Joseph K. Madathil

Interfacial chemistry of Alq3 and LiF with reactive metals

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.

Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes

Organic surface-emitting diodes have been constructed with a multilayer stacked cathode consisting of (1) an ultrathin LiF/Al bilayer acting as an effective electron injector, (2) an optically low-loss and electrically conducting silver intermediate layer for sheet resistance reduction, and (3) a transparent and nonconducting capping layer for refractive index matching to optimize optical transmission. The entire cathode structure is prepared by conventional thermal evaporation without incurring radiation damage, and the resulting organic surface-emitting diodes exhibit superior electrical and optical characteristics.

Reduction of ambient-light-reflection in organic light-emitting devices

An organic light-emitting device capable of reducing ambient-light reflection from a cathode includes a light-transmissive substrate, a light-transmissive anode, an organic hole-transporting layer, and an organic electron transporting layer. A reflection-reducing structure disposed between the electron-transporting layer and a light-reflective cathode is capable of providing electron injection into the electron-transporting layer and of substantially reducing reflection of ambient-light entering the device.

Performance enhancement of top- and bottom-emitting organic light-emitting devices using microcavity structures

In order to improve the efficiency of top- and bottom-emitting devices, metallic electrodes have been used to create microcavity effects within the OLED structure. Semi-transparent Ag is used as the anode in bottom-emitting microcavity structures, whereas various reflective opaque metallic anodes are used for the top emitters. The cathode used in both configurations is MgAg - thick and opaque in the case of the bottom emitter and thin and semi-transparent in the case of the top emitter. Modeling and experiments show that for the top-emitting structures, the device efficiency is roughly proportional to the reflectivity of the anode in the low reflectivity range and increases significantly more than predicted by reflectivity alone in the high-reflectivity range. An ultrathin CF x or MoO x hole-injecting layer allows for the use of many metals as anodes and is an important feature of the device structure. With an Ag anode, both the top- and bottom-emitting microcavity devices are about twice as efficient (on axis) as the analogous nonmicrocavity bottom-emitting device. Microcavity devices employing a C545T-doped Alq emitter exhibit efficiencies of 21 cd/A at 6.4 V and 20 mA/cm 2 , with operational stability equivalent to conventional bottom-emitting structures.

31.4: Fabrication of Lithium-Based Alloy Cathodes for Organic Light-Emitting Diodes by D C Magnetron Sputtering

The cathodes for small-molecule based organic light-emitting diodes (OLED) have been prepared by DC magnetron sputtering. This is accomplished by introducing a properly configured bilayer buffer structure comprised of sublayers of LiF and copper phthalocyanine (CuPc). The buffer virtually eliminates the damage to the electroluminescent medium during high power sputtering deposition of Al:Li and Ag:Li alloys. The sputtered cathode devices exhibit luminance efficiency, drive voltage, and operational stability comparable to those of evaporated Mg:Ag cathode devices made of identical organic layers.