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.

Internal quantum efficiency drop induced by the heat generation inside of light emitting diodes (LEDs)

The reasons for low output power of AlGalnP Light Emitting Diodes （LEDs） have been analysed. LEDs with AlGaInP material have high internal but low external quantum efficiency and much heat generated inside especially at a large injected current which would reduce both the internal and external quantum efficiencies. Two kinds of LEDs with the same active region but different window layers have been fabricated. The new window layer composed of textured 0.5 μm GaP and thin Indium-Tin-Oxide film has shown that low external quantum efficiency （EQE） has serious impaction on the internal quantum efficiency （IQE）, because the carrier distribution will change with the body temperature increasing due to the heat inside, and the test results have shown the evidence of LEDs with lower output power and bigger wavelength red shift.

Synthesis and luminescent properties of polycrystalline Gd_2(MoO_4)_3:Dy^(3+) for white light-emitting diodes

Polycrystalline Gd2(MoO4)3:Dy3+ phosphors have been synthesized by high temperature solid-state reaction method. The phosphors were characterized with X-ray diffractometer, thermogravimetric analysis and different scanning calorimeter, scanning electron microscopy, and photoluminescence spectrofluorimeter. Several peaks at 351, 389, 425, 452, and 472 nm appeared in photoluminescence excitation spectrum, which matched well with the emission of the ultraviolet (UV) and blue-light emitting diode (LED) chips. Upon excitation at 389 nm UV light, intense emissions centered at 484, 575 and 668 nm were attributed to the transitions of 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2 of Dy3+, respectively. The chromaticity coordinates and correlative color temperatures have been calculated and presented in the Commission International de I’Eclairage (CIE) diagrams. The results indicated that Gd1.9(MoO4)3:Dy0.13+ with CIE coordinates of (x=0.38, y=0.41) and the correlative color temperature of 4134 K is a potential candidate for white LEDs.

Metal–organic–vapor phase epitaxy of InGaN quantum dots and their applications in light-emitting diodes

InGaN quantum dot is a promising optoelectronic material, which combines the advantages of low-dimensional and wide-gap semiconductors. The growth of InGaN quantum dots is still not mature, especially the growth by metal--organic- vapor phase epitaxy （MOVPE）, which is challenge due to the lack of, itin-situ monitoring tool. In this paper, we reviewed the development of InGaN quantum dot growth by MOVPE, including our work on growth of near-UV, green, and red InGaN quantum dots. In addition, we also introduced the applications of InGaN quantum dots on visible light emitting diodes.

Near-infrared lead chalcogenide quantum dots:Synthesis and applications in light emitting diodes

This paper reviews the recent progress in the synthesis of near-infrared(NIR) lead chalcogenide(PbX;PbX = PbS,PbSe, PbTe) quantum dots(QDs) and their applications in NIR QDs based light emitting diodes(NIR-QLEDs). It summarizes the strategies of how to synthesize high efficiency PbX QDs and how to realize high performance Pb X based NIR-QLEDs.

Potential red-emitting phosphor GdNbO_4:Eu^(3+),Bi^(3+) for near-UV white light emitting diodes

A red-emitting phosphor GdNbO4：Eu3＋,Bi3＋ was prepared by a high temperature solid-state reaction technique. The phosphor was characterized by X-ray diffraction （XRD）, particle size analyzer and fluorescence spectrometer. The single phase of GdNbO4：Eu3＋,Bi3＋ was obtained at 1150~C and the average particle diameter was about 2.30 μm. Excitation and emission spectra reveal that the phosphor can be ef- ficiently excited by ultraviolet （UV） light （394 nm） and emit the strong red light of 612 nm due to the Eu3＋ transition of SD0~TF2. The opti- mum content of Eu3＋ doped in the phosphor GdNbOn：Eu3＋ is 20mo1%. The phosphor Gdo.80NbO4：0.20Eu3＋,0.03Bi3＋ shows much stronger photoluminescence intensity and better chromaticity coordinates （x=0.642, 0.352） than GdNbO4：Eu3＋. It is confirmed that Gdo.80NbO4：0.20Eu3＋,0.03Bi3＋ is a potential candidate for near-UV chip-based white light emitting diodes.

Organic Light Emitting Diodes with Lithium Contained Alq3 as Electron Injection Layer

lNovel lithium doped tris 8 hydroxylquinoline aluminium(Alq3:Li) layer is deposited between emission layer and electron injection aluminium electrode as an electron injection assistant layer in different organic light emitting diodes(OLED) to lower the electron injection barrier. In these devices, Alq3 is used as emission layer, and a bilayer film of N,N bis (1 naphhyl) N,N diphenyl 1,1 biphenyle 4,4' diamine(NPB) and 4,4,4' tris(3 methyl phenylphenylamino) triphenylamine( m MTDATA) used as hole transport layer(HTL). The electroluminescent performance of devices with different thicknesses of Alq3∶Li shows that the insertion of the lithium doped Alq3 layer can reduce the turn on voltage by at least 2 volts, and the stability of devices with this lithium doped Alq3 layer is improved too. It can also change the efficiency of devices. Compared with an ultra thin lithium fluoride(LiF) layer, Alq3∶Li sheet gives similar effects but higher efficiency and can be much thicker and hence it is easier to control the deposition.

