thermal stability, i.e. the integrated emission intensity is still more than 90% of the initial intensity at 480 K. Then, a w-LED device was fabricated from this new green phosphor BLAS: Ce3+ and commercial red phosphor (Ca,Sr)AlSiN3: Eu2+, which shows a quite high color rendering (Ra = 88.2) and a relatively low color temperature 4772 K. Besides, the phosphor exhibits a stable chromaticity under different acceleration voltages. The phosphor may be promising material for the development of solid-state lighting and display systems.

An efficient material via meta-position connection as thermally activated delayed fluorescence emitter for organic light-emitting diodes

A novel compound was designed and synthesized by connecting a dicyanobenzene acceptor and two 9,9-dimethyl-9,10-dihydroacridine donors to the 1,3,5-position of a phenyl ring by meta-position connection. This compound, which is a novel emitter for OLED devices, exhibits preferable heat stability. Moreover, the energy gap between its singlet and triplet states is as small as 0.04eV, resulting in this molecule possesses thermally activated delayed fluorescence. Therefore, the corresponding device showed efficient electroluminescent performances. The maximum external quantum efficiency, maximum current efficiency, maximum power efficiency and maximum luminance were 16.5%, 40.8cd A−1, 45.8lmW−1 and 5120cdm−2, respectively. In addition, the CIEx,y only changed from (0.22, 0.38) to (0.22, 0.39) over the entire operating voltage range, which confirms that the device possesses highly stable chromaticity with respect to the current density. Based on these experimental results, meta-connected type structures may provide a new approach for developing high-performance TADF emitters for OLED applications.

Upconversion mechanisms of Er3+:NaYF4 and thermal effects induced by incident photon on the green luminescence

Wide range concentrations of Er3+ doped NaYF4 nanocrystals are synthesized. Upconversion mechanisms are discussed under 976nm NIR laser excitation on the basis of the fluorescence spectra and lifetimes data of Er3+. It is found that the heat induced by incident laser results in an evident increase of the slope of emission from 2H11/2 energy level in the pump power dependence. Emission color of samples is investigated and stable chromaticity under different pump power from low and heavy doping samples are found. In addition, upconversion mechanisms are also investigated under 966nm excitation.

Highly Twisted Carbazole–Oxadiazole Hybrids as Universal Bipolar Hosts for High Efficiency PhOLEDs

Four bipolar molecules oDOXA, mDOXA, oOXA, and mOXA composed of a hole‐transport carbazole (donor, D) and an electron‐transport 1,3,4‐oxadiazole (acceptor, A) bridged with different π‐spacers (biphenyl or o‐terphenyl) are synthesized, characterized, and used as host materials for various colored phosphorescent OLEDs (PhOLEDs). The highly twisted geometry established via multiple ortho/meta‐connections effectively inhibits direct electronic D–A coupling and gives these bipolar molecules similar high triplet energies (≈2.70 eV). In addition, distinctive bipolar transport capabilities are observed by time‐of‐flight technique (μh ≈ μe ≈ 10−5–10−6 cm2 V−1 s−1). The D/A connection topology is found to subtly govern the physical properties, rendering these new molecules suitable for serving as bipolar host materials. Among the four host materials, oOXA using the tandem ortho‐linkage terphenyl as a linker outperforms the other three hosts in terms of the device efficiency, in which the maximum external quantum efficiencies (ηext) of the corresponding PhOLEDs are as high as 19.4%, 21.3%, 20.9%, 20.1%, and 19.0% for blue, green, yellow, orange, and red PhOLEDs, respectively. Moreover, a single‐host multilayered warm‐white OLED based on oOXA also shows remarkable efficiency (19.8%, 42.4 cd A−1, and 38.6 lm W−1) with high color‐rendering index of 86.8 and stable chromaticity.

