Color-stable,reduced efficiency roll-off hybrid white organic light emitting diodes with ultra high brightness

High-brightness and color-stable two-wavelength hybrid white organic light emitting diodes (HWOLEDs) with the configuration of indium tin oxide (ITO)/ N, N, N, N-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD): tetrafluoro-tetracyanoqino dimethane (F4-TCNQ)/N,N-di(naphthalene-1-yl)-N,N-diphenyl-benzidine (NPB)/ 4,4-N,N-dicarbazolebiphenyl (CBP): iridium (III) diazine complexes (MPPZ) 2 Ir(acac)/NPB/2-methyl-9,10-di(2-naphthyl)anthracene (MADN): p-bis(p-N,N-di-phenyl-aminostyryl)benzene (DSA-ph)/bis(10-hydroxybenzo[h] quino-linato)beryllium complex (Bebq2)/LiF/Al have been fabricated and characterized. The optimal brightness of the device is 69932 cd/m2 at a voltage of 13 V, and the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates are almost constant during a large voltage change of 6-12 V. Furthermore, a current efficiency of 15.3 cd/A at an illumination-relevant brightness of 1000 cd/m2 is obtained, which rolls off slightly to 13.0 cd/A at an ultra high brightness of 50000 cd/m2. We attribute this great performance to wisely selecting an appropriate spacer together with effectively utilizing the combinations of exciton-harvested orange-phosphorescence/blue-fluorescence in the device. Undoubtedly, this is one of the most exciting results in two-wavelength HWOLEDs up to now.     

Efficient blue phosphorescent organic light emitting diodes with host engineering

We demonstrate as much as possible blue color and high efficiency phosphorescent organic light-emitting diodes (PHOLEDs) by using well-known iridium(III)bis[(3,5-difluoro-4-cyanophenyl)-pyridinato-N,C′]picolinate (FCNIrpic) dopant and previously reported good host materials. For the control of blue color and efficiency, various host materials, 1,3-bis(carbazole-9-yl)benzene (mCP), 9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole (mCPPO1), and 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan (DFCz), bis(4-(N-carbazole)phenyl)dimethylsilane (2MCBP) are selected and investigated their performances. A maximum external quantum efficiency (EQE) of 23.9% and power efficiency of 30.2 lm/W are achieved from 2MCBP device with Commision Internationale de L'Eclairage color coordinates (CIE_x,y) of (0.14, 0.21). The deepest color with color coordinate of (0.14, 0.19) is obtained for the mCP device.     

Light outcoupling efficiency enhancement in organic light emitting diodes using an organic scattering layer

Crystallized 4,7‐diphyenyl‐1,10‐phenanthroline (BPhen) films deposited by convenient vacuum thermal evaporation technique have been found to be an efficient means to extract the substrate wave guided light in organic light emitting diodes (OLEDs). The optimized BPhen film working as organic scattering layer was successfully used with OLEDs for light outcoupling efficiency improvement. Enhancement of 26%, 15% and 6% in efficiency of the blue, green and red OLEDs were obtained, respectively. The achievement was found to be advantageous in terms of simplicity of fabrication method and feasibility for large area OLED applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simple-structure white organic light emitting diodes with high color temperature

We present high color temperature white organic light emitting diodes with a simple p-i-n structure. A sky blue phosphorescent dopant of iridium(III) bis[4,6-(difluorophenyl)-pyridinato-N,C^2’] picolinate and a red phosphorescent dopant of bis(2-phenylquinoline)(acetylacetonate)iridium(III) in the emissive layers is employed to make high color temperature devices. Very stable color variation under ?0.02 until a 5000 cd/m² brightness value is realized by efficient carrier control in a multi stacked emitting layer of blue/red/blue colors. Maximum current and power efficiencies of 23.8 cd/A and 22.9 lm/W in forward direction are obtained. With balanced emissions from the two emitters, the white light emission with very high correlated color temperature of 7308 K as well as CIE coordinates of (0.30, 0.33) is achieved.     

