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.     

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.     

Reduced hole loss in organic light emitting diode incorporating two p-doped hole transport layers

The hole-injecting structure of 15 nm MoO₃-doped 4,4-N,N-bis [N-1-naphthyl-N-phenyl-amino]biphenyl (NPB:MoO₃)/5 nm MoO₃-doped 4,4′-N,N′-dicarbazole-biphenyl (CBP:MoO₃) has been used in organic light emitting diodes (OLEDs). It was found that a device using the 15 nm NPB:MoO₃/5 nm CBP:MoO₃/NPB combination was superior to one adopting the 20 nm NPB:MoO₃/NPB combination due to two major causes: the NPB:MoO₃/CBP:MoO₃ interface is a quasi-ohmic contact, and the hole transport barrier from CBP:MoO₃ to NPB is smaller than that from NPB:MoO₃ to NPB; moreover, it outperformed the device employing the 20 nm CBP:MoO₃/NPB combination, mostly because of the higher conductivity of NPB:MoO₃ compared to CBP:MoO₃. We demonstrate that using a structure resulting from uniting two p-doped hole transporters is a beneficial, simple method of achieving an improved trade-off between high conductivity and small hole transport barrier, thereby leading to a significantly reduced ohmic loss in the hole current conduction in the OLEDs, relative to the single p-doped layers.     

Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

发光层和空穴传输层对白色电致发光器件性能的影响

利用电子传输性能良好的苯并噻唑螯合锌(Zn(BTZ)₂)作为蓝光层，通过设计不同类型的空穴传输层并试验不同厚度的发光层后，制作了一种最佳厚度的双发光层白色电致发光器件：氧化铟锡(ITO)/N-N′-双(3-甲基苯基)-N-N′-二苯基-1-1′-二苯基-4-4′-二胺(TPD)∶N,N′-二(1-萘基)-N,N′-二苯基-1,1′-联苯-4-4′-二胺(NPB)(1∶0.0 关键词： 厚度 空穴传输层 白光 载流子     

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.     

Effects of hole injection layer thickness on the luminescent properties of white organic light-emitting diodes

This work investigates how the thickness of the hole injection layer (HIL) influences the luminescent characteristics of white organic light-emitting diodes (WOLED). Experimental results indicate that inserting a thin HIL (<200 Å) into a WOLED without an HIL reduces the brightness and clearly changes the chromaticity because the surface of the 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine) (m-MTDATA) film is extremely rough. In contrast, a dense film structure and the fine surface morphology of m-MTDATA of moderate thickness (350-650 Å) provides a uniform conducting path on which holes cross the indium tin oxide (ITO)/HIL interface, improving luminescent performance, associated with the relatively stable purity of the color of the emission, with Commission Internationale 1′Eclairage (CIE) coordinates of (x = 0.40, y = 0.40). However, inserting a thick HIL (>650 Å) reduces the luminescent performance and causes red-shift, because the holes and electrons in the effective emissive confinement region become less optimally balanced. Moreover, optimizing the device structure enables a bright WOLED with CIE coordinates of (x = 0.34, y = 0.33) to reach a luminance of 7685 cd/m² at a current density of 100 mA/cm², with a maximum luminous efficiency of 1.72 lm/W at 5.5 V.     

Efficiency enhancement of fluorescent blue organic light-emitting diodes using doped hole transport and emissive layers

The electroluminescent characteristics of blue organic light-emitting diodes(BOLEDs) fabricated with doped charge carrier transport layers are analyzed. The fluorescent blue dopant BCzVBi is doped in an emissive layer,hole transport layer(HTL) and electron transport layer(ETL), respectively, to optimize the probability of exciton generation in the BOLEDs. The luminance and luminous efficiency of BOLEDs made with BCzVBi-doped HTL and ETL increase by 22% and 17% from 11,683 cd/m2 at 8.5 V and 6.08 cd/A at 4.0 V to 14, 264 cd/m2 at8.5 V and 7.13 cd/A at 4.0 V while CIE coordinates of(0.15, 0.15) of both types of BOLEDs remained unchanged. The electron mobility of BCzVBi is estimated to be 1.02 x 10_o cm2/Vs by TOF.     

On the stimulation of luminescence with green light emitting diodes

The present state of development of a green light emitting diode system, devised for convenient and economical optically stimulated luminescence dating, is described. The performance of the system is assessed in relation to use with feldspar and with quartz, both for material in which the latent signal was zeroed by heating and for material in which it was zeroed by bleaching. Aspects of assessment of the system include consideration of the measured signal-to-noise ratio and the consequently estimated minimum detectable dose, as well as the accuracy achieved in equivalent dose determination when simulating the dating process with laboratory dosed samples. A trial on quartz of equivalent dose determination by the single aliquot method is compared with use of the traditional multiple aliquot approach.     

