M. G. Mason

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.

Anode modification in organic light-emitting diodes by low-frequency plasma polymerization of CHF3

Plasma polymerization of CHF3 at low frequencies was utilized for anode modification in organic light-emitting diodes. The polymerized fluorocarbon films have a high ionization potential and a relatively low resistivity. The devices with a polymer-coated anode of indium–tin–oxide exhibited enhanced hole injection and superior operational stability.

Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces

We have investigated the interfaces of aluminum on tris-(8-hydroxyquinoline) aluminum (Alq3) and aluminum on LiF/Alq3, using x-ray and ultraviolet photoemission spectroscopy (UPS). Aluminum appears to react destructively with Alq3 causing significant modification of the oxygen, nitrogen, and aluminum spectra. The well-defined UPS spectrum of Alq3 is quickly destroyed by very low coverages of aluminum. With only a 5 A layer of LiF on the Alq3, the reaction with aluminum is significantly suppressed. The Alq3 molecular orbital features in the UPS shift to higher binding energy but remain easily recognizable. In addition, a well-defined gap-state forms which is significantly different from that produced without LiF. Both the core-level spectra and the gap-state suggest that the Alq3 anion is formed in the presence of aluminum and LiF.

Investigation of the interface formation between calcium and tris-(8-hydroxy quinoline) aluminum

X-ray and ultraviolet photoemission spectroscopy investigations reveal strong interactions between Ca and tris-(8-hydroxy quinoline) aluminum (Alq3) during the Ca/Alq3 interface formation. The details of the interaction depend on the direction of the interface formation. For the case of Ca deposited on Alq3, a staged interface reaction is observed. For low Ca coverages (ΘCa⩽4 A), negatively charged Alq3 radical anions are formed by electron transfer from the Ca. The emergence of new states in the energy gap is observed in the UPS spectra. At higher coverages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule. On the other hand, for the case of Alq3 deposited on Ca, a strong chemical reaction takes place as soon as Alq3 is deposited, and Ca attacks every constituent of Alq3. Finally, no interaction occurs between Alq3 and the Ca substrate if the substrate has been passivated by oxygen prior to the Alq3 deposition.X-ray and ultraviolet photoemission spectroscopy investigations reveal strong interactions between Ca and tris-(8-hydroxy quinoline) aluminum (Alq3) during the Ca/Alq3 interface formation. The details of the interaction depend on the direction of the interface formation. For the case of Ca deposited on Alq3, a staged interface reaction is observed. For low Ca coverages (ΘCa⩽4 A), negatively charged Alq3 radical anions are formed by electron transfer from the Ca. The emergence of new states in the energy gap is observed in the UPS spectra. At higher coverages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule. On the other hand, for the case of Alq3 deposited on Ca, a strong chemical reaction takes place as soon as Alq3 is deposited, and Ca attacks every constituent of Alq3. Finally, no interaction occurs between Alq3 and the Ca substrate if the substrate has been passivated by oxygen prior to the Alq3 deposition.

Energy level alignment at Alq/metal interfaces

The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

Transient photocurrents across organic–organic interfaces

Hole photocurrent transients in organic–organic bilayers are described. Transitions in the photocurrents for holes moving across the organic–organic interfaces are observed. The magnitude of the photocurrent increases (or decreases) when holes transfer from a lower (or higher) mobility material into a higher (or lower) one. These results demonstrate a novel technique for studying energetic barriers and hole injection dynamics at organic–organic interfaces.

Photoemission study of energy alignment at the metal/Alq3 interfaces

Abstract We have investigated the interface formation and the energy alignment between different metals and Alq3 using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) simultaneously. While, the UPS results show that the vacuum level changes during interface formation, implying the possibility of surface dipole layer, our XPS results show more detailed and complex behavior. At the low work function, metal/Alq3 interfaces a gap state is observed in UPS. At the same time, a new peak can be observed in the N 1s XPS core level at a lower binding energy. These results can be well characterized as charge transfer from the low work function metal to Alq3. Shifting of core levels is also observed, which may be due to charge diffusion or doping from metal atoms. These interfaces are drastically different from the Al/Alq3 interface, which has very poor electron injection. At the Al/Alq3 interface, there is a destructive chemical reaction and much smaller core level shifts is observed.

Voltage reduction in organic light-emitting diodes

For practical applications, it is important to operate organic light-emitting devices at low voltages and low power consumption. When both the cathode and anode are perfectly injecting, low electron mobility in electron-transport materials, such as tris-(8-hydroxyquinoline)aluminum (Alq), becomes a limiting factor on voltage reduction. In this letter copper phthalocyanine (CuPc) is replaced for Alq as an electron-transport layer, and interfacial modification is utilized to enhance electron injection from the CuPc electron-transport layer into the Alq emissive layer. The outcome of this structure significantly facilitates electron transport through the organic materials, thus resulting in substantial reduction in operating voltages and power consumption.