Chromaticity Stability Research Articles

Efficiency improvement in solution-processed multilayered phosphorescent white organic light emitting diodes by silica coated silver nanocubes

Localized surface plasmon resonance (LSPR) effect of metal nanoparticles (MNs) has been widely applied in organic light-emitting diodes (OLEDs) to improve the radiation of excitons. The LSPR wavelength and intensity of MNs and the coupling between MNs and excitons greatly affect the LSPR effect on exciton radiation. In this work, silica coated silver nanocubes (Ag@SiO<sub>2</sub> NCs) were doped in the electron transport layer (ETL) of a solution-processed multilayered white OLED (WOLED). Due to the sharp edges and corners, Ag NCs have strong LSPR effect and can effectively enhance the radiation of nearby excitons. With an appropriate concentration of Ag@SiO<sub>2</sub> NCs, the WOLED achieved two fold improvement in the current efficiency comparing with the control device without Ag@SiO<sub>2</sub> NCs incorporated. The working mechanism of the Ag@SiO<sub>2</sub> NCs based WOLED was investigated in detail. For the solution-processed OLED, excitons usually form and recombine near the interface of emission layer and electron transport layer (EML/ETL) because the commonly used host material (such as polyvinylcarbazole, PVK) has the unipolar hole transport property. So the Ag@SiO<sub>2</sub> NCs in ETL greatly enhanced the radiation of the excitons located near the EML/ETL interface, which mostly contributed to the performance enhancement of the Ag@SiO<sub>2</sub> NCs based WOLED. Study on a group of devices with Ag@SiO<sub>2</sub> NCs doped in different locations indicated that Ag@SiO<sub>2</sub> NCs in ETL showed more effective LSPR effect than those in hole injection layer. Electroluminescence and photoluminescence spectra of the WOLEDs declared that the Ag@SiO<sub>2</sub> NCs simultaneously improved the radiation intensities of the blue and yellow excitons and helped the WOLED maintain the good chromaticity stability, which was mainly attributed to the wide LSPR wavelength range (450–650 nm) of the Ag@SiO<sub>2</sub> NCs. The SiO<sub>2</sub> coating layer of the Ag@SiO<sub>2</sub> NCs played the important role in the LSPR enhanced emission. On the one hand, it formed an appropriated distance between the Ag NCs and the extions, helping to generate the strong coupling between them. On the other hand, it suppressed the effect of Ag NCs on charge trapping, keeping the stability of the carrier transport in the device. Our research demonstrate MNs can effectively improve the performance of OLEDs by carefully designing the device structure.

A single-phase full-visible-spectrum phosphor for white light-emitting diodes with ultra-high color rendering.

Excellent luminous performance and high color rendering are the keys to white light-emitting diode (WLED) illumination. This work reports a single-phase full-visible-spectrum Y2Mg2Al2Si2O12(YMAS):Eu2+,Mn2+ phosphor for WLEDs with ultra-high color rendering. The luminescence of a single Mn2+ doped YMAS phosphor is very weak due to the spin-forbidden transition of Mn2+, while it can be dramatically enhanced in the YMAS:Eu2+,Mn2+ system through efficient energy transfer from the sensitizer Eu2+. Meanwhile, the luminescent color of this phosphor can be tuned from cyan to cold white, to warm white, and finally close to the yellow region by controlling the activator concentration and energy transfer process. Its good thermal and chromaticity stability meet the requirements of application in WLEDs. Its stable photochromic performance at different excitation wavelengths (365-395 nm) indicates that it can be used in different ultraviolet chips. The YMAS:0.03Eu2+,0.30Mn2+ phosphor-converted WLED achieves an ultra-high color rendering index (Ra = 93.3), near-standard chromaticity coordinates (CIE = (0.3343, 0.3388)) and a suitable correlated color temperature (CCT = 5417 K).

