Temperature effects on photoluminescence of YAG:Ce^(3+) phosphor and performance in white light-emitting diodes

The well crystalline YAG：Ce^3＋ phosphor was synthesized by sold-state method, and the temperature dependence of excitation and emission spectra of YAG：Ce^3＋ phosphor were investigated in the temperature range from room temperature to 573 K. With temperature increasing, it was noted that the emission intensity of as-repared phosphors decreased considerably more rapidly when pumped by 460 nm than by 340 nm. The temperature-intensity curves under different excitation wavelengths were obtained using an Arrhenius function, and the corresponding activation energies were also obtained respectively. Thus, the experimental phenomenon was discussed in terms of nonradiative decay rate. The effects of as-prepared phosphors on the performance of the white LED with changing temperature were also studied.

Highly efficient emission and high-CRI warm white light-emitting diodes from ligand-modified CsPbBr_(3) quantum dots

All-inorganic CsPbBr_(3) perovskite quantum dots(QDs)have received great attention in white light emission because of their outstanding properties.However,their practical application is hindered by poor stability.Herein,we propose a simple strategy to synthesize excellent stability and efficient emission of CsPbBr_(3) QDs by using 2-hexyldecanoic acid(DA)as a ligand to replace the regular oleic acid(OA)ligand.Thanks to the strong binding energy between DA ligand and QDs,the modified QDs not only show a high photoluminescence quantum yield(PLQY)of 96%but also exhibit high stability against ethanol and water.Thereby warm white light-emitting diodes(WLEDs)are constructed by combining lig-and modified CsPbBr_(3) QDs with red AgInZnS QDs on blue emitting InGaN chips,exhibiting a color rendering index of 93,a power efficiency of 64.8 lm/W,a CIE coordinate of(0.44,0.42)and correlated color temperature value of 3018 K.In ad-dition,WLEDs based on ligand modified CsPbBr_(3) QDs also exhibit better thermal performance than that of WLEDs based on the regular CsPbBr_(3) QDs.The combination of improved efficiency and better thermal stability with high color quality indicates that the modified CsPbBr_(3) QDs are ideal for WLEDs application.

Structure and luminescence of Ca_2Si_5N_8:Eu^(2+) phosphor for warm white light-emitting diodes

We have synthesized Ca2Si5N8：Eu^2＋ phosphor through a solid-state reaction and investigated its structural and luminescent properties. Our Rietveld refinement of the crystal structure of Ca1.9Eu0.1Si5N8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions. Between 10 K and 300 K, Ca2Si5N8：Eu^2＋ phosphor shows a broad red emission band centred at -1.97 eV-2.01 eV. The gravity centre of the excitation band is located at 3.0 eV 3.31 eV. The centroid shift of the 5d levels of Eu^2＋ is determined to be -1.17 eV, and the red-shift of the lowest absorption band to be - 0.54 eV due to the crystal field splitting. We have analysed the temperature dependence of PL by using a configuration coordinate model. The Huang-Rhys parameter S = 6.0, the phonon energy hv = 52 meV, and the Stokes shift △S = 0.57 eV are obtained. The emission intensity maximum occurring at -200 K can be explained by a trapping effect. Both photoluminescence （PL） emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.

Very-High Color Rendering Index Hybrid White Organic Light-Emitting Diodes with Double Emitting Nanolayers

A very-high color rendering index white organic light-emitting diode(WOLED) based on a simple structure was successfully fabricated. The optimized device exhibits a maximum total efficiency of 13.1 and 5.4 lm/W at 1,000 cd/m2. A peak color rendering index of 90 and a relatively stable color during a wide range of luminance were obtained. In addition, it was demonstrated that the 4,40,400-tri(9-carbazoyl) triphenylamine host influenced strongly the performance of this WOLED.These results may be beneficial to the design of both material and device architecture for high-performance WOLED.