Study of different roles phosphorescent material played in different positions of organic light emitting diodes

Phosphorescent materials are crucial to improve the luminescence and efficiency of organic light emitting diodes (OLED), because its internal quantum efficiency can reach 100%. So the studying of optical and electrical properties of phosphorescent materials is propitious for the further development of phosphorescent OLED. Phosphorescent materials were generally doped into different host materials as emitting components, not only played an important role in emitting light but also had a profound influence on carrier transport properties. We studied the optical and electrical properties of the blue 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi)-based devices, adding a common yellow phosphorescent material bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2′] iridium(acetylacetonate) [(t-bt)₂Ir(acac)] in different positions. The results showed (t-bt)₂Ir(acac) has remarkable hole-trapping ability. Especially the ultrathin structure device, compared to the device without (t-bt)₂Ir(acac), had increased the luminance by about 60%, and the efficiency by about 97%. Then introduced thin 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host layer between DPVBi and (t-bt)₂Ir(acac), and got devices with stable white color.     

3,4,9,10-Perylenetetracarboxylicdiimide as an interlayer for ultraviolet organic light emitting diodes

Ultraviolet organic light emitting diodes with 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI) interlayer have been achieved. The emission spectrum and intensity were strongly dependent on the thickness of PTCDI interlayer, in spite of the fact that PTCDI has neither much lower HOMO nor much higher LUMO level, which is considered necessary for efficient charge blocking layers. The influence of PTCDI layer was investigated in three different device configurations and obtained results are discussed. For optimal device configuration, OLED with emission centered at 370 nm and turn-on voltage of 4.5 V is obtained.     

利用银纳米立方增强效率的多层溶液加工白光有机发光二极管

金属纳米粒子的局域表面等离子体共振效应常被用于增强有机发光二极管中激子辐射强度,其增强效果与金属纳米粒子的共振波长、共振强度及其与激子之间的耦合密切相关.本文将具有较强局域表面等离子体共振效应的银纳米立方引入多层溶液加工白光有机发光二极管中提升器件性能.在传统的溶液加工有机发光二极管中,发光层主体一般具有较强的空穴传输性,因此激子主要在发光层/电子传输层界面附近复合.本文将银纳米立方掺入电子传输层中,使银纳米立方与激子之间产生充分的耦合作用,提高激子发光强度.对银纳米立方包裹二氧化硅外壳,一方面优化纳米立方与激子之间的距离,另一方面减小其对器件中电荷传输的影响.通过优化银纳米立方的浓度,多层溶液加工白光有机发光二极管的电流效率达到30.0 cd/A,是基础器件效率的2倍.另外,由于银纳米立方的等离子体共振光谱较宽,同时增强了白光中蓝光和黄光的强度,因此引入银纳米立方基本没有影响白光的色度.研究结果表明引入金属纳米粒子是提升多层溶液加工发光二极管性能的有效方法.     

Enhancement of light extraction efficiency of organic light emitting diodes using nanostructured indium tin oxide

Improved outcoupling efficiency of organic light emitting diodes (OLEDs) is demonstrated by incorporating a nanostructured indium tin oxide (NSITO) film between a conducting anode and a glass substrate. NSITO film was fabricated using rf-sputtering at oblique angle (85°). Significant reduction in refractive index and improved transmission of NSITO film was observed. OLEDs were then fabricated onto NSITO film to extract the ITO-glass waveguided modes. Extraction efficiency was enhanced by 80% without introducing any detrimental effects to operating voltage, current density, and angular invariance of emission spectra of OLEDs.     

Design and development of organic light emitting diodes

A highly luminescent thiophene based conjugated polymer, i.e., poly[2-(3-thienyl) ethanol butoxy carbonyl–methyl urethane] (PURET) has been used for fabricating polymeric light emitting diodes in the present investigations. PURET has been doped with varying amount of (4-dicyano methylene-2 methyl-6-(p-dimethyl amino styryl)-4H-pyran) dye. Enhanced electroluminescence (EL) and quantum efficiency has been observed by incorporating small amount of dye. An attempt has been made to understand the mechanism of charge transport, which helped in the understanding of the possible reasons for enhancement of EL emission as a function of dye concentration and allowed for further optimization of device performance. Based on capacitance–voltage (C–V) analysis it is proposed that the devices in the present investigations, may be modeled as a resistance and capacitor in parallel for the frequency range of 20 Hz–1 MHz. The enhancement in EL intensity and external quantum efficiency of PURET has been observed in addition of small amount of dye which is attributed to the trapping of excitons and enhanced probability electron–hole recombination in EL layer. In addition, voltage tunable color emission has also been observed. This is attributed to the charge transport among the various layers depending upon the applied voltage.     