Facile synthesis of the desired red phosphor Li2Ca2Mg2Si2N6:Eu2+ for high CRI white LEDs and plant growth LED device

AbstractThe red emission with suitable peak wavelength and narrow band is acutely required for high color rendering index (CRI) white LEDs without at the cost of the luminous efficacy. Herein, the Li2Ca2Mg2Si2N6:Eu2+ red phosphor was prepared with facile solid‐state method using Ca3N2, Mg3N2, Si3N4, Li3N, and Eu2O3 as the safety raw materials under atmospheric pressure for the first time, which shows red emission peaking at 638 nm with full width at half maximum (FWHM) of 62 nm under blue light irradiation and becomes the desired red phosphor to realize the balance between luminous efficacy and high CRI in white LEDs. The morphology, structure, luminescence properties, thermal quenching behavior, and chromaticity stability of the Li2Ca2Mg2Si2N6:Eu2+ phosphor are investigated in detail. Concentration quenching occurs when the Eu2+ content exceeds 1.0 mol%, whereas high‐temperature photoluminescent measurements show a 32% drop from the room‐temperature efficiency at 423 K. In view of the excellent luminescence performances of Li2Ca2Mg2Si2N6:Eu2+ phosphor, a white LEDs with CRI of 91 as a proof‐of‐concept experiment was fabricated by coating the title phosphor with Y3Al5O12:Ce3+ on a blue LED chip. In addition, the potential application of the title phosphor in plant growth LED device was also demonstrated. All the results indicate that Li2Ca2Mg2Si2N6:Eu2+ is a promising red‐emitting phosphor for blue LED‐based high CRI white LEDs and plant growth lighting sources.

The effect of partial substitution of Bi on colour properties and thermal stability of BixPr1−xFeO3 pigments

The samples of BixPr1−xFeO3 (x = 0–0.3) were prepared by conventional ceramic method. The main aim of this work was to focus on determination of influence of partial Bi substitution for Pr on chromaticity and thermal stability. The formation temperature of the orthoferrites was chosen according to the results of DTA/TG analysis. X-ray powder diffraction analysis showed that solid solutions with orthorhombic structure were created. Results of the work proved that Bi substitution for Pr in PrFeO3 has significant impact on colour properties and thermal stability. Start of sintering of PrFeO3 was detected at 1086 °C and due to substitution of Bi3+ decreased to 956 °C (for x = 0.3). Thermal stability of the samples with x = 0.2 and x = 0.3 was limited at 1371 °C and 1360 °C, respectively. However, final colour was positively affected by the addition of Bi3+. Colour shades of powder shifted from yellowish brown to reddish brown with increasing amount of Bi ions. Very interesting colours of different deep yellowish and reddish brown shades were obtained after their application into organic binder. Mean particle size for all milled compounds prepared at 1000 °C was around 1 μm and for samples calcined at 1100 °C ranged between 1 and 1.6 μm.

Study of K+ doping on structure and properties of γ-Ce2S3

In this study, K+-doped γ-Ce2S3 was successfully prepared via a gas–solid reaction method using CeO2, K2CO3, and CS2 as raw materials. The effects of the suitable sulfide system and different molar ratios of K to Ce (nK/Ce = 0–0.30) on the phase composition, crystal structure, chromaticity and thermal stability of γ-Ce2S3 were systematically investigated. Pure γ-Ce2S3 was obtained by calcining the doped samples at 840 °C for 150 min. After calcination at the same temperature the undoped K+ samples exhibit a pure α-phase. Samples with a K/Ce molar ratio (nK/Ce) of 0.10–0.25 comprise only the γ-phase; and when nK/Ce exceeds 0.25, a new heterogeneous phase, KCeS2, emerges. For values of nK/Ce in the range of 0–0.25, the γ-Ce2S3 lattice parameters gradually increases with increasing K+ content. When nK/Ce exceedes 0.25, the lattice parameters remains unchanged. As nK/Ce increased, the synthesized color gradually changes from red to orange–red and finally, to yellow. The redness value a* reaches the maximum (L* = 33.86, a* = 36.68, b* = 38.15) when nK/Ce = 0.10. The nK/Ce = 0.10 composition continues to exhibit the γ-phase after heat treatment at 420 °C for 10 min in air. The K+ doping fills the internal vacancies of γ-Ce2S3 and formed a solid solution, which is beneficial for the stability of its lattice, thus improving the thermal stability of γ-Ce2S3 (from 350 to 420 °C).

