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.     

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes （OLEDs） are developed. A concept of using multiple-emissive layer （EML） configuration is adopted. In this letter, bis（2-methyl-8-quinolinolate）-4- （phenylphenolato）A1 （BAlq） and 9,10-di（naphtha-2-yl）anthracene （ADN）, which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL＇eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN.     

Improved Performance of Organic Light-Emitting Diodes with MgF2 as the Anode Buffer Layer

Organic light-emitting diodes （OLEDs） based on N,N＇-bis（1-naphthyl）-N,N＇-diphenyl-1,l1＇-biphenyl-4,4＇-diamine （NPB） and tris （8-hydroxyquinoline） aluminium （Alq3 ） are improved by using a thin MgF2 buffer layer sandwiched between the indium tin oxide （ITO） anode and hole transporting layer （HTL） of NPB. The current-voltage curves of the OLEDs with MgF2 buffers shift to lower voltages, which can be explained by the tunnelling effect. Under 10 V bias, the current density and brightness for the optimized OLED with a 1.0-nm MgF2 are 196A/m^2 and 517cd/m^2, respectively, while for the OLED without anode buffer layer are only 109A/m^2 and 156cd/m^2, The atomic force microscopy shows that the rms roughness of NPB on ITO/MgF2 is only 1/3 of NPB on bare ITO. The improved morphology of the HTL would lead to more robust OLEDs. The OLED with a 1.0-nm MgF2 layer has a long lifetime of more than five times of the MgF2-free reference device due to the combined electrical and morphological effects of the MgF2 layer.     

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes

This paper investigates the dependence of current voltage characteristics of AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes （RTDs） on spacer layer thickness. It finds that the peak and the valley current density J in the negative differential resistance （NDR） region depends strongly on the thickness of the spacer layer. The measured peak to valley current ratio of RTDs studied here is shown to improve while the current density through RTDs decreases with increasing spacer layer thickness below a critical value.     

Influence of optical interference and carrier lifetime on the short circuit current density of organic bulk heterojunction solar cells

Based on simple analytical equations, short circuit current density (J_SC) of the organic bulk heterojunction solar cells has been calculated. It is found that the optical interference effect plays a very important role in the determination of J_SC; and obvious oscillatory behaviour of J_SC was observed as a function of thickness. At the same time, the influence of the carrier lifetime on J_SC also cannot be neglected. When the carrier lifetime is relatively short, J_SC only increases at the initial stage and then decreases rapidly with the increase of active layer thickness. However, for a relatively long carrier lifetime, the exciton dissociation probability must be considered, and J_SC behaves wave-like with the increase of active layer thickness. The validity of this model is confirmed by the experimental results.     

The influence of interface exchange coupling on the demagnetization process for perpendicularly oriented FePt/α-Fe/FePt trilayers

The influence of the interface exchange coupling on the magnetization reversal process for a FePt/α-Fe/FePt tri-layer structure has been studied through a micromagnetic approach.The analytical formula of the nucleation field has been derived.It is found that the nucleation field increases as the interface coupling constant rises.Especially when the thickness of the soft layer is small,the influence of the exchange coupling on the nucleation field is significant.The angular distributions of the magnetization for various exchange coupling constants have been obtained by numerical calculation.It is found that the angular distribution of the magnetization is discontinuous at the interface of the hard and soft layers.In the meantime,the pinning field decreases with the increase of the thickness of the soft layer and the exchange coupling constant.     

Investigation of gate oxide traps effect on NAND flash memory by TCAD simulation

The effects of gate oxide traps on gate leakage current and device performance of metal–oxide–nitride–oxide–silicon(MONOS)-structured NAND flash memory are investigated through Sentaurus TCAD. The trap-assisted tunneling(TAT)model is implemented to simulate the leakage current of MONOS-structured memory cell. In this study, trap position, trap density, and trap energy are systematically analyzed for ascertaining their influences on gate leakage current, program/erase speed, and data retention properties. The results show that the traps in blocking layer significantly enhance the gate leakage current and also facilitates the cell program/erase. Trap density ~10¹⁸ cm^-3 and trap energy ~ 1 eV in blocking layer can considerably improve cell program/erase speed without deteriorating data retention. The result conduces to understanding the role of gate oxide traps in cell degradation of MONOS-structured NAND flash memory.     

