Organic Light Emitting Diodes Using Doped Alq3 as the Hole-transport Layer

Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane （F4TCNQ） doping on the hole conductivity of Alq3 layer are measured. In the hole-only device of Alq3, the current densities increase in 1-3 orders of magnitude upon doping with F4TCNQ, suggesting that the doping can effectively enhance the hole-injection and hole- transport ability of Alq3. An organic light-emitting device using an F4TCNQ doped Alq3 layer as the hole- injection and hole-transport layer, and pristine Alq3 as the electron-transport and emitting layer is fabricated and characterized. Bright emission is achieved in the simple OLED with p-doped Alq3 as the hole-transport layer and the intrinsic Alq3 as the electron-transport and emitting layer. The emitting efficiency and brightness of the device are further improved by inserting a thin electron block layer to confine the carrier recombination zone in the middle of the organic layers.     

Red and Near-Infrared Electroluminescences from Metal-Free Phthalocyanine

Organic light emitting diodes are fabricated based on metal-free phthalocyanine (H₂Pc) doped into tris-(8-hydroxyquinoline) aluminium (Alq₃). The device structure is ITO/NPB (30nm)/Alq₃: H₂Pc(30nm)/BCP(20nm)/Alq₃(20 nm)/Al. In the light-emitting layers, H₂Pc concentrations are varied from 0wt% to 100wt%. The emissions around 708nm and 800nm appear at low concentrations, while the emissions around 910nm and 930nm appear at high concentrations. The emissions around 708nm and 800nm are from H₂Pc monomers. The emissions around 910nm and 930nm are from H₂Pc aggregates. The dominant mechanism in the doped devices is direct chargetrapping.     

Optimized Performances of Thick Film Organic Lighting-Emitting Diodes

The performance of organic light-emitting diodes (OLEDs) with thick film is optimized. The alternative vanadium oxide (V₂O₅) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) layers are used to enhance holes in the emissive region, and 4,7-dipheny-1,10-phenanthroline (Bphen) doped 8-tris-hydroxyquinoline aluminium (Alq₃) is used to enhance electrons in the emissive region, thus ITO/V₂O₅ (8nm)/NPB (52nm)/V₂O₅ (8nm)/NPB (52nm)/Alq₃ (30 and 45nm)/Alq₃:Bphen (30wt%, 30 and 45nm)/LiF (1nm)/Al (120nm) devices are fabricated. The thick-film devices show the turn-on voltage of about 3V and the maximal power efficiency of 4.5lm/W, which is 1.46 times higher than the conventional thin-film OLEDs.     

Modified Spontaneous Emission from Dye Molecules inside a Photonic Crystal Microcavity

We fabricate a photonic crystal microcavity containing Alq3 in a sandwiched structure by the self-assemble method. The angle-dependent photoluminescence （PL） spectra and the variation of the PL lifetime demonstrate the effect of the photonic band gap on the spontaneous emission of Alq3 in the photonic crystals.     

Organic Light-Emitting Devices with a LiF Hole Blocking Layer

We introduce a thin LiF layer into tris-8-hydroxyquinoline aluminium （Alq3 ） based bilayer organic light-emitting devices to block hole transport. By varying the thickness and position of this LiF layer in Alq3, we obtain an electroluminescent efficiency increase by a factor of two with respect to the control devices without a LiF blocking layer. By using a 10nm dye doped Alq3 sensor layer, we prove that LiF can block holes and excitons effectively. Experimental results suggest that the thin LiF layer may be a good hole and exciton blocking layer.     

Influence of Substrate on the Transportation Properties of Co/Alq3 Granular Films on a Si Wafer

A series of Co0.48 （Alq3）0.52 granular films were deposited on silicon substrates using the co-evaporating technique. A crossover of magnetoresistance （MR） from negative to positive was observed in the samples, due to conducting channel switching. The transport properties of samples are greatly influenced by hydrofluoric acid pretreatment, as a result, positive MR decreases drastically and the temperature dependence of resistance changes a lot near room temperature. The result indicates that the native oxide layer plays an important role in the transport mechanism. Moreover, different resistivities of Si substrates influence the current distribution of conducting channels, leading to different transport behaviors accordingly.     