Influence of Ba2+ doping on the properties and thermal stability of γ-Ce2S3

In this study, Ba2+-doped γ-Ce2S3 was successfully prepared via a co-precipitation reaction method using Ce (NO3)3·6H2O, Ba (NO3)3, (NH4)2CO3, and CS2 as raw materials. The effects of different molar ratios of Ba2+ to Ce3+ (nBa/Ce = 0–0.6) on the phase composition, chromaticity, and thermal stability of γ-Ce2S3 were systematically investigated. Pure γ-Ce2S3 was obtained by calcining samples with the Ba2+/Ce3+ molar ratio 0.1–0.5 at 900 °C for 150 min. Under the same conditions, the undoped samples exhibited a pure α phase, while for the Ba2+/Ce3+ ratio of 0.6, a new heterogeneous phase, BaS formed. As nBa/Ce increased, the band gap of γ-Ce2S3 increased from 2.06 to 2.31 eV, which led to the color change from red to orange–red and finally, to yellow. The redness value a* reaches a maximum (L* = 33.51, a* = 37.07, b* = 27.18) when nBa/Ce = 0.1. The samples with nBa/Ce = 0.1 still presented the γ phase and showed excellent chromaticity (L* = 30.10, a* = 25.33, b* = 12.71) after heat treatment at 440 °C for 10 min in air. The results fully show that Ba2+ doping filled the internal vacancies of γ-Ce2S3 solid solution, which was beneficial for the stability of its lattice, thus improving the thermal stability of γ-Ce2S3 (from 350 to 440 °C).

Chromaticity Stability of Beta-Carotene from Pumpkin (Cucurbita Moschata)

Chromaticity Stability of Beta-Carotene from Pumpkin (Cucurbita Moschata)

Preparation of γ-Ce2S3@ZrO2 Red Pigment by Hydrolysis Combined Hydrothermal Two Steps Method

The core-shell red pigment γ-Ce2S3@ZrO2 was prepared by a two step method. γ-Ce2S3 was pre-coated at 30°C with hydrolysis, followed by hydrothermal reaction at 180 °C for further coating and crystallization. The effect of preparation method, hydrolysis dropping speed and the number of hydrothermal process on the microstructure of the ZrO2 layer was observed. The influence of coating layer to chromaticity, heat resistance and acid stability was also investigated. The results show that a complete layer at nearly 90 nm coated on the surface of the γ-Ce2S3 particles. The as-prepared pigment exhibited pure red color (L* = 44.38, a* = 31.94, b* = 23.14). Moreover, this coating layer can provide protection to pigment particles. The core-shell pigment can remain red color (L* = 42.71, a* = 26.23, b* =18.38) after calcined in air at 380 °C, while uncoated pigment turned to grey dark at 360 oC.

Enhanced red emission for BaSiF6:Mn4+ hexagonal nanorod phosphor via using spherical silica

In this work, a narrow-band red-emitting BaSiF6:Mn4+ (BSFM) phosphor with regular hexagonal nanorod morphology were successfully prepared through using SiO2 spheres as raw materials in a hydrothermal process (S-BSFM). Interestingly, the as-prepared BSFM nanorods presented better luminescence intensity (about 1.4 times) than that of BSFM sample prepared by using bulk SiO2 as raw material (B-BSFM). Moreover, compared to the B-BSFM samples, the S-BSFM phosphor not only maintained the high thermal stability and chromaticity stability but also improved the color purity. The corresponding luminescence improvement mechanism was proposed that the surface functionalization of SiO2 spheres induced the formation of regular hexagonal nanorods with smooth surface, and thus reduced the non-radiative transitions.

Concentration-Driven Selectivity of Energy Transfer Channels and Color Tunability in Ba3La(PO4)3:Tb3+, Sm3+ for Warm White LEDs.