Enhancing The Efficiency of White Organic Light-emitting Diode Using Energy Recyclable Photovoltaic Cells

We demonstrate that power recycling is feasible by using a semi-transparent stripped Al electrode as interconnecting layer to merge a white organic light-emitting devices(WOLED) and an organic photovoltaic(OPV) cell.The device is called a PVOLED.It has a glass / ITO / CuPc / m-MTDATA ∶ V 2 O 5 / NPB / CBP ∶ FIrpic ∶ DCJTB / BPhen / LiF / Al / P3HT∶ PCBM / V 2 O 5 / Al structure.The power recycling efficiency of 10.133% is achieved under the WOLED of PVOLED operated at 9 V and at a brightness of 2 110 cd / m 2,when the conversion efficiency of OPV is 2.3%.We have found that the power recycling efficiency is decreased under high brightness and high applied voltage due to an increase input power of WOLED.High efficiency(18.3 cd / A) and high contrast ratio(9.3) were obtained at the device operated at 2 500 cd / m 2 under an ambient illumination of 24 000 lx.Reasonable white light emission with Commission Internationale De L'Eclairage(CIE) color coordinates of(0.32,0.44) at 20 mA / cm 2 and slight color shift occurred in spite of a high current density of 50 mA / cm 2.The proposed PVOLED is highly promising for use in outdoors display applications.

Pure blue and white light electroluminescence in a multilayer organic light-emitting diode using a new blue emitter

We characterized the 6,12-bis{[N-（3,4-dimethylphenyl）-N-（2,4,5-trimethylphenyl）]amino} chrysene （BmPAC）, which has been proven to be a blue fluorescent emission with high EL efficiency. The blue fluorescent device exhibits good performance with an external quantum efficiency of 5.8% and current efficiency of 8.9 cd/A, respectively. Using BmPAC, we also demonstrate a hybrid phosphorescence/fluorescence white organic light-emitting device （WOLED） with high efficiency of 36.3 cd/A. In order to improve the relative intensity of blue light, we plus a blue light-emitting layer （BEML） in front of the orange light emitting layer （YEML） to take advantage of the excess singlet excitons. With the new emitting layer of BEML/YEML/BEML, we demonstrate the fluorescence/phosphorescence/fluorescence WOLED exhibits good performance with a current efficiency of 47 cd/A and an enhanced relative intensity of blue light.

Regulating Charge and Exciton Distribution in High-Performance Hybrid White Organic Light-Emitting Diodes with n-Type Interlayer Switch

The interlayer(IL) plays a vital role in hybrid white organic light-emitting diodes(WOLEDs); however,only a negligible amount of attention has been given to n-type ILs. Herein, the n-type IL, for the first time,has been demonstrated to achieve a high efficiency, high color rendering index(CRI), and low voltage trade-off.The device exhibits a maximum total efficiency of 41.5 lm W^(-1), the highest among hybrid WOLEDs with n-type ILs. In addition, high CRIs(80–88) at practical luminances(C1000 cd m^(-2)) have been obtained, satisfying the demand for indoor lighting. Remarkably, a CRI of 88 is the highest among hybrid WOLEDs. Moreover, the device exhibits low voltages, with a turn-on voltage of only 2.5 V([1 cd m^(-2)), which is the lowest among hybrid WOLEDs. The intrinsic working mechanism of the device has also been explored; in particular, the role of n-type ILs in regulating the distribution of charges and excitons has been unveiled. The findings demonstrate that the introduction of n-type ILs is effective in developing high-performance hybrid WOLEDs.

White organic light-emitting diodes based on electroplex from polyvinyl carbazole and carbazole oligomers blends

White organic light-emitting diodes with a blue emitting material fluorene-centred ethylene-liked carbazole oligomer （Cz6F） doped into polyvinyl carbazole （PVK） as the single light-emitting layer are reported. The optical properties of Cz6F, PVK, and PVK：Cz6F blends are studied. Single and double layer devices are fabri- cated by using PVK： Cz6F blends, and the device with the configuration of indium tin oxide （ITO）/PVK：Cz6F/ tris（8-hydroxyquinolinate）aluminium （Alq3）/LiF/A1 exhibits white light emission with Commission Internationale de l＇Eclairage chromaticity coordinates of （0.30, 0.33） and a brightness of 402 cd/m^2. The investigation reveals that the white light is composed of a blue-green emission originating from the excimer of Cz6F molecules and a red emission from an electroplex from the PVK：Cz6F blend films.