Organic Light-Emitting Diodes with Magnesium Doped CuPc as an Efficient Electron Injection Layer

Bright organic electroluminescent devices are developed using a metal-doped organic layer intervening between the cathode and the emitting layer. The typical device structure is a glass substrate/indium-tin oxide （ITO）/copper phthalocyanine （CuPc）/N,N＇-bis-（1-naphthl）-diphenyl-1,1＇-biphenyl-4,4＇-diamine （NPB）/Tris（8-quinolinolato） aluminum（Alq3）/Mg-doped CuPc/Ag. At a driving voltage of 11 V, the device with a layer of Mg-doped CuPc （1：2 in weight） shows a brightness of 4312cd/m^2 and a current efficiency of 2.52cd/A, while the reference device exhibits 514 cd/m^2 and 1.25 cd/A.     

Studies on influence of light on fluorescence of Tris-(8-hydroxyquinoline)aluminum thin films

Tris-(8-hydroxyquinoline)aluminum (Alq₃) thin films, the most widely used electron transport/emissive material in the organic electroluminescent (EL) devices, have been deposited on glass substrates by thermal evaporation process. Alq₃ thin films were exposed to light for various time periods under normal ambient. The fluorescence of as-prepared and light exposed Alq₃ thin films and formation of luminescent quencher have been studied using fluorescence, Mass, MALDI-ToF-MS, ¹H & ¹³C NMR, and FT-IR spectroscopy. It is observed that among the three 8-hydroxyquinoline (HQ) units in Alq₃ molecule, one HQ unit is affected during the light exposure in the normal ambient. It is found that the affected resultant Alq₃ molecule containing the carbonyl group acts as fluorescent quencher and the energy of excitons in the Alq₃ molecule in the light exposed Alq₃ thin films can be non-radiatively transferred to the neighboring fluorescent quencher, quenching the fluorescence of light exposed Alq₃ thin films in the normal ambient.     

Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

The origination of ill-defined layer in organic spin valves

The origination of ill-defined layer in organic spin valves was investigated by using atomic force microscopy (AFM) and Rutherford backscattering (RBS) analysis. It was found that conductive bulges of LSMO film and self-grown pinholes in Alq₃ film other than Co inclusions could lead to the formation of ill-defined layer. The morphology of LSMO substrate had a strong influence on that of Alq₃ film, LSMO/Alq₃ and Alq₃/Co interfaces. Moreover, Alq₃ film with the thickness of 1-4 nm could be barriers which was explained by small active area and added insulated layer in organic magnetic tunnel junctions.     

Photoluminescence spectroscopy study on tris(8-hydroxyquinoline) aluminum film

In situ photoluminescence spectroscopy (PL) measurements of tris(8-hydroxyquinoline) aluminum (Alq₃) film were carried out. Upon deposition of Alq₃ on the glass substrate, the PL intensity changes dramatically, while the peak position of Alq₃ emission shows a sharp red-shift from 524 nm at the initial deposition of Alq₃, and tends to a saturation value of 536 nm for the film thickness range from 2 to 500 nm. This red-shift is associated with the change from the 2D to 3D exciton state with increasing Alq₃ film thickness. Temperature dependent PL spectra of Alq₃ films showed, besides the changes in the PL intensity, clearly a blue-shift of Alq₃ emission about 9 nm for the film annealing up to 150 °C, while no any shift of Alq₃ emission was observed for the film annealing below 130 °C. Both changes in PL intensity, and especially in the peak position of Alq₃ emission were attributed to crystallization (thermal) effect of Alq₃ film upon annealing.     