Here, we report the large-scale emission color tunability in Ba3La(PO4)3:Tb3+, Sm3+ (BLPO:TS) system based on the detailed discussion on the concentration-driven selectivity of energy transfer (ET) channels from Tb3+ to Sm3+. It is induced by the concentration-dependent 5D3 and 5D4 emissions of Tb3+ and the different interaction mechanisms of ET from Tb3+ to Sm3+ via 5D3 and 5D4 channels. In the diluted Tb3+ scenario, the red emission of Sm3+ is efficiently sensitized via the 5D3 channel, while in the concentrated Tb3+ case, the contribution of 5D4 channel is dominant. Therefore, by simply adjusting the doping concentrations of Tb3+ and Sm3+, the emission color of the phosphors can be tuned from green to red. In view of the phosphors with red emissions are critical to the warm white light-emitting diodes (WLEDs), an orange-red Tb3+, Sm3+ coactivated phosphor Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S) with good thermal and chromaticity stability and internal quantum efficiency ∼67% is developed in the system. Then, a near-UV WLED (CCT ≈ 4500 K, Ra ≈ 81) is fabricated using this phosphor. These findings not only indicate that the orange-red phosphor BLPO:5T5S is available for near-UV warm white LEDs but also deliver new insights into the ET processes in Tb3+ and Sm3+ activated phosphors.

Microstructure, chromaticity and thermal stability of SS/TiC-WC/Al2O3 spectrally selective solar absorbers

A new spectrally selective solar absorber coating of TiC-WC/Al2O3 tandem layer is designed and prepared on stainless steel (SS) substrate by magnetron sputtering. The coating consists of an absorptance layer (TiC-WC) and an antireflectance layer (Al2O3) from substrate to top. The new SS/TiC-WC/Al2O3 coating exhibits excellent selective absorbing properties with a high solar absorptance of 0.92 and a low thermal emittance of 0.11. The SS/TiC-WC/Al2O3 coating presents a purple color, which is further investigated by chromaticity study. The thermal stability of the coating is evaluated and characterized by SEM, XRD, UV–vis–NIR spectrophotometer, Fourier transform infrared spectroscopy, and Raman spectroscopy. Experimental results indicate that the coating is highly stable in vacuum at 500°C for 100h with a good solar selectivity of 0.92/0.11.

Tunable chromaticity stability in solution-processed organic light emitting devices

Efficient solution-processed color-stable and color-tunable white organic light emitting diodes (OLEDs) have been realized by judicious selection of the host materials for the emission layers. The color-tunable OLED demonstrates the unique characteristic of modulating the electroluminescence by using the applied voltage of the device and displays color temperatures ranging from 1600K to 4600K around the daylight locus, with a peak external quantum efficiency of 13.6% and a peak current efficiency of 22.5cdA−1. On the other hand, the chromaticity-stable device shows a negligible color change, from 300 to 2000nits. The manipulation of chromaticity is attributed to the energy transfer dynamics of the hosts and dopants under different electric fields.

Improvement of the color-stability in top-emitting white organic light-emitting diodes by utilizing step-doping in emission layers

Chromatic stability and purity cannot be acquired easily in the top-emitting white organic light-emitting diode (TWOLED) due to its special device structure with metal film as both anode and cathode. Blue/red/blue emission layers are utilized to optimize the chromaticity and to improve the stability. Then step-doping is introduced into the red emission layer to further improve the device stability. In order to explain the mechanism of the improvement in chromaticity stability, effects of step-doping on the device performances are analyzed in detail. Experimental results refleal that the built-in electric field induced by the step-doping changes the carrier drifting and exciton recombination in emission layers. When the doping concentration gradually increases from the anode side to the cathode side (increasing step-doping), the built-in electric field is facilitated to suppress the electron drafting and the exciton diffusing, which helps control the exciton recombination zone. Color stability can be improved in the TWOLED with increasing step-doping. Additionally, as an important factor influencing the emission spectra of TWOLEDs, microcavity resonance is calculated and used to explain the variation of spectra induced by device structural changes.

Photoluminescence properties of Ca-doped BaMgAl10O17:Eu2+,Mn2+ blue phosphor using BaF2 and CaF2 as co-flux