Multifunctional silicon-based light emitting device in standard complementary metal oxide semiconductor technology

A three-terminal silicon-based light emitting device is proposed and fabricated in standard 0.35 μm complementary metal-oxide-semiconductor technology. This device is capable of versatile working modes： it can emit visible to near infra-red （NIR） light （the spectrum ranges from 500 nm to 1000 nm） in reverse bias avalanche breakdown mode with working voltage between 8.35 V-12 V and emit NIR light （the spectrum ranges from 900 nm to 1300 nm） in the forward injection mode with working voltage below 2 V. An apparent modulation effect on the light intensity from the polysilicon gate is observed in the forward injection mode. Furthermore, when the gate oxide is broken down, NIR light is emitted from the polysilicon/oxide/silicon structure. Optoelectronic characteristics of the device working in different modes are measured and compared. The mechanisms behind these different emissions are explored.

Organic light emitting diodes using magnesium doped organic acceptor as electron injection layer and silver as cathode

Organic light emitting diodes employing magnesium doped electron acceptor 3, 4, 9, 10 perylenetetracarboxylic dianhydride （Mg：PTCDA） as electron injection layer and silver as cathode were demonstrated. As compared to Mg ： Ag cathode, the combination of the Mg ： PTCDA layer and silver provided enhanced electron injection into tris （8- quinolinolato） aluminium. The device with 1 ： 2 Mg ： PTCDA and Ag showed an increase of about 12% in the maximum current efficiency, mainly due to the improved hole-electron balance, and an increase of about 28% in the maximum power efficiency, as compared to the control device using Mg ： Ag cathode. The properties of Mg ： PTCDA composites were studied as well.

Effects of ultrasonic bonding parameters on reliability of flip chip GaN-based light emitting diode

This work applied the ultrasonic bonding to package flip chip GaN-based light emitting diodes （flip chip LEDs） on Si substrates. The effects of ultrasonic bonding parameters on the reliability of flip chip GaN-based LED were investigated. In the sequent aging tests, samples were driven with a constant current of 80 mA for hundreds hours at the room temperature. It was found that the electroluminescence （EL） intensity variation had a large correlation to the ultrasonic power, and then to the bonding temperature and force. A high bonding temperature and ultrasonic power and a proper bonding force improved the EL intensity significantly. It was contributed to a strong atom inter-diffusion forming a stable joint at the bonding interface, The temperature fluctuation in the aging test was the main factor to generate a high inner stress forming delamination at the interface between the chip and Au bump. As a result, delamination had retarded the photons to emit out of the LED packaging and decay its EL intensity.

A green-yellow emitting β-Sr_2SiO_4:Eu^(2+) phosphor for near ultraviolet chip white-light-emitting diode

Sr2SiO4：xEu^2＋ phosphors were synthesized through the solid-state reaction technique. The crystal phase of Sr2SiO4：xEu^2＋ phosphor manipulated by Eu^2＋ concentration was studied. The phase transited from β to α＇ in Sr2SiO4：xEu^2＋ phosphor with increasing europium concentration. The single β phase was formed as x≤005 and changed α＇ phase when x〉0.01. The emission spectrum of the β-Sr2SiO4：Eu^2＋ phosphor consisted of a green-yellow broadband peaking at around 540 nm and a blue band at 470 nm under near ultraviolet excitation. The white LEDs by combining near ultraviolet chips with β-Sr2SiO4：Eu^2＋ phosphors were fabricated. The luminous efficiency （15.7lm/W） was higher than α＇-Sr2SiO4：Eu^2＋ phosphor white LED.

Review of blue perovskite light emitting diodes with optimization strategies for perovskite film and device structure

Perovskite light emitting diodes(PeLEDs)have attracted considerable research attention because of their external quantum efficiency(EQE)of>20%and have potential scope for further improvement.However,compared to red and green PeLEDs,blue PeLEDs have not been extensively investigated,which limits their commercial applications in the fields of luminance and full-color displays.In this review,blue-PeLED-related research is categorized by the composition of perovskite.The main challenges and corresponding optimization strategies for perovskite films are summarized.Next,the novel strategies for the design of device structures of blue PeLEDs are reviewed from the perspective of transport layers and interfacial layers.Accordingly,future directions for blue PeLEDs are discussed.This review can be a guideline for optimizing perovskite film and device structure of blue PeLEDs,thereby enhancing their development and application scope.