Organic solution-processible electroluminescent molecular glasses for non-doped standard red OLEDs with electrically stable chromaticity

Organic light-emitting molecular glasses (OEMGs) are synthesized through the introduction of nonplanar donor and branched aliphatic chain into electroluminescent emitters. The target OEMGs are characterized by 1H NMR, 13C NMR, IR, UV–vis and fluorescent spectra as well as elemental analysis, TG and DSC. The results indicated that the optical, electrochemical and electroluminescent properties of OEMGs are adjusted successfully by the replacement of electron-donating group. The non-doped OLED device with a standard red electroluminescent emission is achieved by spin-coating the THF solution of OEMG with a triphenylamine moiety. This non-doped red OLED device takes on an electrically stable electroluminescent performance, including the stable maximum electroluminescent wavelength of 640nm, the stable luminous efficiency of 2.4cd/A and the stable CIE1931 coordinate of (x, y)=(0.64, 0.35), which is basically in accord with the CIE1931 coordinate (x, y)=(0.64, 0.33) of standard red light in PAL system.

Efficient solution-processed green and white phosphorescence organic light-emitting diodes based on bipolar host materials

Four carbazole-based bipolar host materials are utilized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). These bipolar materials consist of an electron-donor unit (carbazole) linking to a fluorene unit bearing various electron-acceptor units (oxadiazole, cyano, and benzimidazole) via a saturated carbon, giving sufficiently high triplet energies due to the lack of direct electronic coupling between the donor and acceptor(s). The resulting physical properties and bipolar characteristics render the realization of efficient solution-processed green and white OLEDs feasible. The best green light-emitting device based on bipolar host CzFCBI incorporating a stepwise hole-injection/transporting system exhibit a low drive voltage, a maximum external quantum efficiency of 14.0%, a current efficiency of 49.0cd/A, and a power efficacy of 55.0lm/W. Moreover, the CzFOXa-based two-component (blue–orange) white light-emitting device shows a warmish-white emission with a maximum external quantum efficiency of 6.9% and stable chromaticity coordinates at different luminance levels and yield a high color rendering index (CRI) reaching 76 at a luminance of 1000cd/m2.

A high color purity red emitting phosphor CaYAlO4:Mn4+ for LEDs

Abstract An intense red phosphor, CaYAlO4:Mn4+, was developed by solid state reaction. The photoluminescence excitation and emission spectra, concentration effect, thermal-dependent luminescence quenching properties and decay curves are investigated. The results show Mn4+ ion exhibits a broadband excitation extending from 250 to 550 nm and emits an intense red light at 710 nm with high color purity and stable chromaticity coordinates. These results demonstrate that Mn4+ ion can play a role of activator in narrow red emitting phosphor potentially useful in UV (~370 nm) GaN-base or Blue (~460 nm) InGaN-base LED.

Bipolar Hosts Based on a Rigid 9,10‐Dihydroanthracene Scaffold for Full‐Color Electrophosphorescent Devices

AbstractTwo bipolar hosts, CzDPO and CzDOxa, comprising a hole‐transporting carbazole and an electron‐transporting diphenyl phosphine oxide or oxadiazole with a saturated linker (9,10‐dihydroanthracene) have been synthesized and characterized. The saturated linker limited the effective extension of π conjugation, leading to high triplet energies (ET=2.97–2.98 eV). The diastereomers with a rigid configuration provided an amorphous thin film with high thermal (Td=415–444 °C) and morphological stability (Tg=188–224 °C). The high triplet energies and bipolar carrier transport characteristics (ambipolariy) of CzDPO and CzDOxa can facilitate exothermic energy transfer to the dopants and balance carrier injection/transport in the emission layers. As a result, they were utilized as universal hosts for various phosphorescent OLEDs (from blue to red), showing average external quantum efficiencies (ηext=8.2–13.1 %) and low efficiency roll‐off. In addition, we also fabricated dual‐emitter white organic light‐emitting diodes with a co‐doped single emissive layer. WOLED hosted by CzDOxa exhibited satisfactory device efficiencies (14.4 %, 25.7 cd A−1, 27 lm W−1) with highly stable chromaticity (CIEx=0.26–0.28 and CIEy=0.38) at brightnesses from 560 to 15200 cd m−2.

