L. S. Hung

Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode

A bilayer is used as an electrode for organic electroluminescent devices. The bilayer consists of an ultrathin LiF layer adjacent to an electron-transporting layer and an aluminum outerlayer. Devices with the bilayer electrode showed enhanced electron injection and high electroluminescence efficiency as compared with a Mg0.9Ag0.1 cathode. Similar effects were observed when replacing MgO for LiF. The improvements are attributed to band bending of the organic layer in contact with the dielectrics.

CHARACTERIZATION OF TREATED INDIUM-TIN-OXIDE SURFACES USED IN ELECTROLUMINESCENT DEVICES

The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.

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.

Metal diffusion from electrodes in organic light-emitting diodes

Metal diffusion from magnesium–silver cathodes and indium–tin–oxide anodes in organic light-emitting diodes has been investigated. Magnesium showed no substantial diffusion under device operation and had no significant effects on luminance decay with operation time. Indium was immobile in storage at room temperature, while indium penetration into organic layers was observed after device operation. The presence of indium in organic films showed a correlation with performance degradation.

INTERFACE ENGINEERING IN PREPARATION OF ORGANIC SURFACE-EMITTING DIODES

Surface-emitting organic light emitting diode (OLED) was prepared by sputter deposition of indium-tin-oxide on a buffered organic layer structure. Confirming a previous report, a thin film of copper phthalocyanine (CuPc) was found to be a useful buffer layer in preventing sputter damage to the OLED layer structure, particularly the underlying Alq emissive layer. However, the CuPc layer forms an electron-injection barrier with the Alq layer, resulting in increased electron-hole recombination in the nonemissive CuPc layer, and thus a substantial reduction in electroluminescence efficiency. Incorporation of Li at the CuPc/Alq interface was found to reduce the injection barrier at the interface and recover the overall device efficiency with good surface emission characteristics.

High-efficiency red electroluminescence from a narrow recombination zone confined by an organic double heterostructure

Red light-emitting diodes (LEDs) with both a conventional bilayer structure and a double heterostructure (DH) have been investigated. In these LEDs, N,N′-bis-(1-naphthl)-diphenyl-1, 1′-biphenyl-4,4′-diamine (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-quinolinolato) aluminum (Alq3) were used as hole-transporting, hole-blocking, and electron-transporting layers, respectively. The bilayer and DH LEDs had a configuration of ITO/NPB/Alq3:red dopant/Alq3/MgAg and ITO/NPB/Alq3:red dopant/BCP/Alq3/MgAg, respectively. Three kinds of red fluorescent dyes—nile red, DCJTB, and DCM—were used as dopants. Compared with the bilayer structures, the luminance efficiencies of the DH LEDs were found to increase as much as 100%. We attribute the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined. High charge concentrations in the emissive layer resulted in efficient collision capture in the electron–hole recombination proc...

A bis-salicylaldiminato Schiff base and its zinc complex as new highly fluorescent red dopants for high performance organic electroluminescence devices

Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (BDPMB) and its zinc complex (BDPMB-Zn) with donor–acceptor–donor (D–A–D) type ICT properties in the neutral form were used as novel red-emitting dopants in OLEDs; bright saturated red-emitting EL devices with excellent colour chromaticity coordinates (x, y = 0.670, 0.325 for BDPMB; x, y = 0.655, 0.325 for BDPMB-Zn) and good efficiency (1.35 cd A−1 for BDPMB; 0.50 cd A−1 for BDPMB-Zn) were obtained.

Sputter deposition of cathodes in organic light emitting diodes

Sputter deposition was employed for cathode preparation in organic light emitting diodes (OLEDs). A thin film of copper phthalocyanine (CuPc) was found to be an effective buffer layer in preventing sputter damage to the OLED layer structure. However, the CuPc layer forms an electron-injection barrier with the underlying Alq layer, resulting in increased electron-hole recombination in the nonemissive CuPc layer, and thus a substantial reduction in electroluminescence efficiency. Incorporation of Li at the CuPc/Alq interface from a sputter-deposited Al (Li) cathode was found to reduce the injection barrier at the interface and make the overall device efficiency comparable to a device having an evaporated MgAg cathode. The devices exhibited good operational stability with a half lifetime greater than 3800 h at 20 mA/cm2.

Improved efficiency by a graded emissive region in organic light-emitting diodes

Electrical and optical properties of organic light-emitting diodes (OLEDs) with a stepwise graded bipolar transport emissive layer for a better control of charge transport and recombination are presented. The graded bipolar transport layer was formed by co-evaporating a hole-transporting material N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) and an electron-transporting/emissive material tris-(8-hydroxyquinoline) aluminum (Alq3) in steps, where each step has a different concentration ratio of NPB to Alq3. Compared to a conventional heterojunction OLED, electroluminescence efficiency was enhanced by a factor of more than 1.5, whereas the turn-on voltage remained unchanged in the graded structure.