L.S. Liao

ENHANCED HOLE INJECTION IN A BILAYER VACUUM-DEPOSITED ORGANIC LIGHT-EMITTING DEVICE USING A P-TYPE DOPED SILICON ANODE

We report the fabrication of a vacuum-deposited light-emitting device which emits light from its top surface through an Al cathode using p-type doped silicon as the anode material. Enhanced hole injection is clearly demonstrated from the p-Si anode as compared to the indium–tin–oxide (ITO) anode. The mechanisms of hole injection from both the p-Si and ITO anodes into the organic layer are investigated and a possible model based on anode surface band bending is proposed. During the operation of the organic light-emitting device, the surface band bending of the anode plays a very important role in modifying the interfacial barrier height between the anode and the organic layer.

Effect of deposition rate on the morphology, chemistry and electroluminescence of tris-(8-hydroxyqiunoline) aluminum films

Abstract The effect of Alq 3 deposition rate on the performance of the devices has been investigated by using the organic light-emitting diodes of indium-tin-oxide/N,N′-bis-(1-naphthyl)-N,′-diphenyl-1,1′-biphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum (Alq 3 )/Mg:Ag. When the Alq 3 deposition rate decreased from about 1.33 to 0.05 A/s, the luminance efficiency of the devices decreased from 4.75 to 2.0 cd/A. Atomic force microscopy observations showed that Alq 3 films prepared at the deposition rates of 1.33, 0.05, and 0.01 A/s had a root-mean-square roughness of 12.0, 32.0, and 36.6 A, respectively. X-ray photoelectron spectroscopy measurements showed that as Alq 3 deposition rate decreased from 1.33 to 0.01 A/s, the film contained more N-containing species. These changes in film morphology and chemistry are suspected to be responsible for the change in the electroluminescent performance of the devices.

Bubble formation in organic light-emitting diodes

Bubbles in organic light-emitting diodes can be formed from gas release due to Joule heating effect at localized electrical shorts during operation, which could be simulated by a rapid thermal annealing. The gases in the bubbles consist of not only adsorbed moistures but also the decomposed organic species, which are detected in situ in an ultrahigh vacuum chamber. In the device of Al/tris-(8-hydroxyquinoline) aluminum (Alq/N,N′-diphenyl-N.N′-bis-{3-methylphenyl}{1,1′biphenyl}-4,4′-diamine/indium tin oxide (ITO), the gases released from ITO surface were mainly of adsorbed moistures, while those released from the organic layers were of both the decomposed products from Alq and the trapped moistures. The decomposition of Alq could not be easily avoided if there were severe localized electrical shorts in the devices.

Electronic structure and energy level alignment of Alq3/Al2O3/Al and Alq3/Al interfaces studied by ultraviolet photoemission spectroscopy

The electronic structures at the interface of aluminum tris(8-hydroxyquinoline) (Alq3)/Al2O3/Al have been determined by ultraviolet photoemission spectroscopy measurements and compared to similar measurements of the Alq3/Al interface. In the Alq3/Al2O3/Al study, shift of the highest occupied molecular orbital level of the Alq3 layer was observed when compared to that of Alq3/Al. An energy level alignment diagram was proposed, showing that the lowering of the driving voltage achieved in organic electro-luminescent devices with a thin Al2O3 layer between the aluminum cathode and the Alq3 film can be attributed to the reduction of the barrier height for electron injection. The electronic structures of Alq3 grown on Ga and its oxide have also been studied. q 2000 Elsevier Science S.A. All rights reserved.

Ambient effect on the electronic structures of tris-(8-hydroxyquinoline) aluminum films investigated by photoelectron spectroscopy

Abstract Thin films of tris-(8-hydroxyquinoline) aluminum (Alq 3 ) were exposed to trace amounts of O 2 , CO 2 , H 2 O, or to ambient air. Evolution of electronic structures of Alq 3 films with increasing gas exposure was measured using ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). The vacuum energy level, the highest occupied molecular orbital, and XPS core levels of the constituting elements in Alq 3 shifted according to the kind of gas exposure. Chemical reaction between oxygen and the Alq 3 films was observed upon oxygen exposure. Moreover, it was found that the dominant influence of ambient conditions on the electronic structures of the Alq 3 films was from H 2 O.

Effects of O, H and N passivation on photoluminescence from porous silicon

Abstract A simple but effective passivation method for porous silicon (PS) has been developed. Immersion of as-etched PS in dilute (NH4)2S/C2H5OH solution followed by ultraviolet light irradiation in air can lead to an enhancement of photoluminescence (PL) up to more than 20 times. Infrared absorption and Auger electron spectroscopic measurements show that the formation of SiH(O3), SiOSi and SiN bonds are formed during the post-treatment process. However, the PL intensity cannot be enhanced if the solution-treated sample is exposed to the laser beam in vacuum. It is thus concluded, that the PL enhancement can be attributed to the presence of compact passivation films consisting of the oxides and the nitride on both external and internal surfaces of the sponge-like PS samples.

Damage study of ITO under high electric field

Abstract Photoetching was used to obtain a narrow gap (about 10 μm) on a slip of indium tin oxide (ITO) which was coated on a glass plate. A high electrical field (about 10 6 V/cm) was applied between the two sides of the gap, so as to study the characteristics of ITO. Under this field, there is an intermittent current between two sides of the gap. We found that ITO decomposed, became opaque and its surface became very rough. The atomic concentrations of In and Sn in the dark region were higher than those in the original ITO slip. The resistance of the ITO slip increased accordingly. There were some gases, such as oxygen, carbon dioxide (or nitrogen), carbon monoxide, which evolved from the surface when ITO decomposed. After quick annealing under an oxygen atmosphere, the sample became as transparent as the original ITO. These observations show that ITO decomposes under a high electric field.

The interface analyses of inorganic layer for organic electroluminescent devices

Abstract High efficiency organic electroluminescence devices with an inorganic layer (SiO 2 ) near ITO were fabricated. The buffer layer was studied by UPS, XPS and AES. It is found that the energy offset can be changed by inserting an inorganic buffer layer and that the contamination of the interface could be decreased with ozone treatment, especially the carbon contamination can be eliminated, which increases the stability and the emission intensity of the devices.

Interface formation between poly(9,9-dioctylfluorene) and Ca electrode investigated using photoelectron spectroscopy

Abstract Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS) have been used to study the interface formation between poly(9,9-dioctylfluorene) (PFO) and Ca electrode. As the Ca coverage increases, the vacuum energy level of PFO decreases gradually in order to match the Fermi level ( E F ) of Ca. The original highest occupied molecular orbital of PFO moves away from the E F , and vanishes eventually. Bipolaron states with 1.8 eV in peak interval are formed in the former energy gap. The original lowest occupied molecular orbitals of PFO are broadened resulting in a featureless shake-up in the XPS C1s core level.

Photoluminescence from C+-implanted SiNxOy films grown on crystalline silicon

Carbon ions at an energy of 35 keV with a dose of 5×1016 cm−2 were implanted into SiNxOy films grown on crystalline silicon by plasma enhanced chemical vapor deposition. Intense photoluminescence (PL) peaked at about 550 nm is observed in the implanted films under an excitation of 441.6 nm laser line. The PL intensity varies with annealing temperature, and reaches a maximum at the annealing temperature of 600 °C. The luminescence may originate from the complex of Si, N, O, and C in the films.