Light extraction efficiency enhancement in organic light emitting diodes based on optimized multilayer structures

The main focus of this study is to improve the light extraction efficiency, as well as directionality of organic light emitting diodes (OLEDs) using multi-layer structures between Indium tin Oxide (ITO) and glass layers in a typical OLED. In conventional OLEDs, only about half of the light generated in the emission zone can reach to the glass substrate due to refractive index mismatch in ITO (n = 1.8?i0.01)/glass (n = 1.51) interface. The main attempt is to reduce the share of total internal reflection (TIR) and hence, the effect of different structures such as Thue-Morse and Fibonacci have been investigated and optimized with suitable layer thickness and materials based on Transfer Matrix Method (TMM). The most effective Multi-layer structures have been added to conventional OLED and have been analyzed the extraction efficiency using Finite Difference Time Domain (FDTD) method. Results show large enhancement of extraction efficiency (about 40%) in ITO/glass interface. Using this idea and applying micro-lenses array to glass substrate at the same time, one can get even higher extraction efficiency in OLED. The interesting aspect of this project is its easy fabrication process in order to commercialize the product with highest extraction efficiency and low fabrication cost.     

Transparent manganite films as hole injectors for organic light emitting diodes

Organic light-emitting diodes (OLEDs) are nowadays one of the most attractive devices based on organic semiconductors due to their successful application in the display technology. Electroluminescence in OLEDs is mainly governed by the fluorescence from excited singlet states, which have large transition probabilities providing the major radiative pathway. The “forbidden” triplet state emission can be activated by increasing spin–orbit coupling via dye doping. The singlet–triplet exciton formation statistics is usually given by 1:3 partition due to the quantum constrains.

Injection of carriers with finite spin polarisation should influence and modify the recombination statistics and can be used for tuning of the device efficiency. In this context, the development of a new class of electrodes able to guarantee both efficient charge and spin injection becomes of paramount importance. We show that strongly spin polarised colossal magnetoresistance manganite La_0.7Sr_0.3MnO₃ (LSMO) can successfully substitute conventional ITO electrodes in OLEDs. Highly transparent, metallic and ferromagnetic LSMO layers were used in combination with standard Al and spin polarised Co top electrodes. Electrical and optical characterisations of the OLEDs with spin polarised electrodes indicate the applicability of the new manganite electrodes for organic light-emitting devices.     

Numerical and experimental studies of mixed-host organic light emitting diodes

Electrical characteristic and luminance of three mixed-host organic light emitting diodes (OLEDs): namely the uniformly mixed, step-wise graded and mixed, and continuously graded and mixed, were compared with the conventional hetero-junction OLED in both numerical and experimental studies. These mixed-host OLEDs were fabricated by a mixed-source thermal evaporation process, and half-cell devices were also fabricated to provide some input parameters for OLED simulations. The current efficiencies were largely influenced by their device structures and strongly agreed with the computed current balance factors. The improved mixed-host OLED performances can be discussed with aid from simulations, which include spatial distributions of electron and hole, carrier mobility, electric field profiles, the total recombination rates in the light emitting layer.     

Exciton dissociation in organic light emitting diodes at the donor-acceptor interface

Experimental in situ photoluminescence and transient photovoltage results show that the interface formed by N, N{'}-Bis(naphthalene-1-yl)-N, N{'}-bis(phenyl) benzidine (NPB) and tris(8-hydroxyquinoline) aluminum (Alq{3}) acts as an exciton dissociation site. Because of this dissociation effect, excitons formed in NPB at or within a diffusion length of the interface tend to dissociate before they radiatively decay to generate blue light. This suggests that the action of the "hole-blocking layer" used in indium tin oxide\NPB\hole-blocking layer\Alq{3}\aluminium to promote blue light emission from the NPB is more "exciton dissociation inhibition" than "hole blocking."     

Improved light extraction of organic light emitting diodes with a nanopillar pattering structure

We have investigated the properties of organic light emitting diodes(OLEDs)with a nanopillar patterning structure at organic–metal or organic–organic interfaces.The results demonstrate that the introduction of a nanopillar structure can improve the light extraction efficiency greatly.We also find that the number,height,and position of nanopillars all affect the light extraction of OLEDs.The maximum power efficiency of a device with an optimized nanopillar patterning mode can be improved to 2.47 times that of the reference device.This enhancement in light extraction originates from the improved injected carriers,the broadened charge recombination zone,and the intensified wave guiding effects.     

