L. Li Yan

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.

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.

Thermodynamic equilibrium and metal-organic interface dipole

We determined the interface dipoles at a number of metal-organic interfaces using ultraviolet and x-ray photoelectron spectroscopy. A linear dependence of the dipole on the metal work function is observed. This is consistent with the theory based on the charge transfer and thermodynamic equilibrium across the interface. The agreement suggests that charge transfer is one major factor in the formation of interface dipole. In addition, we find that the pushing back of the electron cloud tail that extends out of the metal surface and the permanent dipole moment of the organic molecule affect the interface dipole.

Direct observation of Fermi-level pinning in Cs-doped CuPc film

The electronic structures of pristine and Cs-doped CuPc films are investigated using photoemission spectroscopy and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital can be directly observed by IPES and ultraviolet photoemission spectroscopy simultaneously. We found that the Fermi-level position in organic film can be modified by Cs doping. The observed onset of the LUMO of the CuPc film is shifted by Cs doping to less than 0.2 eV above the Fermi level. The result indicates that the energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Interface formation between NPB and processed indium tin oxide

We have investigated the interface formation between ITO and N,N 0 -bis-(1-naphthyl)-N,N 0 -diphenyl-1,1 0 -biphenyl-4,4 0 -diamine (NPB), an organic materials often used as hole transport layer in organic light-emitting devices (OLED), by using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and atomic force microscopy (AFM). Acid or base treatment of indium tin oxide (ITO) surfaces can significantly alter the surface work function which, in the case of acid treatment, points to an improved energy level alignment with NPB and, therefore, enhanced hole injection efficiency. We found no significant reactions nor level bending for NPB deposited on standard ITO. In contrast, for acid-treated ITO, reaction of NPB nitrogen with the proton of the dipole layer on the ITO surface is observed. At low NPB coverages, AFM images reveal uniform island growth of NPB on ITO. Further deposition leads to a more complete covering of the ITO surface by NPB layer, corresponding to a laminar growth mode. q 2000 Published by Elsevier Science S.A. All rights reserved.

X-ray photoelectron spectroscopy and atomic force microscopy investigation of stability mechanism of tris-(8-hydroxyquinoline) aluminum-based light-emitting devices

Stability is an essential issue in the application of organic light-emitting devices (OLEDs). We have investigated the indium tin oxide (ITO) surface for operated and unoperated OLEDs using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques. The device structure consists of ITO/phenyl-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3)/Mg:Ag with NPB thickness varied from 0 to 300 A. The ITO surface was exposed by removing the organic and metal layers with dichloromethane, an organic solvent in which NPB and Alq3 are highly soluble. Electroluminescence characterization demonstrates that the NPB layer substantially enhanced the stability. XPS analysis shows that for the device made without NPB and after 90 h of operation, there exists an insoluble organic material on the ITO surface. This organic material is not observed on the ITO of unoperated devices. Lateral force AFM also shows a striking difference between the ITO surface of devices with and without NPB after oper...

Photoemission study of interfaces in organic light-emitting diodes

Abstract The importance of the interfacial properties in organic light-emitting devices (OLEDs) is well recognized. We have investigated the interface formation between a metal, namely Al or Ca and tris-(8-hydroxyquinoline) aluminum (Alq 3 ) using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). In the case of Al/Alq 3 , the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq 3 . UPS spectra show a quick disappearance of the Alq 3 features as early as 1 A of Al deposition, and also suggest the formation of a rather poorly defined gap state induced by Al. On the other hand, in the case of Ca/ Alq 3 , the interface is characterized by a staged interface reaction: for low Ca coverages ( 3 molecule.

Fermi level pinning in Cs doped CuPc

We have investigated the electronic structures of pristine and alkali metal doped organic films using photoemission and inverse photoemission spectroscopy (PES and IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) were directly observed simultaneously. We found that the Fermi level position in the organic film was modified by alkali metal doping, and the observed LUMO of the CuPc film was shifted by the Cs doping to less than 0.2eV above the Fermi level. The observation is a direct confirmation of widely used assumption that the LUMO can be inferred from HOMO position in organic films. The result indicates that energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Energy alignment and trap states in dye-doped tris-8-(hydroxyquinoline) aluminum light-emitting devices

Thermally stimulated luminescence has used to directly measure the trap states in intrinsic Alq3 and Alq3 doped with coumarin 6. For Alq3 doped with coumarin 6, we observe an increase in the trap energy from 0.25eV for undoped Alq3 to 0.32eV for doping concentrations as high as 2 percent. The origin of these trap states may be related to the relative energy level alignment between the Alq3 host and coumarin dopant. Using UV photoemission spectroscopy, we have measured the solid state energy alignment of the highest occupied molecular orbitals between Alq3 and coumarin 6. Finally, we report I-V curves for single layer devices as a function of doping with Al/LiF top and bottom contacts. The charge transport shows the trap states induced in the Alq3 films due to the presence of the coumarin 6 molecules decrease the carrier mobility and increase the energetic disorder. These results can be directly observed from the measured I-V curves.

Interface formation between Al and Ca with tris-(8-hydroxyquinoline) aluminum

Using x-ray and UV photoemission spectroscopy (XPS and UPS), we have investigated the early stages of the interface formation between metals, namely Al and Ca, and tris-(8- hydroxyquinoline) aluminum (Alq3). Both interfaces show signs of reaction between the metal and Alq3. However, the detailed behaviors of the two interfaces are very different. In the case of Al/Alq3 interface, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq3. No evidence of reaction with the carbon was found. Unlike with Ca, little interaction between Al and nitrogen of the pyridyl was observed. UPS spectra show a quick disappearance of the Alq3 features as early as 0.7 angstrom of Al deposition, and also suggest the formation of a gap state induced by Al. In the case of Ca/Alq3, the interface is characterized by a staged interface reaction: for low Ca coverages, 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 overages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule.