Cathode In Organic Light-emitting Diodes Research Articles

A highly transparent laminated composite cathode for organic light-emitting diodes

Transparent conductive electrodes are one of the important components of organic optoelectronic devices. Mg:Ag alloys have been widely used as a semi-transparent cathode for top-emitting organic light-emitting diodes (OLEDs) or transparent OLEDs (TrOLEDs) due to their high conductivity and relatively low work function. However, their low transmittance makes the light-extraction efficiency to be comparatively low. In this study, a highly transparent (84.6% in the visible light range), conductive, laminated cathode with the structure of ZnO/Mg:Ag/ZnO was fabricated via atomic layer deposition (ALD). ALD-deposited ZnO is used to protect the vulnerable ultrathin Mg:Ag-alloy layer, while preserving its low work function for the entire structure. In particular, the bottom ZnO layer is used as an ETL, which can provide reasonable energy alignment between the cathode layer and the emission layer and prevent metal particles from penetrating into the underlying organic layer. The top-emission OLED, which is prepared in this study by using the laminated cathode, showed an 85.8% increase in brightness, with a current density of 11.2 mA/cm3 at 4.0 V, which is an ultra-low voltage compared with devices made with Mg:Ag cathodes. In addition, the total luminance of TrOLEDs increases 34.4% compared with devices using Mg:Ag cathodes. Furthermore, the average transmittance of the device was still as high as 77.19% for visible light.

Mechanistic Understanding of Improved Performance of Graphene Cathode Inverted Organic Light-Emitting Diodes by Photoemission and Impedance Spectroscopy.

Modification of multilayer graphene films was investigated for a cathode of organic light-emitting diodes (OLEDs). By doping the graphene/electron transport layer (ETL) interface with Li, the driving voltage of the OLED was reduced dramatically from 24.5 to 3.2 V at a luminance of 1000 cd/m2. The external quantum efficiency was also enhanced from 3.4 to 12.9%. Surface analyses showed that the Li doping significantly lowers the lowest unoccupied molecular orbital level of the ETL, thereby reducing the electron injection barrier and facilitating electron injection from the cathode. Impedance spectroscopy analyses performed on electron-only devices (EODs) revealed the existence of distributed trap states with a well-defined activation energy, which is successfully described by the Havriliak-Negami capacitance functions and the temperature-independent frequency dispersion parameters. In particular, the graphene EOD showed a unique high-frequency feature as compared to the indium tin oxide one, which could be explained by an additional parallel capacitance element.

Sub-micron pinhole detection in the cathode of organic light-emitting diodes

A technique is presented on tracing the sub-micron pinholes that result in black spots in organic light-emitting diodes (OLED) when exposed to ambient atmosphere. The mystery about the type and nature of tiny pinholes present in the OLED cathode that allow oxygen and/or moisture ingress in minute quantities causing black spot formation, is clarified. The technique can accurately locate nanodefects or pinholes in the center of black spots of various sizes, even on a centimeter scale. Pinholes in the investigated OLEDs were shown to be caused by different particles originating from various locations in the device stack. Defects in the Ba-Al cathode of a solution processed polymer LED (PLED) and pinholes in the LiF-Al cathode of a thermally evaporated small molecule organic LED (smOLED) were investigated and compared. The technique is a powerful tool for inspection and can, thereby, accelerate the process optimization for OLED fabrication.

The Li3PO4/Al electrode: An alternative, efficient cathode for organic light-emitting diodes

In this contribution an alternative, efficient cathode for organic light-emitting diodes (OLEDs) is introduced consisting of a sub-nanometer thin lithium phosphate (Li3PO4) layer capped with aluminium. This cathode is competitive to the LiF/Al state-of-the-art cathode and performs equally well in small molecule and polymer OLEDs but yields a higher device lifetime. The functional principle of the Li3PO4/Al cathode is based on the in situ formation of a dipolar interface. Partly oriented AlPO4 dipoles are the reason for a potential drop at the interface and a work function lowering of about 500meV. The work function of the Li3PO4/Al cathode is found to be comparable to the work function of calcium resulting in a promotion of electron injection. As the reaction products of the interface reaction are environmentally stable and chemically attached to the interface diffusion of these species into the organic layer is unlikely. This might be the reason for the observed enhanced device lifetime for devices with a Li3PO4/Al cathode.

Efficiency dependence on alkali metal compound/Al bilayer cathode in organic light-emitting diodes

The use of alkali metal compound/Al bilayer cathodes of LiF/Al and Liq/Al caused a dramatic luminous efficiency change of one order of magnitude in organic light-emitting diodes (OLEDs) comprising 2-methyl-9,10-bis(naphthalene-2-yl)anthracene (MADN) as an electron-transporting layer (ETL). In contrast, the use of the same two electrodes yielded similar efficiency in OLEDs comprising of tris(8-hydroxyquinoline) aluminum(III) (Alq3) as an ETL. The dramatic change is attributed to different behaviors of LiF/Al and Liq/Al cathodes toward the ETL. It is revealed that organic alkali metal complex/Al, such as Liq/Al, is superior to inorganic alkali metal compound/Al, such as LiF/Al, as a standard bilayer cathode to investigate the effectiveness of electron-transporting materials for high-efficiency OLEDs.

