The enhanced electron injection by fluorinated tris-(8-hydroxy-quinolinato) aluminum derivatives in high efficient Si-anode OLEDs

Fabrication of organic light-emitting diodes (OLEDs) and lasers on silicon substrates is a feasible route to integrate microelectronic chips with optical devices for telecommunications. However, the efficiency of Si-anode based OLEDs is restricted by the imbalance of hole-electron injection because a p-type Si anode owns better hole injection ability than ITO. We have used fluorinated tris-(8-hydroxy-quinolinato) aluminum (FAlq₃) derivatives to prepare Si-anode based OLEDs. We observed that, when tris-(5-fuloro-8-hydroxyquinolinato) aluminum (5FAlq₃) is used as the electron-transporting material instead of Alq₃, the cathode electron injection is enhanced due to its lower lowest unoccupied molecular orbital (LUMO) compared to the Alq₃. The device can keep the relative carrier balance even when a Si anode capable of stronger hole injection was used. Further optimization of the device structure by introducing 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole blocking layer showed significant increase in the device power efficiency from 0.029 to 0.462 lm/W. This indicates that use of fluorinated Alq₃ derivatives is an effective way to improve the performance of Si-anode based OLEDs.     

具有Au/MoO_3空穴注入层的有机发光二极管

研究了单层MoO3(5nm)和复合Au(4nm)/MoO3(5nm)HILs对OLEDs器件性能的影响,器件结构为ITO/HIL/NPB(40nm)/Alq3(60nm)/LiF(1nm)/Al(100nm)。与单层MoO3HIL的器件相比,具有复合Au/MoO3HIL的器件具有较大的电流和亮度。这是由于Au的功函数介于ITO和MoO3之间,导致Au的引入提高了空穴的注入效率。     

Enhanced brightness of organic light-emitting diodes based on Mg:Ag cathode using alkali metal chlorides as an electron injection layer

Different thicknesses of cesium chloride (CsCl) and various alkali metal chlorides were inserted into organic light-emitting diodes (OLEDs) as electron injection layers (EILs). The basic structure of OLED is indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1.1′-biphenyl-4.4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Mg:Ag/Ag. The electroluminescent (EL) performance curves show that both the brightness and efficiency of the OLEDs can be obviously enhanced by using a thin alkali metal chloride layer as an EIL. The electron injection barrier height between the Alq₃ layer and Mg:Ag cathode is reduced by inserting a thin alkali metal chloride as an EIL, which results in enhanced electron injection and electron current. Therefore, a better balance of hole and electron currents at the emissive interface is achieved and consequently the brightness and efficiency of OLEDs are improved.     

Tetraalkyl-substituted zinc phthalocyanines used as anode buffer layers for organic light-emitting diodes

Two soluble tetraalkyl-substituted zinc phthalocyanines(ZnPcs) for use as anode buffer layer materials in tris(8-hydroxyquinoline)aluminum(Al_(q3))-based organic light-emitting diodes(OLEDs) are presented in this work. The holeblocking properties of these Zn Pc layers slowed the hole injection process into the Al_(q3) emissive layer greatly and thus reduced the production of unstable cationic Al_(q3)(Alq_3~+) species. This led to the enhanced brightness and efficiency when compared with the corresponding properties of OLEDs based on the popular poly-(3, 4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT: PSS) buffer layer. Furthermore, because of the high thermal and chemical stabilities of these Zn Pcs, a nonaqueous film fabrication process was realized together with improved charge balance in the OLEDs and enhanced OLED lifetimes.     

