Sang Wan Cho

The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.

The electronic structure of C60/ZnPc interface for organic photovoltaic device with blended layer architecture

The interfacial electronic structures of fullerene (C60)/zinc-phthalocyanine (ZnPc) and C60/ZnPc:C60 (50 wt %) containing a blended layer were investigated by in situ ultraviolet photoelectron spectroscopy (UPS), in an attempt to understand the role of the blended layer in improving the performance of organic photovoltaic devices that contain such layers. From the UPS spectra, the band bending found to be 0.30 eV in the ZnPc layer and 0.43 eV in the C60 layer at the C60/ZnPc interface. On the other hand, the band bending was 0.25 eV in both of the organic layers at the ZnPc:C60/ZnPc interface and no significant band bending in the C60 layer at the C60/ZnPc:C60 interface was found. The observed interface dipole was 0.06 eV at the C60/ZnPc interface and 0.26 eV at the ZnPc:C60/ZnPc interface. The offset between the highest unoccupied molecular orbital of ZnPc and the lowest occupied molecular orbital of C60 was 0.75 eV at C60/ZnPc and was 1.04 eV at the ZnPc:C60/ZnPc interface. The increased offset can be a...

The interface state assisted charge transport at the MoO 3 /metal interface

The interface formation between a metal and MoO(3) was examined. We carried out in situ ultraviolet and x-ray photoemission spectroscopy with step-by-step deposition of MoO(3) on clean Au and Al substrates. The MoO(3) induces huge interface dipoles, which significantly increase the work functions of Au and Al surfaces. This is the main origin of the carrier injection improvement in organic devices. In addition, interface states are observed at the initial stages of MoO(3) deposition on both Au and Al. The interface states are very close to the Fermi level, assisting the charge transport from the metal electrode. This explains that thick MoO(3) layers provide good charge transport when adopted in organic devices.

Buffer layer effect on the structural and electrical properties of rubrene-based organic thin-film transistors

The structural and electrical properties of organic thin-film transistors with rubrene/pentacene and pentacene/rubrene bilayered structures were investigated using x-ray diffraction, atomic force microscopy, and x-ray emission spectroscopy. High-quality rubrene thin films with orthorhombic structure were obtained in the rubrene/pentacene bilayer while the pentacene/rubrene bilayer only had an amorphous rubrene phase present. The rubrene/pentacene thin-film transistor shows more desirable current-voltage characteristics compared to the pentacene/rubrene transistor. The overall results suggest that the presence of a chemically active organic buffer layer and its associated crystal structure are crucial in enhancing the structural and electrical properties of rubrene-based transistors.

Electronic structure of In2O3 from resonant x-ray emission spectroscopy

The valence and conduction band structures of In2O3 have been measured using a combination of valence band x-ray photoemission spectroscopy, O K-edge resonant x-ray emission spectroscopy, and O K-edge x-ray absorption spectroscopy. Excellent agreement is noted between the experimental spectra and O 2p partial density of states calculated within hybrid density functional theory. Our data are consistent with a direct band gap for In2O3.

High concentration of nitrogen doped into graphene using N2 plasma with an aluminum oxide buffer layer

We performed plasma doping of nitrogen into single-layer graphene on SiO2. Using aluminum oxide as a buffer layer to reduce the plasma damage, up to 19.7% nitrogen was substitutionally doped into graphene. The nitrogen doping of graphene was confirmed by Raman and X-ray photoemission spectroscopy analyses. The n-doping property of the N-doped graphene was measured by Raman spectroscopy. Raman mapping was carried out to statistically confirm the Dirac cone shift of graphene resulting from the N-doping. The Dirac cone shift was directly measured by ultraviolet photoemission spectroscopy (UPS). The UPS result was consistent with the value calculated from the Raman G peak shift.

Interfacial reaction of atomic-layer-deposited HfO2 film as a function of the surface state of an n-GaAs (100) substrate

The characteristics of interfacial reactions and the valence band offset of HfO2 films grown on GaAs by atomic layer deposition were investigated by combining high-resolution x-ray photoelectron spectroscopy and high-resolution electron transmission microscopy. The interfacial characteristics are significantly dependent on the surface state of the GaAs substrate. Polycrystalline HfO2 film on a clean GaAs surface was changed to a well-ordered crystalline film as the annealing temperature increased, and a clean interface with no interfacial layer formed at temperatures above 600°C. The valence band offset of the film grown on the oxidized GaAs surface gradually increased with the stoichiometric change in the interfacial layer.

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O K-edge and Mn L{sub 3,2}-edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn L{sub 3} resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment (800 C ambient pressure of O{sub 2}) alone results in La deficiency (severest near the surface with Sr doping >0.55) and a stronger Mn{sup 4+} contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O K-edge X-ray absorption measurements support Sr/La enrichment nearer the surface, along with the formation of mixed Sr{sub x}Mn{sub y}O{sub z} and/or passive MnO{sub x} and SrO species.

The effect of ZnO surface conditions on the electronic structure of the ZnO/CuPc interface

The interfacial electronic structures of zinc oxide (ZnO)/copper-phthalocyanine (CuPc) were investigated by in situ x-ray and ultraviolet photoelectron spectroscopy (UPS) to determine the effects of air contamination on the ZnO substrate. UPS spectra showed that the 0.2 eV of the interface dipole is generated at the interface of the air exposed ZnO/CuPc while the interface of the annealed ZnO/CuPc generated −0.2 eV. In both cases, no band bending was observed. On the other hand, band bending at 0.3 eV and an interface dipole of 0.2 eV were observed at the interface of the sputter cleaned ZnO/CuPc. The energy offset between the conduction band maximum of ZnO and the highest occupied molecular orbital of CuPc was determined to be 0.6–0.7 eV for the contaminated ZnO interface while the offset was 1.0 eV for the cleaned ZnO interface. Contaminating moisture has little effect on the offset while the charge transfer was blocked and the offset was decreased in the presence of hydrocarbons.

Interfacial electronic structure of N, N′ -bis(1-naphthyl)- N, N′ -diphenyl-1, 1′ -biphenyl-4, 4′ -diamine/copper phthalocyanine: C60 composite/Au studied by ultraviolet photoemission spectroscopy

The interfacial electronic structures of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/copper phthalocyanine (CuPc)∕Au, NPB∕C60∕Au, and NPB∕CuPc:C60 composite/Au were investigated by in situ ultraviolet photoelectron spectroscopy to understand the highly efficient hole injection in organic light-emitting diode. The hole-injection barrier of CuPc:C60∕Au was 0.52eV, while those of CuPc∕Au and C60∕Au were 0.96 and 1.62eV, respectively. The lowered injection barrier is attributed to the smaller interface dipole of CuPc:C60 compared to that of pristine CuPc. This small interface dipole pulled up the highest occupied molecular orbital of CuPc in composite, which results in the decreased hole-injection barrier.