修饰阴极界面提高聚合物白光二极管发光效率

利用聚合物的不同溶解性,研究用旋涂方法制备双层高分子白光二极管(WPLED),采用器件结构为：ITO/PEDOT(50nm)/PVK:PFO-BT: PFO-DBT(40nm)/PFO(40nm)/Ba(4nm) /Al(120nm),当相对比例为PVK: PFO-BT:PFO-DBT=1∶4%：3%时,得到标准白光,最大电流效率为2.4 cd/A,最大亮度为3215 cd/m²,色坐标为(0.33,0.34).用水溶性的聚电介质层修饰阴极界面,器件效率可以进一步提高到5.28 cd 关键词： 聚合物发光二极管 白光 双发光层结构     

高效率和色稳定性的双超薄发光层的黄光有机电致发光器件

主要对rubrene黄光发光材料制作0.1nm厚度的超薄发光层的有机电致发光器件作了研究,并配合BCP空穴阻挡层探讨了对器件效率和色坐标稳定性的影响。双超薄rubrene发光层配合BCP空穴阻挡层的有机电致发光器件的性能得到了很好的改善,外加电压6V时,器件电流效率为6.35cd.A-1;外加电压10V时,器件发光亮度达到了7068cd.m-2。另外,在较大的外加电压驱动范围内,器件的色坐标一直保持在(0.49,0.49)。增加的发光效率和良好的色坐标稳定性主要是取决于空穴与电子的注入与输运平衡以及激子在超薄rubrene发光层中稳定性的复合平衡。     

Effect of CuPc and MoO_3 co-evaporated layer on the conductivity of organic light emitting diodes

Devices with copper phthalocyanine(CuPc):molybdenum trioxide(MoO3) co-evaporated layer were fabricated and the current–voltage(I–V) and capacitance–voltage(C–V) characteristics were measured. It has been found that for a given voltage, the current of the device with a co-evaporated layer is higher than those without the co-evaporated layer and it reaches the highest value if the ratio of MoO3 to CuPc is 1:1. Meanwhile, the C–V characteristics showed that only free holes exist in the function layer consisting of pure CuPc. However, charge transfer(CT) complexes exist in the function layer of a CuPc:MoO3 mixture. The charge transfer complexes do not contribute to the transport of the device efficiently under low applied fields but are disassociated into free carriers rapidly at applied fields higher than 1.7×105V/cm, which greatly increases the conductivity.     

Alternating current organic light emitting diodes based on polymer heterojunction

Most alternating current (ac) polymer EL (electroluminescent) devices to date are based on symmetrical structure. Here novel alternating current EL devices with asymmetric structure are successfully fabricated by using a hole type polymer PDDOPV [poly (2,5-bis (dodecyloxy)-phenylenevinylene)] and an electron type polymer PPQ [poly (phenyl quinoxaline)]. We report that performance of polymer devices with heterojunction in ac operation is not so sensitive to thickness of the two polymer layers as in direct current (dc) operation. This new advantage of ac operation mode over dc means easy production and cheap facilities in large-scale production in the near future. Different emission spectra are obtained when our ac devices operate in ac mode, forward and reverse bias. Emission spectrum at reverse bias includes two parts: one is from PDDOPV, the other is from PPQ.     

表面InGaN厚度对GaN基发光二极管特性的影响

通过调整GaN基发光二极管(LED)表面InGaN层的厚度,发现在20 mA电流驱动下,LED器件的正向压降有明显差距.本文考虑了极化效应的影响,通过求解InGaN/GaN三角形势阱内二维空穴气浓度以及空穴隧穿概率的变化,求得了表面InGaN层厚度不同时器件正向压降的变化趋势,发现理论结果与实验结果有很好的吻合.同时得到了获得最低正向压降的表面InGaN厚度. 关键词： 极化 二维空穴气 隧穿概率     

表面InGaN厚度对GaN基发光二极管特性的影响

通过调整GaN基发光二极管(LED)表面InGaN层的厚度，发现在20 mA电流驱动下，LED器件的正向压降有明显差距.本文考虑了极化效应的影响，通过求解InGaN/GaN三角形势阱内二维空穴气浓度以及空穴隧穿概率的变化，求得了表面InGaN层厚度不同时器件正向压降的变化趋势，发现理论结果与实验结果有很好的吻合.同时得到了获得最低正向压降的表面InGaN厚度.     

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.     

Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures

Improvement in the light extraction efficiency (LEE) of GaN-based green light emitting diodes (LEDs) with ZnO nanostructures synthesized by a hydrothermal method is reported. Formation of ZnO nanorods, hemispheres, and cones was controlled by varying the pH of the aqueous synthesis solution. The shape of the ZnO nanostructures integrated onto the LEDs shows a strong relationship with the LEE characteristics of GaN-based green LEDs. The electroluminescence (EL) intensity of LEDs covered by ZnO nanostructures increased compared to conventional LEDs. In terms of LEE, LEDs with surface-textured ZnO hemispheres showed the highest EL intensity, which can be attributed to an increase in the effective critical angle, the escape cone, and multiple scattering. Finite difference time domain (FDTD) simulation was conducted to theoretically confirm the experimental results.     

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."     

Influence of organic capping layers on the performance of transparent organic light-emitting diodes

We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05.     

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.