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.     

Research of Trap and Electron Density Distributions in the Interface of Polyimide/Al_2O_3 Nanocomposite Films Based on IDC and SAXS

The distributions of traps and electron density in the interfaces between polyimide(PI) matrix and Al_2O_3 nanoparticles are researched using the isothermal decay current and the small-angle x-ray scattering(SAXS) tests.According to the electron density distribution for quasi two-phase mixture doped by spherical nanoparticles, the electron densities in the interfaces of PI/Al_2O_3 nanocomposite films are evaluated. The trap level density and carrier mobility in the interface are studied. The experimental results show that the distribution and the change rate of the electron density in the three layers of interface are different, indicating different trap distributions in the interface layers. There is a maximum trap level density in the second layer, where the maximum trap level density for the nanocomposite film doped by 25 wt% is 1.054×10~(22) eV·m~(-3) at 1.324 eV, resulting in the carrier mobility reducing. In addition, both the thickness and the electron density of the nanocomposite film interface increase with the addition of the doped Al_2O_3 contents. Through the study on the trap level distribution in the interface, it is possible to further analyze the insulation mechanism and to improve the performance of nano-dielectric materials.     

Tandem organic light-emitting diode with a molybdenum tri-oxide thin film interconnector layer

A 10-nm-thick molybdenum tri-oxide(MoO3) thin film was used as the interconnector layer in tandem organic lightemitting devices(OLEDs).The tandem OLEDs with two identical emissive units consisting of N,N-bis(naphthalen-1-yl)N,N-bis(phenyl)-benzidine(NPB) /tris(8-hydroxyquinoline) aluminum(Alq3) exhibited current efficiency-current density characteristics superior to the conventional single-unit devices.At 20 mA/cm2,the current efficiency of the tandem OLEDs using the interconnector layers of MoO3 thin film was about 4.0 cd/A,which is about twice that of the corresponding conventional single-unit device(1.8cd/A).The tandem OLED showed a higher power efficiency than the conventional single-unit device for luminance over 1200cd/m2.The experimental results demonstrated that a MoO3 thin film with a proper thickness can be used as an effective interconnector layer in tandem OLEDs.Such an interconnector layer can be easily fabricated by simple thermal evaporation,greatly simplifying the device processing and fabrication processes required by previously reported interconnector layers.A possible explanation was proposed for the carrier generation of the MoO3 interconnector layer.     

利用p型掺杂技术制备的高效有机电致发光器件

利用氧化钼(MoO_x)作为p型掺杂剂,以掺杂层4,4'-bis(carbazol-9-yl)biphenyl(CBP):MoO_x作为空穴注入层,制备了一种结构为ITO/MoO_x/CBP:MoO_x/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)₃)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al的有机电致发光器件.器件中CBP同时作为空穴注入层、空穴传输层以及发光层母体材料,这种结构具有结构简单同时能有效降低空穴注入势垒等优点.研究发现,随着空穴注入层厚度的增加,器件的电流密度增加,表明p型掺杂层的引入能够有效增强空穴的注入;通过优化器件空穴注入层与空穴传输层厚度,器件性能有所提高,最大电流效率为29.8 cd/A,可以认为合理的优化空穴注入层和空穴传输层的厚度,使载流子在发光层中的分布更加平衡是提高器件发光效率的主要原因.值得指出的是,从电流效率最大值到亮度为 20 000 cd/m²时,优化后器件的效率衰减仅为17.7%,而常规器件的效率衰减则为62.1%,优化后器件效率衰减现象得到了明显的改善.分析认为优化后的器件中未掺杂的CBP有助于展宽激子形成区宽度,进而减弱了三线态-三线态湮灭、三线态-极化子淬灭现象,激子形成区的展宽是改善效率衰减的主要原因.     