利用氧等离子体处理阳极银实现高效率顶发射有机电致发光

主要报道在器件结构为玻璃衬底/Ag(阳极)/NPB(空穴传输层)/Alq3(电子传输及发光层)/Sm(半透明阴极)/Alq3的顶发射有机电致发光器件中,利用氧等离子体对阳极银的表面进行处理来降低阳极和空穴传输层(Ag/NPB)界面处的空穴注入势垒,提高顶发射有机电致发光器件的性能。主要研究了氧等离子体处理时间对阳极银和顶发射有机电致发光器件光电特性的影响。紫外光电子能谱表明,氧等离子体处理能有效降低Ag/NPB界面处的空穴注入势垒。通过优化处理时间获得最佳器件性能,优化后的器件最大效率可达6.14cd/A。     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L2₁结构Co₂FeSi高有序合金之间的Co₅₀Fe_50-xSi_x合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似(GGA+U)方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co₂FeSi合金的分子磁矩和居里温度分别达到5.92μ_B和777 ℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co₂FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内. 关键词： 磁性 Heusler合金 结构相变     

On the improvement of the electroluminescent signal of organic light emitting diodes by the presence of an ultra‐thin metal layer at the interface organic/ITO

The electroluminescent (EL) signal of organic light emitting diodes (OLEDs) based on simple “hole transporting layer/electron transporting layer” (HTL/ETL) structures has been studied as a function of the anode/HTL interface, the anode being an indium tin oxide (ITO) film. It is shown that the electroluminescent (EL) signal increases when a metal ultra‐thin layer is introduced between the anode and the HTL. Experimental results show that the work function value of the metal is only one of the factors which allow improving the EL signal via better hole injection efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Recombination zone and efficiency in bipolar single layer light-emitting devices: a numerical study

The efficiency of organic light-emitting devices (OLEDs) is closely related to the position and width of recombination zone (RCZ) in the emission layer. Based on the drift–diffusion theory of carrier motion in semiconductors, we developed a numerical model for the position and width of the RCZ in bipolar single layer OLEDs. The calculation results show that for a given operation voltage, the position and width of the RCZ are determined by the mobility difference of electrons and holes, and the energy barrier at the two contacts. When the anode and cathode contact are both ohmic, then RCZ will be near the electrode, from which the low-mobility carriers are injected, and the smaller the mobility difference, the wider the RCZ, and the width of RCZ will be maximal when the mobility of holes and electrons are equal. When the anode contact is Schottky, while the cathode contact is ohmic, then the position and width of RCZ will be determined by both the mobility difference and hole–injection energy barrier. When μ _p<μ _n, the RCZ will be at the anode side. When μ _p>μ _n, then RCZ will move away from the anode and become wider, with the increase of the hole injection barrier. For a given hole–injection barrier and mobility of holes and electrons, the position and width of RCZ change with the applied voltage.     

Enhanced performance of the OLED with plasma treated ITO and plasma polymerized thiophene buffer layer

In this work, indium-tin-oxide (ITO) electrode in organic light emitting device (OLED) was modified by using an O₂ plasma treatment and plasma polymerized thiophene buffer layers were inserted between ITO (anode) and organic layer in order to improve the hole injection efficiency. Furthermore, electron injection to cathode (Al) in the test OLED seemed to be improved due to introduction of quantum well in the cathode. The plasma-polymerized thiophene buffer layer on the O₂ plasma-treated transparent ITO electrode seemed to result in formation of a stable interface and consequently, reduction the hole mobility, which in turn caused enhanced recombination of hole and electron in the emitting layer. Compared with the test device without buffer layer, the turn-on voltage of the test device with the buffer layer was lowered by 1.0 V.     

Progress in Modification of Indium-Tin Oxide/Organic Interfaces for Organic Light-Emitting Diodes

The oxide/organic interfaces play crucial roles in the hole injection from the anode electrodes to the emitting organics in organic light-emitting diodes (OLEDs), and hence have strong impacts on the efficiencies and other properties of the devices. Indium-tin oxide (ITO) is currently the most popular anode material used in OLEDs due to several merits, such as good etch ability, good adherence, high transparency, low resistivity, and high work function. Interfacial engineering between the ITO electrode and the overlying organic layers is an important process to obtain the high performance of the diode devices. In this article, recent progress in modification of the ITO/organic interfaces is reviewed, as these interfaces are important to the development of the technologies aiming at improving the electroluminescence, and efficiencies as well as reducing the operation voltages of OLEDs. ITO/Organic interfacial properties can be controlled or modified by simply changing the surface properties of ITO using chemical or physical treatments, and by adding a buffer layer (e.g., metal, oxide, or organic thin films) between the ITO and hole transport or emitting organic layers. The literature data showed that the electroluminescence, efficiencies, and lifetimes of the OLEDs could be greatly increased and the operation voltage considerably decreased when the ITO/organic interfaces have been properly improved.     