Degradation behaviors of high power GaN-based blue light emitting diodes

The degradation mechanism of high power InGaN/GaN blue light emitting diodes(LEDs)is investigated in this paper.The LED samples were stressed at room temperature under 350-mA injection current for about 400 h.The light output power of the LEDs decreased by 35%during the first 100 h and then remained almost unchanged,and the reverse current at 5 V increased from 10 9A to 10 7A during the aging process.The power law,whose meaning was re-illustrated by the improved rate equation,was used to analyze the light output power-injection current(L–I)curves.The analysis results indicate that nonradiative recombination,Auger recombination,and the third-order term of carriers overflow increase during the aging process,all of which may be important reasons for the degradation of LEDs.Besides,simulating L–I curves with the improved rate equation reveal that higher-than-third-order terms of carriers overflow may not be the main degradation mechanism,because they change slightly when the LED is stressed.     

A potential red-emitting phosphor with high color-purity for near-UV light emitting diodes

New red tungstates phosphors, Na₅La_1?xLn_x(WO₄)₄ (Ln = Eu, Sm) and Na₅Eu_1?xSm_x(WO₄)₄, were prepared by solid-state reaction technique. And their structure and photo-luminescent properties were investigated. The introduction of Sm³⁺ broadened the excitation band around 400 nm of the phosphors, and strengthened the red emission. And the possible energy transfer process from Sm³⁺ to Eu³⁺ is discussed. The single red LED was fabricated by combining InGaN chip with Na₅Eu_0.94Sm_0.06(WO₄)₄ as red phosphor, intense red light can be observed by naked eyes. Then the phosphor Na₅Eu_0.94Sm_0.06(WO₄)₄ may be a good candidate for red component of near-UV InGaN-based W-LEDs, because of efficient red-emitting with broadened absorption around 400 nm and appropriate CIE chromaticity coordinates (x = 0.65, y = 0.34).     

Effect of ZnO film thickness on the performance of organic light emitting diodes

The electrical and optical properties of the ZnO layers and of the ITO/ZnO bilayers are investigated. We show that a ZnO layer of about 120 nm is the best compromise to obtain simultaneously a high transmittance and conductivity. Moreover an X-ray diffraction analysis underscores that an amorphous ITO deposited on a polycrystalline ZnO could change into a polycrystalline ITO. The modifications of the ITO layer by a preliminary deposition of a 120 nm thick ZnO underlayer enables us to decrease the threshold voltage of organic light emitting diodes.     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effects of parylene C layer on high power light emitting diodes

20 μm parylene C layer was deposited on silicone film to enhance the oxygen and water barrier properties, deposited at typical rate of 2.0 and 5.6 μm/h by controlling different deposited pressures. Surface morphology and roughness were observed by atomic force microscopy (AFM), surface morphology images show lower deposited rate that can lead to better quality film. 20 μm parylene C layer was deposited on silicone encapsulation of high power light emitting diodes (LEDs), the samples were tested by power temperature cycling (PTC) test from −40 to 85 °C, 15 min each extreme, and no corrosion, discoloration, cracks was found on the LEDs after the 1000 h PTC reliability test, and PTC test is intended to simulate worst case conditions encountered in typical applications, and parylene C floating membrane structure can stand such high stress and strain. Optical test on white and red LED samples with and without 20 μm parylene C layer, measurement result shows the optical transmittance more than 95%. 1000 h temperature humidity bias life test (T&HB) is performed for the purpose of evaluating the reliability of LEDs in humid environments, energy-dispersive X-ray (EDX) analysis revealed much lower content of C (carbon) and O (oxygen) on the lead frame of LED with parylene C coating after T&HB test on, and no oxidation was found in the LED package.     

Local indium segregation and bang gap variations in high efficiency green light emitting InGaN/GaN diodes

High efficient green light emitting diodes (LED) on the basis of GaN/InGaN exhibit indium-rich nanoclusters inside the quantum wells (QW) due to InN-GaN phase decomposition. By direct measurements of the variations in the electronic structure, we show for the first time a correlation between indium-rich nanoclusters and local energy band gap minima. Our investigations reveal the presence of 1-3 nm wide indium rich clusters in these devices with indium concentrations x as large as x∼0.30-0.40 that narrow the band gap locally to energies as small as 2.65 eV. These clusters are able to act as local traps for migrating photon-emitting carriers and seem to boost the overall device performance.