Ca-doped BaMgAl10O17:Eu2+,Mn2+ (BAM) blue phosphors were synthesized by flux assisted solid-state reaction method using CaF2 and BaF2 as co-flux. Good dispersity and particle size homogenization of hexagonal pure phase BAM were obtained by sintering at 1400 °C. The effects of the Ca2+ ions content on the structure, morphology and photoluminescence properties of the phosphors were studied. The results indicated that the incorporation of Ca could decrease the lattice constant, improve the homogeneity and dispersity and enhance the photoluminescence (PL) intensity of the phosphor effectively. The optimum Ba0.86Ca0.04Mg0.97Al10O17:0.1Eu2+,0.03Mn2+ PL intensity was enhanced for about 30% and relative brightness was improved about 4%. Furthermore, the synthesized BAM and commercial BAM phosphors were annealed for 30 min at 600 °C in air. The Ca-doped phosphors had stronger emission intensity, higher brightness and better chromaticity stability than that of the commercial phosphor. These results indicated that Ca-doped blue phosphors had good potential applications in the commercial tricolor fluorescent lamps as well as in other display and lamps.

Chromaticity stability of phosphor-converted white light-emitting diodes with an optical filter

The thermal stability of blue LEDs and yellow YAG phosphor for blue-pumped white LEDs is critical to the performance of white LEDs, including the color properties and luminous efficacy. To investigate the thermal effect on the color properties of emitted white light, the junction temperature of the excitation blue LED was raised by increasing the supplied current. However, by choosing an optical filter with the appropriate transmittance spectrum for processing a portion of the blue light output and reexciting the secondary yellow emission, we observed a reduction in color instability and lower color temperature across the current range of 20-200 mA. Moreover, the color temperature distribution was improved.

Temperature Measurement Technique for Stabilizing the Light Output of RGB LED Lamps

The efficiency of light-emitting-diode (LED) lights approaches that of fluorescent lamps. LED light sources find more applications than conventional light bulbs due to their compactness, lower heat dissipation, and real-time color-changing capability. Stabilizing the colors of red-green-blue (RGB) LED lights is a challenging task, which includes color light intensity control using switching-mode power converters, color point maintenance against LED junction temperature change, and limiting LED device temperature to prolong the LED lifetime. In this paper, we present a LED junction temperature measurement technique for a pulsewidth modulation diode forward current controlled RGB LED lighting system. The technique has been automated and can effectively stabilize the color without the need for using expensive feedback systems that involve light sensors. Performance in terms of chromaticity and luminance stability for a temperature-compensated RGB LED system will be presented.

Luminescence properties of Eu 2+- and Ce 3+-doped CaAl 2S 4 and application in white LEDs

The Eu 2+- and Ce 3+-doped CaAl 2S 4 phosphors were comparatively synthesized by conventional solid-state reaction and the evacuated sealed quartz ampoule. The X-ray diffraction (XRD) patterns show that the sample with better crystalline quality was prepared by the evacuated sealed quartz ampoule, resulting in the enhancement of the emission intensity of Eu 2+ ion by a factor of 1.7. The intensive green LEDs were also fabricated by combining CaAl 2S 4:Eu 2+ with near-ultraviolet InGaN chips ( λ em=395 nm). The dependence of as-fabricated green LEDs on forward-bias currents shows that it presents good chromaticity stability and luminance saturation, indicating that CaAl 2S 4:Eu 2+ is a promising green-emitting phosphor for a near-UV InGaN-based LED. In addition, the optical properties of CaAl 2S 4:Ce 3+ were systematically investigated by means of diffuse reflectance, photoluminescence excitation and emission, concentrating quenching and the decay curve.

A novel orange phosphor of Eu 2+-activated calcium chlorosilicate for white light-emitting diodes

Novel orange phosphor of Eu 2+-activated calcium chlorosilicate was synthesized at 1273 K by conventional solid-state reactions under reductive atmosphere and investigated by means of photoluminescence excitation, diffuse reflectance and emission spectroscopies. These results show that this phosphor can be efficiently excited by the incident light of 300–450 nm, well matched with the emission band of 395 nm-emitting InGaN chip, and emits an intense orange light peaking at 585 nm. By combining this phosphor with a 395 nm-emitting InGaN chip, an intense orange light-emitting diode (LED) was fabricated. Under 20 mA forward-bias current, its CIE chromaticity coordinates are (0.486, 0.446). The dependence of as-fabricated orange LED on forward-bias current indicates that it shows excellent chromaticity stability and luminance saturation. These results show that this Eu 2+-activated calcium chlorosilicate is a promising orange-emitting phosphor for near-ultraviolet (UV) InGaN-based white LED.