Patterning cathode for organic light-emitting diode by pulsed laser ablation

In this paper, nanosecond pulsed laser is introduced as a method to pattern the metal cathode of organic light-emitting diodes (OLEDs) without shadow mask or cathode separator. This method can selectively ablate away cathode metal without damage on the underlying indium tin oxide anode or the surrounding organic layers. The width and morphology of laser ablation trenches were studied as a function of energy fluence as well as pulse number of laser. The uniform ablation trench was obtained with 16 pulses at the laser fluence of 0.15J/cm2. It was found that the characteristics of OLED prepared with laser ablation under optimum ablation process were almost the same with that of OLEDs prepared with other two patterning methods.

Photoelectron Spectroscopic Study of C12A7:e- and Alq3 Interface: The Formation of a Low Electron-Injection Barrier

12CaO·7Al2O3 electride (C12A7:e-) is a promising material for the cathode of organic light-emitting diodes (OLEDs), because it has a low work function (φWF = 2.4 eV), comparable to metal potassium,...

Molecular monolayer modification of the cathode in organic light-emitting diodes

The effects of alkanethiol self-assembled monolayers (SAMs) attached to the gold cathode of organic light-emitting diodes made by soft contact lamination are investigated. In spite of reported work function lowering by alkanethiol SAMs, the results from this work showed that their primary effect in carrier transport is to act as a thin insulating layer, causing current reduction. At the same time, the luminescence efficiency was enhanced because the SAMs reduce exciton quenching by the metal cathode. A two-order-of-magnitude enhancement at light emission onset was observed for a hexadecanethiol modified device.

Improved performance and stability by an Al/Ni bilayer cathode in organic light-emitting diodes

Al/Ni bilayer cathode was used to improve the electroluminescent (EL) efficiency and stability in N, N′-bis(1-naphthyl)- N, N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq 3)-based organic light-emitting diodes. The device with LiF/Al/Ni cathode achieved a maximum power efficiency of 2.8 lm/W at current density of 1.2 mA/cm 2, which is 1.4 times the efficiency of device with the state-of-the-art LiF/Al cathode. Importantly, the device stability was significantly enhanced due to the utilization of LiF/Al/Ni cathode. The lifetime at 30% decay in luminance for LiF/Al/Ni cathode was extrapolated to 400 h at an initial luminance of 100 cd/m 2, which is 10 times better than the LiF/Al cathode.

Voltammetric study of Bphen electron-transport layer in contact with LiF∕Al cathode in organic light-emitting diodes

We report that the similarities of I-V characteristics of organic light-emitting diodes (OLEDs) incorporating Bphen (4,7-diphenyl-1,10-phenanthroline) electron-transport layer, with or without lithium doping, do not stem from the analogous compositions of the respective electron-transport layers. Voltammetric studies of OLED devices of the following general structure: anode ∣ NPB (N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine) ∣ TBADN (9,10-bis(2-naphthyl)-2-t-butylanthracene) ∣ Bphen ∣ LiF ∣ Al, indicate that the bulk of the Bphen electron-transport layer has insulating rather than semiconducting properties. A previously unknown feature is also detected by voltammetry: the presence of a macroscopic dipole comprised of a negative interfacial charge at the NPB ∣ TBADN interface and a positive charge in the Bphen layer and∕or at the TBADN ∣ Bphen interface, which could result from the diffusion of lithium ions. We also find that, in contrast to the bulk of the Bphen layer, its thin sublayer has, in fact, semiconducting properties, which also could be reasonably explained by the diffusion of lithium ions along with the migration of a countercharge, e.g., electron hopping between Bphen molecules.

Ohmic cathode for low-voltage organic light-emitting diodes

It is found that voltage-current characteristics of Al∕C60 interfaces are fundamentally altered from a strong rectifying junction to an Ohmic junction where a LiF interlayer is used. Al∕LiF∕C60 has been used as an Ohmic cathode for organic light-emitting diode (OLED). The Ohmic cathode together with a high electron mobility of the fullerene layer have lead to a drastic reduction in the driving voltage and consequently a significant increase in the power efficiency of OLED.

Optical Interference Effects by Metal Cathode in OrganicLight-Emitting Diodes

The dependence of light intensities of organic light-emittingdiodes (OLEDs) on the distance of emission zone to metal cathode isinvestigated numerically. The investigation is based on the half-spaceoptical model that accounts for optical interference effects of metalcathode. We find that light intensities of OLEDs are functions of thedistance of emission zone from the metal cathode because of the effectof interference of the metal cathode. This interference leads to anoptimal location of emission zone in OLEDs for the maximum of lightintensities. Optimal locations of emission zone are numerically shownin various emitting colour OLEDs with different metal cathodes andthese results are expected to give insight into the preparation of highefficiency full colour or white light OLEDs.