High color rendering index white organic light-emitting diode using levofloxacin as blue emitter

Levofloxacin （LOFX）, which is well-known as an antibiotic medicament, was shown to be useful as a 452-nm blue emitter for white organic light-emitting diodes （OLEDs）. In this paper, the fabricated white OLED contains a 452-nm blue emitting layer （thickness of 30 nm） with 1 wt% LOFX doped in CBP （4,4＇-bis（carbazol-9-yl）biphenyl） host and a 584-nm orange emitting layer （thickness of 10 nm） with 0.8 wt% DCJTB （4-（dicyanomethylene）-2-tert-butyl-6-（1,1,7,7- tetramethyljulolidin-4-yl-vinyl）-4H-pyran） doped in CBE which are separated by a 20-nm-thick buffer layer of TPBi （2,2＇,2＂-（benzene-1,3,5-triyl）-tri（1-phenyl-lH-benzimidazole）. A high color rendering index （CRI） of 84.5 and CIE chromaticity coordinates of （0.33, 0.32）, which is close to ideal white emission CIE （0.333, 0.333）, are obtained at a bias voltage of 14 V. Taking into account that LOFX is less expensive and the synthesis and purification technologies of LOFX are mature, these results indicate that blue fluorescence emitting LOFX is useful for applications to white OLEDs although the maximum current efficiency and luminance are not high. The present paper is expected to become a milestone to using medical drug materials for OLEDs.

Tandem white organic light-emitting diodes adopting a C_(60) :rubrene charge generation layer

Organic bulk heterojunction fullerence(C60) doped 5, 6, 11, 12-tetraphenylnaphthacene(rubrene) as the high quality charge generation layer(CGL) with high transparency and superior charge generating capability for tandem organic light emitting diodes(OLEDs) is developed. This CGL shows excellent optical transparency about 90%, which can reduce the optical interference effect formed in tandem OLEDs. There is a stable white light emission including 468 nm and 500 nm peaks from the blue emitting layer and 620 nm peak from the red emitting layer in tandem white OLEDs. A high efficiency of about 17.4 cd/A and CIE coordinates of(0.40, 0.35) at 100 cd/m2 and(0.36, 0.34) at 1000 cd/m2 have been demonstrated by employing the developed CGL, respectively.

Effects of Light-Emitting Diode (LED) Red and Blue Light on the Growth and Photosynthetic Characteristics of <i>Momordica charantia</i>L.

With andromonoecious Momordica charantia L. (bitter gourd) as material, three light qualities (50 μmol·m-2·s-1) including white LED light (WL), blue monochromatic light (B, 465 nm), and red monochromatic light (R, 650 nm) were carried out to investigate their effects on seed germination, physiological and biochemical parameters, sex differentiation and photosynthetic characteristics of bitter gourd. The results showed that compared to the WL treatment, the R treatment significantly promoted seed germination, seedling height elongation and soluble sugar content, the B treatment significantly increased seedling stem diameter, reducing sugar content and soluble protein content, the R and B treatments both significantly reduced sucrose content, but their POD activity showed no significant difference. Compared with the R treatment, the B treatment significantly increased the total female flower number and female flower nod ratio in 30 nods of main stems. The study of photosynthetic characteristics found that the R and B treatments could effectively increase the stomatal conductance (GS) of leaves, significantly improved the net photosynthetic rate (Pn) compared to the WL treatment, and the effect of the B treatment was better. Compared to the R and WL treatments, the B treatment increased the maximum photosynthetic rate (Pmax), apparent quantum efficiency (AQE) and light saturation point (LSP), and reduced the dark respiration rate (Rd) and light compensation point (LCP) of the leaves. Fit light response curves showed that the adaptability and utilization of weak light in bitter gourd were middle or below, but it showed higher adaptability and utilization of strong light. Thus, it suggests that Momordica charantia is a typical sun plan with lower Rd. In summary, it is concluded that blue light has a positive effect on the seed germination, seedling growth, sex differentiation and improving the photosynthetic performance, and this will lay the foundation for artificially regulating optimum photosynthesis using specific LEDs wavelength, and help to elucidate the relationship how light quality influences the sex differentiation of plant.