Observation of ferromagnetic ordering in Mn‐doped 8‐hydroxy‐quinoline aluminum

The ferromagnetic property of Mn‐doped 8‐hydroxy‐quinoline aluminum (Alq₃), synthesized by thermal co‐evaporation of pure Mn metals and Alq₃ powders, was investigated. The weak ferromagnetic property was observed in 5%‐doped Alq₃, with saturation magnetization of around 0.05μ_B/Mn. The doped Mn chemically interacted with O atoms, producing a new gap state at 0.34 eV above the highest occupied molecular orbital and reducing the effective electron concentration. This led to the decrease of the electron affinity and increase of the optical bandgap, resulting in the reduction of the hole‐injection barrier in comparison with the electron‐injection barrier to the Alq₃ layer. From these, the origin of the observed ferromagnetism is suggested. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

In this paper, the roles of zinc selenide (ZnSe) sandwiched between organic layers, i.e. organic/ZnSe/aluminum quinoline (Alq₃), have been studied by varying device structure. A broad band emission was observed from ITO/poly(N-vinylcarbazole)(PVK)(80 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al under electric fields and it combined the emissions from the bulk of PVK, ZnSe and Alq₃, however, emission from only Alq₃ was observed from trilayer device ITO/N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1, 1^′-biphenyl-4, 4^′-diamine (NPB) (40 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al. Consequently the luminescence mechanism in the ZnSe layer is suggested to be charge carrier injection and recombination. By thermal co-evaporating Alq₃ and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), we get white light emission with a Commission Internationale de l’E clairage (C.I.E) co-ordinates of (0.32, 0.38) from device ITO/PVK(80 nm)/ZnSe(120 nm)/ Alq₃:DCJTB(0.5 wt% DCJTB)(15 nm)/Al at 15 V and the device performs stably with increasing applied voltages.     

Deep‐level characterization of emissive interface states in Alq3‐based OLEDs

We have succesfully investigated emissive interface states in fabricated indium‐tin‐oxide (ITO)/N,N′‐di‐1‐naphthyl‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′diamine (α‐NPD)/tris(8‐hydroxyquinoline) aluminum (Alq₃)/LiF/Al organic light‐emitting diodes (OLEDs) by a modified deep‐level optical spectroscopy (DLOS) technique. In the vicinity of the α‐NPD/Alq₃ emissive interface, a discrete trap level was found to be located at ～1.77 eV below the conduction band of Alq₃, in addition to band‐to‐band transitions of carriers from α‐NPD to Alq₃. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

激子复合区厚度对有机磁效应的影响

在常规型有机发光二极管的基础上, 通过改变发光层tri-(8-hydroxyquinoline) aluminum (III) (Alq₃)厚度, 研究了激子复合区厚度对有机发光二极管磁效应的影响.测量了器件在不同温度及偏压下电致发光及注入电流在外加磁场作用下的变化, 着重研究了低温下的有机磁电导效应和有机磁电致发光效应.实验发现, 低温(50 K)高磁场 (500 mT)下, 器件表现出随Alq₃厚度的减薄, 磁电导值由正到负再到正的非单调变化.利用磁场调控的超精细相互作用、 磁场抑制的三重态激子-电荷反应以及激子在界面的淬灭效应, 对有机磁电导在低温下表现出的现象进行了定性的解释.实验结果表明, 通过改变激子复合区的厚度, 可以实现对激子浓度的有效调节, 进而实现对有机磁电导和磁电致发光效应的调节. 该研究进一步丰富了有机磁效应的实验现象, 同时提供了一种调控有机磁效应的手段. 关键词： 激子复合区 激子浓度 有机磁电导 有机磁电致发光     

Investigation of charge carrier mobility in 5,6,11,12-tetraphenylnapthacene (rubrene) and coumarin 6 doped Alq3 films

