Numerical investigations on the current conduction in bilayer organic light-emitting devices with ohmic injection of charge carriers

A numerical model for bilayer organic light-emitting diodes (OLEDs) has been developed on the basis of trapped charge limited conduction. The dependences of the current density on the operation voltage, the thickness and trap properties of the hole transport layer (HTL) and emission layer (EML) in bilayer OLEDs of the structure anode/HTL/EML/cathode have been numerically investigated. It has been found that, for given values of reduced trap depth, total trap density, and cv~rrier mobility of HTL and EML, there exists an optimum thickness ratio of HTL to the sum of HTL and EML, by which a maximal current density, and hence maximal quantum efficiency and luminance,can be achieved. The current density decreases quickly with the mean trap density, and decreases nearly exponentially with the mean reduced trap depth.     

Numerical analysis of the mechanism of carrier transport in organic light—emitting devces

The mechanism of carrier transport in organic light-emitting devices is numerically studied,on the basis of trappedcharge-limited conduction with an exponential trap distribution.The spatial distributions of the electrical potential,field and carrier density in the organic layer are calculated and analysed.Most carriers are distributed near the two electrodes,only a few of them are distributed over the remaining part of the orgaic layer,The carriers are accumulated near the electrodes,and the remaining region is almost exhausted of carriers.When the characteristic energy of trap distribution is greater than 0.3eV.it leads to a reduction of current density.In order to improve the device efficiency,organic materials with minor traps and low characteristic energy should be chosen.The diffusion current is the dominant component near the injection electrode.whereas the drift current dominates the remaining region of the organic layer.     

Numerical model of multilayer organic light-emitting devices

A numerical model of multilayer organic light-emitting devices is presented in this article. This model is based on the drift-diffusion equations which include charge injection, transport, space charge effects, trapping, heterojunction interface and recombination process. The device structure in the simulation is ITO/CuPc (20 nm)/NPD (40 nm)/Alq3 (60 nm)/LiF/Al. There are two heterojunctions which should be dealt with in the simulation. The I--V characteristics, carrier distribution and recombination rate of a device are calculated. The simulation results and measured data are in good agreement.     

Dependence of charge trapping of fluorescent and phosphorescent dopants in organic light-emitting diodes on the dye species and current density

This paper utilizes multilayer organic light-emitting diodes with a thin layer of dye molecules to study the mechanism of charge trapping under different electric regimes. It demonstrates that the carrier trapping was independent of the current density in devices using fluorescent material as the emitting molecule while this process was exactly opposite when phosphorescent material was used. The triplet--triplet annihilation and dissociation of excitons into free charge carriers was considered to contribute to the decrease in phosphorescent emission under high electric fields. Moreover, the fluorescent dye molecule with a lower energy gap and ionized potential than the host emitter was observed to facilitate the carrier trapping mechanism, and it would produce photon emission.     

一种用于堆叠结构有机发光二极管的新的电荷生成层

本文报道了一种用于堆叠结构有机电致发光器件的新的电荷生成层: LiF/Al/V₂O₅,采用这种电荷生成层的堆叠器件的两个发光单元互相独立,不受影响.说明在外加电场下,这种电荷生成层具有向邻近的发光单元注入电子空穴的能力.而堆叠了两个相同发光单元的器件的电流效率在相同的电流密度下约为普通单层结构的1.7倍.同时这种电荷生成层避免了溅射indium tin oxide(ITO)和金属、有机物共掺,只需要热蒸发,生长工艺简单. 关键词： 堆叠结构 有机电致发光器件 电荷生成层     

Efficient and color-saturated inverted bottom-emitting organic light-emitting devices with a semi-transparent metal-assisted electron injection layer

We report the development of highly efficient and color-saturated green fluorescent 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano-[6,7,8-ij]quinolizin-11-one dye-doped inverted bottom-emitting organic light-emitting diode (IBOLED). This was enabled by the insertion of a silver (Ag) based semi-transparent metal-assisted electron injection layer between the ITO cathode and n-doped electron transporting layer. This IBOLED with ITO/Ag bilayer cathode with its synergistic microcavity effect achieved luminous efficiencies of 20.7 cd/A and 12.4 lm/W and a saturated CIE_x,y of (0.22, 0.72) at 20 mA/cm², which are twice better than those of the conventional OLED and have over 60% improvement on IBOLED without ITO/Ag bilayer cathode.     

微腔结构顶发射有机发光器件

以Ag/ITO为全反射阳极，以Al/Ag为半透明复合阴极，制备了绿色、蓝色两种微腔结构顶发射有机发光器件，研究了微腔效应对顶发射器件颜色的影响，通过调节光程，实现了用同一种有机发光层制备出不同波长的发射.Alq基顶发射器件得到波长峰值从500 nm到584 nm的不同颜色的器件，发光光谱半高宽由传统器件的100 nm窄化到20—40 nm，最高电流效率1.77 cd/A.蓝光顶发射器件发光峰值从464 nm变化到532 nm，半高宽由传统器件的65 nm窄化到17—21 nm，并得到色坐标为(0.141，0.049)的深蓝色顶发射有机发光器件. 关键词： 有机发光 顶发射 微腔效应     

Numerical study of the current conduction in single-layer organic light-emitting devices

A numerical model for the current conduction in single-layer organic light-emitting devices is established under the basis of trapped charge-limited conduction with an exponential trap distribution. The dependences of the current density on the operation voltage, the thickness of the organic layer, and the trap properties are numerically studied. The current density decreases nearly exponentially with the thickness of the organic layer and the relative trap depth (l), and it is inversely proportional to the lth power of the total trap density. The results from simulations for the current–voltage characteristics agree very well with those from experiments. PACS 78.60.Fi; 75.40.Mg; 73.21.Ac     

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.     

有机发光器件的磁电导效应

制备了基于三(8-羟基喹啉)铝(tris-(8-hydroxyquinoline) aluminum (III), Alq₃) 的有机发光二极管, 并在不同偏压下测量了器件的室温磁电导效应.在小偏压下, 发光器件展示出明显的负磁电导效应.偏压增加后, 磁电导由负值变为正值, 出现了正负转变的现象. N, N'-二苯基-N, N'-(1-萘基)-1, 1'-联苯-4, 4'-二胺(N, N′-Di(naphthalen-1-yl)-N, N′ diphenyl-benzidine, NPB) 与铜酞菁 (Copper phthalocyanine, CuPc) 单极器件磁电导的测量结果表明, 发光器件在小偏压下的负磁电导效应来源于器件中的CuPc层. 双极电流的磁电导效应可用电子-空穴对模型进行解释, 而单极电流的磁电导效应可归因于器件中的极化子-双极化子转变. 在注入电流的变化过程中, 发光器件的正负磁电导转变是两种机理共同作用的结果.     

Improved performance of organic light-emitting diodes with dual electron transporting layers

In this study the performance of organic light-emitting diodes(OLEDs) are enhanced significantly,which is based on dual electron transporting layers(Bphen/CuPc).By adjusting the thicknesses of Bphen and CuPc,the maximal luminescence,the maximal current efficiency,and the maximal power efficiency of the device reach 17570 cd/m2 at 11 V,and 5.39 cd/A and 3.39 lm/W at 3.37 mA/cm2 respectively,which are enhanced approximately by 33.4%,39.3%,and 68.9%,respectively,compared with those of the device using Bphen only for an electron transporting layer.These results may provide some valuable references for improving the electron injection and the transportation of OLED.