Monolithic semi-polar(1■01) InGaN/GaN near white light-emitting diodes on micro-striped Si(100) substrate

The epitaxial growth of novel GaN-based light-emitting diode(LED) on Si(100) substrate has proved challenging.Here in this work, we investigate a monolithic phosphor-free semi-polar InGaN/GaN near white light-emitting diode, which is formed on a micro-striped Si(100) substrate by metal organic chemical vapor deposition. By controlling the size of micro-stripe, InGaN/GaN multiple quantum wells(MQWs) with different well widths are grown on semi-polar(1■01)planes. Besides, indium-rich quantum dots are observed in InGaN wells by transmission electron microscopy, which is caused by indium phase separation. Due to the different widths of MQWs and indium phase separation, the indium content changes from the center to the side of the micro-stripe. Various indium content provides the wideband emission. This unique property allows the semipolar InGaN/GaN MQWs to emit wideband light, leading to the near white light emission.

Phosphor-free white light-emitting diodes

The multiple color-matching schemes that could improve the color rendering index for phosphor-free white LEDs are discussed. Then we review a few of the recent research directions for phosphor-free white LEDs, which include the development of monolithic GaN-based white LEDs and hybrid integrated GaN-based and A1GalnP-based white LEDs. These development paths will pave the way toward commercial application of phosphor-free white LEDs in the coming years.

Spectral Characteristics of White Organic Light-emitting Diodes Based on Novel Phosphorescent Sensitizer

White organic light-emitting diodes were fabricated by using a novel phosphorescence bis（1,2-diphenyl-1H-benzoimidazole）iridium（acetylacetonate）[（pbi）2Ir（acac）] as sensitizer and a fluorescent dye of 4- （dicyanomethylene）-2-t-butyl-6-（1,1,7,7-tetramethyljulolidyl-9-enyl）-4H-pyran （DCJTB） codoped into a carbazole polymer of poly（N-vinylcarbazole） （PVK）. Through characterizing the UV-Vis absorption spectra, the photoluminescence spectra of （pbi）2Ir（acac） and DCJTB, and the electroluminescence spectral properties of the WOLEDs, the energy transfer mechanisms of the codoped polymer system were deduced. The results demonstrate that the luminescent spectra with different intensity of （pbi）2Ir（acac） and DCJTB were co-existent in the EL spectra of the blended system, which is ascribed to an incomplete energy transfer process in the EL process. The efficient Forster and Dexter energy transfer between the host and the guests enabled a strong yellow emission from （pbi）2Ir（acac） and DCJTB, where （pbi）2Ir（acac） plays an important role as a phosphorescent sensitizer for DCJTB. With the blue emitting-layer of N,N＇-diphenyl-N,N＇-bis（1- naphthyl）（1,1＇-biphenyl）-4,4＇-diamine, the codoped system device achieved white emission. The codoped system showed that its Commissions Internationale de 1＇Eclairage coordinates were more independent of the variation of bias voltage than those of phosphorescent doped PVK systems.

Color-converted remote phosphor prototype of a multiwavelength excitable borosilicate glass for white light-emitting diodes

We report a unique red light-emitting Eu-doped borosilicate glass to convert color for warm white light-emitting diodes. This glass can be excited from 394 nm-peaked near ultraviolet light, 466 nm-peaked blue light, to 534 nm- peaked green light to emit the desired red light with an excellent transmission in the wavelength range of 400-700 nm which makes this glass suitable for color conversion without a great cost of luminous power loss. In particular, when assembling this glass for commercial white light-emitting diodes, the tested results show that the color rendering index is improved to 84 with a loss of luminous power by 12 percent at average, making this variety of glass promising for inorganic ＂remote-phosphor＂ color conversion.

High-Performance Hybrid White Organic Light-Emitting Diodes Utilizing a Mixed Interlayer as the Universal Carrier Switch

A new interlayer is successfully used to be a universal carrier switch, developing high-performance hybrid white organic light-emitting diodes （WOLEDs）. By dint of this interlayer, the two-color hybrid WOLED shows a maximum total current efficiency （CE） and power efficiency （PE） of 48.1 cd/A and 37.6 Im/W, respectively, while the three-color hybrid WOLED shows a maximum total CE and PE of 33.8 cd/A and 25.7Im/W, respectively. The color rendering index of the three-color hybrid WOLEDs are ≥ 75, which is already a sufficient level for many commercial lighting applications. In addition, both the two-color and three-color hybrid WOLEDs show low efficiency roll-off and stable color. Furthermore, devices with the new interlayer show much higher performance than devices with the most commonly used 4,4-N,N-dicarbazolebiphenyl and N,N＇-di（naphthalene-l-yl）-N,N＇- diphenyl-benzidine interlayers.