Optimization of microcavity OLED by varying the thickness of multi-layered mirror

We optimized the emission efficiency from a microcavity OLEDs consisting of widely used organic materials, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq₃) as emitting and electron transporting layer. LiF/Al was used as a cathode, while metallic Ag was used as an anode material. A LiF/NPB bi-layer or NPB layer on top of the cathode was considered to alter the optical properties of the top mirror. The electroluminescence emission spectra, electric field distribution inside the device, carrier density, recombination rate and exciton density were calculated as a function of the position of the emission layer. The results show that for optimal capping layers thicknesses, light output is enhanced as a result of the increase in both the reflectance and transmittance of the top mirror. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The experimental results have been compared to the simulations and the influence of the thickness of the mirror layers, emission region width and position on the performance of microcavity OLEDs was discussed.     

基于器件结构提高TADF-OLED器件的发光性能

为了提高以TADF材料作为主体、天蓝色荧光材料作为客体的混合薄膜的OLED器件光电性能,我们调整了器件结构,使主体材料发挥其优势。制备了基本结构为ITO/NPB(40 nm)/DMAC-DPS∶x%BUBD-1(40 nm)/Bphen(30 nm)/LiF(0.5 nm)/Al的OLED器件。研究了主-客体材料在不同掺杂浓度下的OLED器件的光电特性。为了提高主体材料的利用率,在空穴传输层和发光层之间加入10 nm的DMAC-DPS作为间隔层;然后,在阳极和空穴传输层之间加入HAT-CN作为空穴注入层,形成HAT-CN/NPB结构的PN结,有效降低了器件的启亮电压(2.7 V)。测量了有无HAT-CN的单空穴器件的阻抗谱。结果表明,在最佳掺杂比例(2%)下,器件的外量子效率(EQE)达到4.92%,接近荧光OLED的EQE理论极限值;加入10 nm的DMAC-DPS作为间隔层,使得器件的EQE达到5.37%;HAT-CN/NPB结构的PN结有效地降低了器件的启亮电压(2.7 V),将OLED器件的EQE提高到5.76%;HAT-CN的加入提高了器件的空穴迁移率,降低了单空穴器件的阻抗。TADF材料作为主体材料在提高OLED器件的光电性能方面具有很大的潜力。     

以ILTO为阳极的高效有机电致发光器件

以高功函数的掺杂钛酸镧的氧化铟薄膜(ILTO)及氧化铟锡(ITO)作为阳极,制备了Glass/anode/NPB/Alq3/LiF/Al结构的有机电致发光器件。得益于ILTO较好的掺杂性、低的表面粗糙度、高的可见光透过率以及高的有效功函数,以ILTO为阳极的有机电致发光器件的开路电压得到降低,最高亮度、电流效率、功率效率以及外量子效率均获得了成倍的提高。研究结果表明,ILTO是一种潜在的光学窗口材料,有望在各种光电器件中得到广泛的应用。     

富勒烯掺杂NPB空穴传输层的有机电致发光器件

报道了不同掺杂浓度NPB：C₆₀（富勒烯）作为空穴传输层对有机电致发光器件性能的影响.采用真空热蒸镀方法,制作了ITO/ NPB：C₆₀ （x % ）/Alq₃/LiF/Mg:Ag结构的四种有机电致发光器件.当NPB：C₆₀的掺杂浓度是15％时,器件的启亮电压是4 V,最大亮度是11000 cd/m².然而,当NPB：C₆₀的掺杂浓度是20％时,器件的最大亮度降     

Improved performance of organic light-emitting devices with plasma treated ITO surface and plasma polymerized methyl methacrylate buffer layer