Formation of a MgAu alloy Cathode by Photolithography and the Application for Organic Light-Emitting Diodes and Organic Field-effect Transistors

To realize an efficient electron injection electrode into organic layers by photolithography, we form a MgAu alloy electrode which has capability of low work function and high durability for humidity and organic solvents during photolithography process. The MgAu alloy thin film shows a work function of 3.7eV which is comparable with that of a neat Mg layer and 0.8eV lower than that of a neat Au layer. The low work function of 3.7eV was maintained even after the photolithography process, suggesting excellent stability for the solvent treatments. We examine a MgAu alloy comb bottom electrode for organic field-effect transistor (OFET) and successfully obtain FET operation of n-type driving. We also demonstrate efficient electron injection characteristics of a MgAu alloy cathode in organic light-emitting diodes (OLEDs). The MgAu alloy cathode can inject electrons efficiently similar to the conventional MgAg cathode.

Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy

Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* transition as observed from the shake-up peaks of the carbon 1s core level in the XPS. The chemistry at the interfaces of Yb/CsF/Au and Yb/CsF/F8BT was also examined by XPS. In both cases, Cs was liberated from the CsF upon Yb deposition and the Yb reacted with the liberated F to form YbF3. In the Yb/CsF/F8BT system, the dissociated Cs did not exist in the metallic state, but reacted with the N atoms and carbon backbone in the F8BT. The low work function of Cs (2.2 eV) may further enhance the injection of electrons into the polymer layer. More importantly, the utilization of Yb/CsF as an electrode is polymer independent, and the CsF layer between the Yb and F8BT could retard Yb diffusion into the bulk polymer. The present results are supported by a calculation of the free energy needed to liberate the Cs in the metal/CsF system using a simple thermodynamic model. It is suggested that the use of Yb/CsF cathode in the polymer offers an advantage over most other metal fluorides because the current electrode has a higher affinity to liberate Cs due to the high negative heat of formation of YbF3, although Yb alone may not be a good electron injector in OLEDs.

Investigation on Copper Liberation in the Multilayer CuPc/Li/Al Cathode of Organic Light-Emitting Diodes

An interlayer Li inserting between copper phthalocyanine (CuPc) and Al improves both electric and luminous properties of an organic light-emitting diode (OLED). X-ray photoelectron spectroscopy (XPS) surface analysis technique is employed to investigate chemical binding conditions of CuPc/Li interface. At varying thicknesses of Li deposited on CuPc, we have studied and XPS of CuPc films. Investigation demonstrates that Cu atoms are liberated from CuPc with increasing Li thicknesses. The results reveal that an OLED with an indium tin oxide glass/triphenyldiamine derivative/tris(8-hydroxyquinoline)aluminum/CuPc/Li/Al structure employing a 2 nm thick Li exhibits optimum performance. It can satisfy commercial demands in not only good reproducibility but also a large area of uniformality. © 2003 The Electrochemical Society. All rights reserved.

Patterning organic light-emitting diodes by cathode transfer

A technique is presented for patterning the metal cathode of organic light-emitting diodes that allows for cathode engineering. The technique involves transferring pretreated and prepatterned metal on a substrate onto the surface of organic layers of the device by pressing, utilizing a difference in the adhesion strength of the metal between the substrate and the underlying organic layer. This cathode transfer technique is applied to fabricating a 20×10 passive matrix display with a pixel size of 250 μm by 250 μm.

A low reflectivity multilayer cathode for organic light-emitting diodes

In this paper, a vacuum-deposited multilayer cathode for organic light-emitting diodes (OLEDs) with reduced reflectivity is described. The reduced reflectivity (58% at 550 nm) is due to the addition of a smooth and compact carbon film between a thin semi-transparent magnesium layer and the top aluminum contact. For the following light-emitting organic structure deposited on indium-tin-oxide substrates: N, N′-diphenyl- N, N′- bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/terbium tris(1-phenyl-3-methyl-4-(tertiarybutyryl)pyrazol-5-one) triphenylphosphine oxide [(tb-PMP) 3Tb(Ph 3PO)]/3-(4-biphenyl)-4-phenyl-5-(4- tert-butylphenyl)-1,2,4-triazole (TAZ), the injection properties of such a multilayer cathode are presented and compared to those of Al/Mg and C/Al cathodes. Such a cathode shows promise for contrast improvements in OLEDs.

Tin Oxide as a Cathode in Organic Light-Emitting Diodes

An alternative to indium tin oxide (ITO) as the cathode in organic light-emitting diodes (LEDs) is suggested: tin oxide. ITO can lead to problems of contamination of the polymer layer with indium, causing a reduction in radiant recombination and, in turn, diminished device brightness. The general consequences of the use of a semiconductor as the charge injection electrode material are analyzed, leading to the conclusion that the formation of an electron accumulation layer facilitates charge injection in the case of tin oxide.