VienE Choong

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

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.

Dramatic photoluminescence quenching of phenylene vinylene oligomer thin films upon submonolayer Ca deposition

We report that Ca atoms severely quench the photoluminescence (PL) of an organic thin film 1,4‐bis[4‐(3,5‐di‐tert‐butylstyryl)stryyl]benzene (4PV). A submonolayer coverage (0.035 monolayer or 0.1 A) of Ca on the 4PV surface reduced the PL of a 300 A layer of 4PV by as much as 50%. Three distinct quenching rates were observed throughout the 200 A Ca deposition. An exciton diffusion length of 200 A in 4PV is estimated assuming that the PL quenching is diffusion controlled.

Energy level bending and alignment at the interface between Ca and a phenylene vinylene oligomer

We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 A. We propose an energy level diagram of the oligomer‐Ca interface based on the information obtained from the photoemission spectra.

Deposition-induced photoluminescence quenching of tris-(8-hydroxyquinoline) aluminum

Ca, Ag, and Ge atoms quench the photoluminescence (PL) of an organic thin film, tris-(8-hydroxyquinoline) aluminum (Alq3), a model organic material for organic light emitting diodes. The observed PL quenching behavior was similar for all three elements, independent of their metallic nature. Due to strong interactions at the Ca/Alq3 interface, the quenching was much less effective at submonolayer coverages. We have also observed that proper oxidation of Ca at the Ca/Alq3 interface can significantly recover the quenched luminescence of Alq3.

Electron spectroscopy studies of interface formation between metal electrodes and luminescent organic materials

Using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), we have studied the formation of metal/organic interfaces in organic electroluminescent devices. Oligo(p-phenylenevinylenes) (OPV) and tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials and Ca was used as the metallic layer. Interfaces are formed differently by depositing organic layer on Ca and Ca on organic substrate. For Ca/OPV, UPS revealed a clear evidence for interface state formation upon Ca deposition. The evolution of XPS core level peak as a function of Ca layer thickness was consistent with the energy level bending picture. The XPS and UPS spectra for OPV/Ca as a function of organic layer thickness also confirmed the energy level bending. The data obtained allowed us to deduce the energy level diagram near the interface. Similar data for Alq3/Ca indicated that no electron injection barrier exists at this interface if the Alq3 optical band gap in the literature was used for estimating the energy positi...

Effects of Al, Ag, and Ca on luminescence of organic materials

Organic materials have been demonstrated to have the necessary attributes for display applications. In typical organic light emitting devices, metallic electrodes are used to inject charged carriers into the organic electroluminescent (EL) medium. We report severe photoluminescence (PL) quenching of organic thin films comprising the most useful materials, namely 1,4-bis[4-(3,5-di-tert-butylstyryl)styryl]benzene (4PV), upon sub-monolayer deposition of Al, Ag, and Ca in an ultrahigh vacuum environment. The severity of the luminescence quenching, which depends on the type of metal used, can greatly affect the EL device performance. For example, a sub-monolayer coverage of the various metals on a 300 A 4PV thin film can reduce the PL by as much as 50%. Depositing the 4PV layer onto a metal substrate also exhibits PL quenching. An exciton diffusion length of 200 A can be estimated from the quenching data.

Photoluminescence quenching of Alq3 by metal deposition: A surface analytical investigation

The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We will describe our recent efforts to understand interfaces in OLEDs using surface/interface analytical techniques in a well controlled ultrahigh vacuum environment. We observed severe photoluminescence quenching of organic thin films comprising of a model OLED material, namely tris-(8-hydroxyquinoline) aluminum (Alq3), upon deposition of a number of metals. Such quenching may severely affect the electroluminescence device efficiency. We have investigated the interfaces using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy, and studied the intriguing process of the interface formation at an atomic/molecular level. We will show that microscopic surface and interface properties are intimately related to the device characteristics and performance.

Molecular quantum chemical calculations of the formation of interfaces between Al, Ca and Mg with poly(phenylene vinylene) oligomers

Molecular quantum chemical calculations were performed both at the ab initio and at the semi-empirical level to model the molecular conformations and electronic structure of oligomers of poly(phenylene vinylene) during the early stages of interface formation with Al, Ca and Mg. We found that the divalent metals, Mg and Ca, disrupt the conformation of the oligomers less than Al does. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) move into the energy gap both for Ca- and for Mg-doped systems, resulting in gap-state formation. This is consistent with the polaron/bipolaron picture. The electron density plots indicate that the de-localization of electrons is reduced more significantly by Al than it is by Ca and Mg. Our simulation results have been confirmed experimentally via XPS and NEXAFS.

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.