效率增强的新型蓝色有机发光器件

使用一种新型空穴传输材料J003制备了不同结构、不同发光层厚度的两组蓝色发光器件,其结构为:ITO/CuPc/J003/JBEM:perylene/Al_q3/LiF/Al和ITO/CuPc/J003/JBEM:perylene/TPBi/Al_q3/LiF/Al,这里CuPc(Copper phthalocyanine)和LiF分别为空穴注入层(HIL)和电子注入层(EIL),J003为空穴传输层(HTL),JBEM(9,10-bis(3,5'-diaryl)phenylylanthracene)为发光层(EML),TPBj(1,3,5-tri(phenyl-2-benzimidazole)-benzene)为空穴阻挡层(HBL),Al_q3(tris(8-quinolinolato)aluminium complex)为电子传输层(ETL).两种结构中前者为无阻挡层的普通型结构,后者在发光层和电子传输层中加入了空穴阻挡层,是新型阻挡层结构.研究了空穴阻挡层的引入在不同厚度发光层时对器件发光性能的影响,结果表明,新型阻挡层结构能明显提高器件的亮度和效率,但依赖于发光层厚度,利用能级图分析了其中的原因.     

Field distribution and criterion for bulk-limited and injection-limited current conduction in single layer organic light-emitting devices

A numerical model for current conduction in single layer OLEDs including both injection and bulk effect is proposed. Based upon this model, a nearly linear distribution of the electric field was found, and the slope of the distribution, or the field at the injection electrode (F₀) is dependent on the energy barrier, mobility, trap density and trap depth. F₀ equals the half of the mean field of the device (F_m), which equals the quotient of the bias to the thickness of organic layer, is proposed as the limit for bulk-limited (BL) and injection-limited (IL) conduction. OLEDs with F₀ greater than F_m/2 are considered as IL-conducting, while those with F₀ less than F_m/2 are considered as BL-conducting. It was found that, the state of current conduction is not only determined by the energy barrier at the injection electrode, but also by the mobility, trap density and trap depth of the organic semiconductor. OLEDs with high injection barrier (>0.7 eV), trap density less than 10¹⁹ cm^-3, and reduced trap depth shallower than 5, will be IL-conducting, while those with low energy barrier (<0.2 eV), low carrier mobility (<10^-6 cm²V^-1s^-1), and trap density higher than 10¹⁷ cm^-3, will be BL-conducting. PACS 78.60.Fi; 75.40.Mg; 73.21.Ac     

基于混合空穴注入层的湿法制备高效蓝光热激活延迟有机发光二极管

在溶液法制备有机电致发光器件(OLEDs)的研究中, PEDOT∶PSS由于具有较好的成膜性与高透光性而常被用作器件的空穴注入层。但相关研究表明, PEDOT∶PSS本身稳定性较差以及功函数较低,这使得溶液法制备OLEDs的性能差且不稳定。蓝色作为全彩色的三基色之一,制备高效的蓝光器件对于实现高质量显示器件和固态照明装置必不可少。而目前溶液法制备蓝光OLEDs的器件效率普遍较差,针对此问题,本文利用传统的蓝光热激活延迟荧光发光(TADF)材料DMAC-DPS作为发光层,用溶液法制备了蓝光TADF OLEDs,通过在PEDOT∶PSS中掺杂PSS-Na制备混合空穴注入层(mix-HIL)来提高空穴注入层的功函数,研究其对于蓝光TADF OLEDs器件性能的影响。首先在PEDOT∶PSS水溶液中掺入不同体积的PSS-Na溶液,在相同条件下旋涂制膜,进行器件制备。通过观测各个实验组器件的电致发光(EL)光谱,发现掺入PSS-Na后器件EL谱存在光谱蓝移的现象,这是由于掺入PSS-Na水溶液后, mix-HIL层的厚度有所降低,使得在微腔效应作用下, EL光谱发生蓝移。通过对比各组器件的电流密度-电压-亮度(J-V-L)曲线及其计算所得器件的电流效率,结果显示随着PSS-Na的掺入,器件的亮度和电流都有所增大,器件的电流效率也得到了提升,当掺杂比例为0.5∶0.5(PEDOT∶PSS/PSS-Na)时提升幅度最大(亮度提升86.7%,电流效率提升34.3%)。通过在瞬态电致发光测试过程中施加或撤去驱动电压观测了器件EL强度的变化,分析了在混合空穴注入层/发光层(mix-HIL/EML)界面处的电荷积累情况。实验证明,通过在PEDOT∶PSS中掺杂PSS-Na制备mix-HIL获得了蓝光TADF OLEDs器件性能的提升,这是一个获得高效率溶液法制备OLEDs的可行方法。     