White organic light-emitting diodes based on a combined electromer and monomer emission in doubly-doped polymers

We report on white organic light-emitting diodes （WOLEDs） based on polyvinylcarbazole （PVK） doped with 1,1-bis（（di-4-tolylamino）phenyl）cyclohexane （TAPC） and perylene, and investigate the luminescence mechanism of the devices. The chromaticity of light emission can be tuned by adjusting the concentration of the dopants. White light with the Commission Internationale de L＇Eclairage （CIE） coordinates of （0.33, 0.34） is achieved by mixing the yellow electromer emission of TAPC and the blue monomer emission of perylene from the device ITO/PVK： TAPC： perylene （100：9：1 in wt.） （100 nm）/tris-（8-hydroxyquinoline aluminum （Alq3） （10 nm）/A1. The device exhibits a maximal luminance of 3727 cd/m2 and a current efficiency of 2 cd/A.

Top-Emitting White Organic Light-Emitting Diodes Based on Cu as Both Anode and Cathode

It is still challenging to obtain broadband emission covering visible light spectrum as much as possible with negligible angular dependence. In this work, we demonstrate a low driving voltage top-emitting white organic light-emitting diode （TEWOLED） based on complementary blue and yellow phosphor emitters with negligible angular dependence. The bottom copper anode with medium reflectance, which is compatible with the standard complementary metal oxide semiconductor （CMOS） technology below 0.13 μm, and the semitransparent multi- layer Cs2CO3/AI/Cu cathode as a top electrode, are introduced to realize high-performance TEWOLED. Our TEWOLED achieves high efficiencies of 15.4callA and 12.1 1m/W at a practical brightness of lO00cd/m2 at low voltage of 4 V.

Nondoped-type White Organic Light-Emitting Diode Using Star-Shaped Hexafluorenylbenzene as an Energy Transfer Layer

White organic light-emitting diodes （WOLEDs） with a structure of indium-tin-oxide （ITO）/N,N＇-bis- （1-naphthyl）-N,N＇-diphenyl- （1, 1＇-biphenyl）-4,4＇-diamine （NPB）/1,2,3,4,5,6-hexakis（9,9-diethyl-9H-fluoren-2- yl）benzene （HKEthFLYPh）/5,6,11,12-tetraphenylnaphtacene （rubrene）/tris（8-hydroxyquinoline） aluminum （Alq3）/Mg：Ag were fabricated by vacuum deposition method, in which a novel star-shaped hexafluorenyl- benzene HKEthFLYPh was used as an energy transfer layer, and an ultrathin layer of rubrene was inserted between HKEthFLYPh and Alq3 layers as a yellow light-emitting layer instead of using a time-consuming doping process. A fairly pure WOLED with Commissions Internationale De L＇Eclairage （CIE） coordinates of （0.32, 0.33） was obtained when the thickness of rubrene was 0.3 nm, and the spectrum was insensitive to the applied voltage. The device yielded a maximum luminance of 4816 cd/m2 at 18 V.

Theoretical Modeling of Luminous Efficacy for High-Power White Light-Emitting Diodes

The photometric characteristics of high-power white light-emitting diode （LED） devices are investigated. A theoretical model for the luminous efficacy o[ high-power white LED devices and LED systems is proposed. With the proposed theoretical model, the mechanism of the luminous efficacy decrease is explained. Meanwhile, the model can be used to estimate the luminous efficacy oF LEDs under general operation conditions, such as different operation temperatures and injection currents. The wide validity of the luminous efficacy model is experimentally verified through the measurements of different types of LEDs. The experimental results demonstrate a high estimation accuracy. The proposed models not only can be applied to estimate the LED photometric performance, but also is helpful for reliability research of LEDs.