Luminescence characteristics of Eu^(3+) activated borate phosphor for white light emitting diode

In this paper, the Sr3Y2 （BO3）4 ：Eu^3＋ phosphor was synthesized by high temperature solid-state reaction method and the luminescence characteristics were investigated. The emission spectrum exhibits one strong red emission at 613 nm corresponding to the electric dipole 5^Do-7^F2 transition of Eu^3＋ under 365 nm excitation, this is because Eu^3＋ substituted for Y^3＋ occupied the non-centrosymmetric position in the crystal structure of Sr3Y2（BO3）4. The excitation spectrum indicates that the phosphor can be effectively excited by ultraviolet （254 nm, 365 nm and 400 nm） and blue （470 nm） light. The effect of Eu^3＋ concentration on the red emission of Sr3Y2（BO3）4 ：Eu^3＋ was measured, the result shows that the emission intensities increase with increasing Eu^3＋ concentration, then decrease. The Commission Internationale del＇Eclairage chromaticity （x, y） of Sr3Y2（BO3）4 ：Eu^3＋ phosphor is （0.640, 0.355） at 15 mol% Eu^3＋.

PEROVSKITE SEMICONDUCTOR OPTOELECTRONIC DEVICES Rational molecular passivation for high-performance perovskite light-emitting diodes

Solution-processed metal halide perovskites (MHPs) have received significant interest for cost-effective, high-performance optoelectronic devices. In addition to the great successes in photovoltaics, their excellent luminescence and charge transport properties also make them promising for light emitting diodes (LEDs). To achieve high-efficiency perovskite LEDs (PeLEDs), extensive efforts have been carried out to enhance radiative recombination rates by confining the electrons and holes. In addition to enhancing radiative recombination rates, it is equally important to decrease the non-radiative recombination for improving the device performance. Passivation of the defects could be an efficient way for reducing the non-radiative recombination.

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.

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.

A recent advances of blue perovskite light emitting diodes for next generation displays

The halide perovskite blue light emitting diodes(PeLEDs)attracted many researchers because of its fascinating optoelectrical properties.This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlayers.The emissive layer covers three types of perovskite structures:perovskite nanocrystals(PeNCs),2-dimensional(2D)and quasi-2D perovskites,and bulk(3D)perovskites.We will discuss about the remaining challenges of blue PeLEDs,such as limited performances,device instability issues,which should be solved for blue PeLEDs to realize next generation displays.

Polymer light-emitting devices using poly(ethylene oxide) as an electron injecting layer

The performance of polymer light emitting devices(PLEDs)based on polyvinyl carbazole(PVK)is improved by introducing a nanoscale interfacial thin layer,made of poly(ethylene oxide)(PEO),between the calcium cathode and the PVK emissive layer.It is believed that the PEO layer plays a key role in enhancing the device performance.In comparison to the device with Ca/Al as the cathode,the performance of the PLED with PEO/Ca/Al cathode,including the driving voltage,luminance efficiency is significantly improved.These improvements are attributed to the introduction of a thin layer of PEO that can lower the interfacial barrier and facilitate electron injection.

Effect of light-emitting diodes with different color rendering indexes on the ocular tissues of rat

AIM:To compare the damage of light-emitting diodes(LEDs)with different color rendering indexes(CRIs)to the ocular surface and retina of rats.METHODS:Totally 20 Sprague-Dawley(SD)rats were randomly divided into four groups:the first group was normal control group without any intervention,other three groups were exposed by LEDs with low(LED-L),medium(LED-M),and high(LED-H)CRI respectively for 12 h a day,continuously for 4 wk.The changes in tear secretion(Schirmer I test,SIt),tear film break-up time(BUT),and corneal fluorescein sodium staining(CFS)scores were compared at different times(1 d before experiment,2 and 4 wk after the experiment).The histopathological changes of rat lacrimal gland and retina were observed at 4 wk,and the expressions of tumor necrosis factor-α(TNF-α)and interleukin-6(IL-6)in lacrimal gland were detected by immunofluorescence method.RESULTS:With the increase of light exposed time,the CFS value of each light exposed group continued to increase,and the BUT and SIt scores continued to decrease,which were different from the control group,and the differences between the light exposed groups were statistically significant.Hematoxylin-eosin(HE)results showed that the lacrimal glands of each exposed group were seen varying degrees of acinar atrophy,vacuoledistribution,increasing of eosinophil granules,etc.;the retina showed obvious reduction of photoreceptor cell layer and changes in retinal thickness;LED-L group has the most significant change in all tests.Immunofluorescence suggested that the positive expressions of TNF-αand IL-6 in the lacrimal glands of each exposed group were higher than those of the control group.CONCLUSION:LED exposure for 4 wk can cause the pathological changes of lacrimal gland and retina of rats,and increase the expression of TNF-αand IL-6 in lacrimal gland,the degree of damage is negatively correlated with the CRI.

Improved blue quantum dot light-emitting diodes via chlorine passivated ZnO nanoparticle layer

In blue quantum dot light emitting diodes(QLEDs),electron injection is insufficient,which would degrade device efficiency and stability.Herein,we employ chlorine passivated ZnO nanoparticles as electron transport layer to facilitate electron injection into QDs effectively.Moreover,it suppresses exciton quenching at the QD/ZnO interface by blocking charge transfer channel.As a result,the maximum external quantum efficiency of blue QLED was increased from 2.55%to 4.60%,and the operation lifetime of blue QLED was nearly 4 times longer than that of the control device.Our work indicates that election injection plays an important role in blue QLED efficiency and stability.