Effect of gold nanoparticles on the performances of the phosphorescent organic light-emitting devices

Gold nanoparticles (GNPs) on the performance of the phosphorescent organic light-emitting devices (OLEDs) were investigated. The green phosphorescent OLEDs with GNPs incorporated in hole transporting layer (HTL) or hole blocking layer (HBL) were fabricated using thermal evaporation technique. The results indicated that the performance of the OLEDs with GNPs were dependent on the position of the GNPs. The optimized device with GNPs in HBL shows enhanced current efficiency and reduced efficiency roll-off. However, the efficiency of the device with GNPs in HTL was decreased. The detailed physical mechanism is investigated in order to unveil such difference.     

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     

一种高电流密度下效率不降低的绿光有机电致发光器件

为了提高有机电致发光器件(OLED)在高电流密度下的发光效率, 在以C545T掺杂Alq₃为发光层的有机小分子绿光器件中的发光层与电子传输层之间插入超薄LiF绝缘层.结果表明, 器件的外量子效率随着电流密度的增加始终没有降低, 直至600 mA/cm²时达到最大值 4.79%, 是相同电流密度下的参考器件的外量子效率的7倍.     

微腔有机发光器件中的电致发光光谱

设计了结构为Glass/DBR/ITO/TPD/Alq3/Ag的微腔有机发光器件。从理论上详细地研究了腔内各层结构对器件电致发光谱性能的影响。结果表明:随着腔长厚度的增加,器件的归一化电致发光谱强度不断减小;在可见光区,器件的EL谱随发光层厚度的增加出现振荡变化。空穴传输层和发光层的界面位置对器件电致发光谱的影响也很大。最后得到,在设计微腔时发光层厚度要尽量窄,并且中心发光区域应位于谐振腔中电场的峰值位置。     

Enhanced Device Performances of Blue‐Emitting PLEDs Coupled with Silver‐Nanoicosahedrons

Silver‐nanoicosahedron particles (AgNIPs) are produced by chemical reduction and photochemical methods and doped into the hole transport layer (HTL) or emissive layer (EML) of blue‐emitting polymer light‐emitting diodes (PLEDs) to improve their luminous efficiency. The optimal distributed‐densities of the AgNIPs are determined from current density–voltage–luminance measurements at different doping concentrations. The AgNIP dopant doses that maximize the average luminous efficiency of the proposed PLED are 6.71 µg cm^?2 in EML (achieving 3.48 cd A^?1) and 6.88 µg cm^?2 in HTL (achieving 3.35 cd A^?1). Although the luminous efficiencies of the blue‐emitting PLEDs fabricated by both doping methods are not significantly different, the maximum plasmonic enhancement (around 30‐fold) of the blue‐emitting PLED with AgNIPs in EML is red‐shifted to the green region (≈530 nm in the electroluminescence spectrum), seriously degrading the luminescent monochromaticity of the blue‐emitting PLED. The maximum plasmonic enhancement (around 33‐fold) of blue‐emitting PLED with AgNIPs in HTL occurred at 430 nm, overlapping the localized surface‐plasmon resonance extinctions of the AgNIPs in HTL (425 nm), thus favoring the enhancement of fluorescence emission. Therefore, to enhance the large‐area emission of blue‐emitting PLEDs, the AgNIPs should be doped in the HTL rather than the EML.