Transparent indium-tin-oxide (ITO) anode surface was modified using O₃ plasma and organic ultra-thin buffer layers were deposited on the ITO surface using 13.56 MHz rf plasma polymerization technique. A plasma polymerized methyl methacrylate (ppMMA) ultra-thin buffer layer was deposited between the ITO anode and hole transporting layer (HTL). The plasma polymerization of the buffer layer was carried out at a homemade capacitively coupled plasma (CCP) equipment. N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) as HTL, Tris(8-hydroxy-quinolinato)aluminum (Alq₃) as both emitting layer (EML)/electron transporting layer (ETL), and aluminum layer as cathode were deposited using thermal evaporation technique. Electroluminescence (EL) efficiency, operating voltage and stability of the organic light-emitting devices (OLEDs) were investigated in order to study the effect of the plasma surface treatment of the ITO anode and role of plasma polymerized methyl methacrylate as an organic ultra-thin buffer layer.     

Surface modification of indium tin oxide anode with self-assembled monolayer modified Ag film for improved OLED device characteristics

Modification of electrodes has attracted much attention in the study of organic semiconductor devices. A self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of indium tin oxide (ITO) to improve the hole injection and the surface morphology. The modified anode was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and UV–vis transmittance spectra. To investigate the effect of the modification on the device characteristics, typical double layer devices with the structure of anode/-naphthylphenylbiphenyl diamine (NPB, 60 nm)/tris-(8-hydroxyquinoline) aluminum (Alq₃, 60 nm)/LiF(0.7 nm)/Al(100 nm) were fabricated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The results revealed that SAM modified ultra-thin Ag film is an effective buffer layer for organic light emitting diode. The device using the ITO/Ag (5 nm)/SAM as anode show improved device characteristics than that of using bare ITO as anode. The enhancements in luminance and efficiency are attributed to enhanced hole injection and smooth surface between anode and the organic material. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance.     

Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.     

Ordered conducting polymer multilayer films and its application for hole injection layers in organic light-emitting devices

We reported a controlled architecture growth of layer-ordered multilayer film of poly(3,4-ethylene dioxythiophene) (PEDOT) via a modified Langmuir-Blodgett (LB) method. An in situ polymerization of 3,4-ethylene dioxythiophene (EDOT) monomer in multilayer LB film occurred for the formation of ordered conducting polymer embedded multilayer film. The well-distribution of conducting polymer particles was characterized by secondary-ion mass spectrometry (SIMS). The conducting film consisting of ordered PEDOT ultrathin layers was investigated as a hole injection layer for organic light-emitting diodes (OLEDs). The results showed that, compared to conventional spin-coating PEDOT film and electrostatic self-assembly (ESA) film, the improved performance of OLEDs was obtained after using ordered PEDOT LB film as hole injection layer. It also indicated that well-ordered structure of hole injection layer was attributed to the improvement of OLED performance, leading to the increase of charged carrier mobility in hole injection layer and the recombination rate of electrons and holes in the electroluminescent layer.     

水溶性CuPc旋涂转速及退火方式对蓝色有机电致发光器件性能的影响

本文以Be(PP)₂为发光层、水溶性酞菁铜(WS-CuPc)为空穴注入层、NPB为空穴传输层,制备了结构为ITO/WS-CuPc/NPB/Be(PP)₂/LiF/Al的蓝色有机发光二极管(OLEDs).研究了WS-CuPc不同旋涂转速对器件性能的影响.并在WS-CuPc最佳旋涂转速的基础上,进一步研究了WS-CuPc薄膜不同退火方式对器件性能的影响.实验中,对WS-CuPc层采用了一种新的退火方式,即对ITO玻璃衬底先加热后旋涂WS-CuPc层,并与传统退火方式 关键词： 水溶性CuPc 蓝色有机电致发光 旋涂转速 退火方式     

空穴传输层掺杂SrF2的高效率蓝色磷光OLED器件

通过在OLED器件的空穴传输层中掺杂不同比例的SrF2制作出了高效率蓝色磷光OLED器件.这种器件能有效提高蓝色磷光OLED器件的空穴注入与传输特性,降低器件的的工作电压,提高流明效率(19.11m/W)、电流效率(26.9 cd/A)以及亮度(22 220 cd/m2),和未经掺杂的参比器件相比,分别提高了85.4％...