The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq₃) was studied by time-of-flight (TOF) measurements. The polar dopant, coumarin 6 (C-6) and extensive π conjugated dopant, 5,6,11,12-tetraphenylnaphthacene (rubrene) were used for this study. The co-doped of rubrene (Rb) with C-6 into Alq₃ improved the carrier mobility compared to the single doped Alq₃:C-6 film. The carrier mobility in single doped Alq₃:C-6 film did not follow the linear relationship of Poole-Frenkel (PF) model with applied electric field. The mobility was in agreement with the PF model at two different ranges of electric fields (F) separated by a critical field . The mobility in co-doped Alq₃:(Rb:C-6) film followed the linear relationship with the PF model. The energetic disorder was found as ∼0.32 eV in co-doped films. It was ∼0.55 and ∼0.27 eV before and after the critical field in Alq₃:C-6 film. The values of positional disorders in co-doped films were estimated as ∼1.8 and it was ∼2 in Alq₃:C-6 film at . The organic light emitting diode performance of the co-doped film was improved compared to single doped film. The luminescence efficiency was improved tremendously to ∼6  Cd/A in co-doped device at 45 mA/cm² current compared to Alq₃:C-6 film device of ∼1  Cd/A.     

Preparation and Luminescence Characteristics of Ca3Y2（BO3）4：Eu^3＋ Phosphor

Ca3Y2 （BO3）4：Eu^3＋ phosphor is synthesized by high temperature solid-state reaction method, and the Iuminescence characteristics are investigated. The emission spectrum exhibits two strong red emissions at 613 and 621 nm corresponding to the electric dipole ^5 Do- ^7F2 transition of Eu^3＋ under 365 nm excitation, the reason is that Eu^3＋ substituting for Y^3＋ occupies the non-centrosymmetric position in the crystal structure of Ca3 Y2 （BO3）4. The excitation spectrum for 613 nm indicates that the phosphor can be effectively excited by ultraviolet （UV） （254 nm, 365nm and 400nm） and blue （470nm） light. The effect of Eu^3＋ concentration on the emission intensity of Ca3 Y2 （BO3）4 ：Eu^3＋ phosphor is measured, the result shows that the emission intensities increase with increasing Eu^3＋ concentration, then decrease. The CIE colour coordinates of Ca3Y2 （BO3）4：Eu^3＋ phosphor is （0.639, 0.357） at 15mol% Eu^3＋.     

Influence of Interface Structure of Co/Cu （100） Superlattices on Electronic Structure and Giant Magnetoresistance

Electronic structures and magnetoresistance （MR） of Co3 Cu5 and Co3 Cur models as well as their interface atom exchange models Co2 CuCoCu4 and Co2 CuCoCu6 are investigated by the tlrst-principles pseudopotential planewave method based on density functional theory. Charge transfer, magnetic moment, density of states, spin asymmetry factor, and MR ratio are discussed. The results show that the values of charge transfer and spin asymmetry factor at the Fermi level of Co layers are closely related to the neighbouring background of the Co layer. The Co layer with two sides adjacent to the Cu layer would transfer more charge to the Cu layer than other neighbouring background and have the largest spin asymmetry factor at the Fermi level. The Co layer with two neighbouring Co layers （interior Co） would gain a little charge and have the smallest spin asymmetry factor at the Fermi level. Two-current model is used to evaluate the MR ratio of Co2CuCoCu4 （Co2CuCoCu6） to be larger than that of Co3 Cu5 （Co3 CUT）, which can be explained by the charge transfer and spin asymmetry factor.     

Li掺杂8-羟基喹啉铝的密度泛函理论研究

采用密度泛函理论研究了Li原子掺杂8-羟基喹啉铝(Alq₃)分子的几何构型、 前线分子轨道及电子转移特性. 研究结果表明, Li原子掺杂Alq₃后, Li原子与Alq₃的O, N原子键合, 形成电子转移复合物. Li原子将部分电子转移到Alq₃的吡啶环上, 在Alq₃的带隙内形成施主能级, 这种n型掺杂结构有效地提高了电子的传输效率; 但过多的Li原子的掺杂会使Alq₃分解, 从而减弱其电子传输能力. 为使Alq₃的电子传输能力达到最高, Li原子的掺杂应保持在2:1左右的比例.