Flexible top-emitting warm-white organic light-emitting diodes with highly luminous performances and extremely stable chromaticity

The flexible top-emitting white organic light-emitting diode (FTEWOLED) with a very high efficiency but a significant color alteration is achieved with a blue/red/blue sandwiched tri-emission-layer. The voltage-dependent recombination region alternation and the emission mechanism are systematically investigated through a delta-doping method and the time-resolved transient photoluminescence lifetime measurement. By locating the main exciton recombination region at the 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and 9,9-spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1) interface, replacing the carrier-trapping red dopant guest with an orange guest that utilizes energy transfer mechanism, and using a P–I–N structure together with the FIrpic blue guest dopant to balance the electron and hole carriers, an extremely color stable and a very high efficient FTEWOLED is fabricated, with the resulting high current and power efficiencies of 22.7cd/A and 14.27lm/W, and a warm white illumination with a small chromaticity variation of (−0.0087, +0.0015) over a broad luminance range of more than four orders of magnitude. In addition, the performances can be further improved to 23,340cd/m2, 24.49cd/A and 15.39lm/W with a slight concentration alteration of the orange emitter.

Research on the Color Performance Changes of iPad Affected by Brightness Settings

The relationship was derived between the brightness setting and the corresponding color characteristics of the new pad display. Chromaticity domain coverage ratio , channel additivity and the constant channel chromaticity under difference brightness values were measured and compared in detail. The experimental results showed that brightness settings had a great influence on channel additivity, Chromaticity gamut coverage and the constant channel chromaticity meanwhile stable Chromaticity gamut coverage and good channel additivity and the constant channel chromaticity, respectively corresponding to different brightness range . conclusion: When brightness was setted in the range of 200cd/m2 ~ 353 cd/m2 , the new pad had better color performance and more stable.

Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence

Two new molecules, CzFCBI and CzFNBI, have been tailor-made to serve as bipolar host materials to realize high-efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole-transporting block and N-phenylbenzimidazole as the electron-transporting block hybridized through the saturated bridge center (C9) and meta-conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N-phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy-level alignment allows CzFCBI to outperform its N-connected counterpart CzFNBI while employing other long-wavelength-emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue-to-red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low-roll-off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish-green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish-green with a sky-blue emitter and a red emitter, a CzFNBI-hosted single-emitting-layer all-phosphor three-color-based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A(-1) , 28.3 lm W(-1) ) and highly stable chromaticity (CIE x=0.43-0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color-rendering index of 91.6.

Mechanoluminescent and efficient white OLEDs for Pt(ii) phosphors bearing spatially encumbered pyridinyl pyrazolate chelates

Three Pt(II) complexes with a formula of [Pt(Lx)2], x = 1, 2 and 3, in which the pyrazole chelate (Lx)H represents 4-(2,6-diisopropylphenyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (1), 5-(2,6-diisopropylphenyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (2) and 4-(tert-butyl)-2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (3), were synthesized, and their photophysical properties were examined. Single-crystal X-ray diffraction studies of 1 and 2 reveal a planar molecular geometry with both Lx chelates adopting a mutually trans configuration. Distinctive packing arrangements within the crystal lattices are observed, e.g., complex 2 shows structural repetitions that prohibit formation of intimate π–π stacking, while complex 1 exhibits an ordered alignment along the axis of the square planar core framework and, remarkably, shows mechanoluminescence and concentration dependent photoluminescence in the poly(methyl methacrylate) matrix (PMMA). In sharp contrast, derivatives 2 and 3 show only the luminescence derived from the monomeric and aggregated state, respectively, which is attributed to the imposed substituent effect. For application, organic light emitting diodes (OLEDs) were also fabricated using the aforementioned blue dopant 1, attaining the peak external quantum efficiency, current and power efficiencies of ηext = 9.1%, ηc = 18.4 cd A−1 and ηp = 16.6 lm W−1 respectively. With the same device structure, devices based on 3 exhibit an extremely high efficiency orange emission (20.0%, 47.6 cd A−1, 50.8 lm W−1) due to the occurrence of extensive π–π-stacking. Upon combining blue-emitting phosphor 1 and orange-emitting phosphor 3, two white OLED devices are fabricated, one of which obtained by sequentially doping 1, 3 and then 1 into separated layers exhibits performance efficiencies of 11.3%, 28.3 cd A−1, and 21.2 lm W−1, highly stable chromaticity (CIEx = 0.41–0.40 and CIEy = 0.43–0.41), and an adequate color-rendering index (CRI) of 80 at brightness of 370 to 10 800 cd m−2. The other device obtained by co-doping 1 and 3 into a single emissive layer shows improved efficiencies of 15.0%, 44.8 cd A−1, and 46.2 lm W−1, but with a lower CRI of 63.