Yeonjin Yi

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.

Insertion of an organic interlayer for hole current enhancement in inverted organic light emitting devices

We report the enhancement of hole current density in the hole transport part of an inverted top-emission organic light emitted diode by applying an organic insertion layer of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN). Poor hole transporting performance of Al/4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB)/indium tin oxide is greatly improved by the HAT-CN insertion between Al and NPB layer. The highest occupied molecular orbital level onset of the NPB bends toward Fermi level at the HAT-CN/NPB interface. This extra charge generation layer made of pure organic molecules substantially enhances hole injection from Al anode as revealed by the results of ultraviolet photoelectron spectroscopy and J-V measurement data.

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.

Effective work function lowering of multilayer graphene films by subnanometer thick AlOx overlayers

A simple method for controlling the effective work function (WF) of conductive multilayer graphene (MLG) film, synthesized by using chemical vapor deposition and transferred to a dielectric substrate, was developed. The WFs of the MLG during the step-by-step deposition of aluminum (Al) were measured using in situ ultraviolet photoelectron spectroscopy. Core-level spectra were also collected to investigate the chemical reaction that occurred when a small amount of Al was deposited onto MLG in a stepwise manner. The measurements revealed that the effective WF of the conductive MLG film could be controlled from 3.77 to 4.40 eV by the deposition of an Al layer less than 0.6 nm thick.

The characteristics and interfacial electronic structures of organic thin film transistor devices with ultrathin (HfO2)x(SiO2)1−x gate dielectrics

Pentacene-based thin film transistors with ultrathin (6nm) (HfO2)x(SiO2)1−x gate dielectric layers (x=0.25 and 0.75) were fabricated for low-voltage operation. The devices with ultrathin (HfO2)x(SiO2)1−x as the gate dielectric layer were operated at a gate voltage lower than −4.0eV. However, the threshold voltage and drain current have different values depending on the composition of the (HfO2)x(SiO2)1−x gate dielectric layer. The device with (HfO2)0.75(SiO2)0.25 gate dielectrics, having larger capacitance, shows a higher drain current than that with (HfO2)0.25(SiO2)0.75 gate dielectrics. On the other hand, the device with (HfO2)0.25(SiO2)0.75 gate dielectrics, which has a larger work function, shows a lower threshold voltage. The in situ ultraviolet photoelectron spectroscopy shows that this is caused by the difference in electronic structures and by changes in band alignment of the interface between the pentacene and dielectric layers.

Direct evidence of Al diffusion into tris-(8-hydroquinoline) aluminum layer: medium energy ion scattering analysis

The diffusion of Al into tris-(8-hydroquinoline) aluminum (Alq3) was studied using in situ medium energy ion scattering (MEIS) spectroscopy. Al was thermally deposited on an Alq3 thin film in a stepwise manner, with MEIS performed after each deposition step. At the initial stage of interface formation, Al diffuses deep into the Alq3 layer and reaches the bottom of the Alq3 layer of thickness 20 nm. Some Al is stacked at the surface of Alq3 and starts to form an Al layer. The deep diffusion of Al is diminished when sufficient Al aggregates at the surface. After this stage, Al is stacked only at the surface, but does not diffuse into the Alq3 film.

Electronic structure of pentacene/ultrathin gate dielectric interfaces for low-voltage organic thin film transistors

This paper describes the fabrication of pentacene-based thin film transistors (TFTs) with ultrathin (4.5nm) SiO2 and SiON gate dielectric layers for low-voltage operations. The device with the SiON gate dielectric layer operated at gate voltages lower than −3.0V, showing a threshold voltage of −0.45V, which was lower than the threshold voltage of the SiO2 device (−2.5V). The electronic structures of the interface between the pentacene and dielectric layers were investigated by in situ ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) to determine the reason for the lower operating voltage. The UPS and XPS results demonstrated that the interface dipole modified the potential of the dielectric layer, explaining the lower operating voltage. The electronic structure allowed for band bending at the interface, resulting in complete energy level diagrams for pentacene on SiO2 and SiON. The shifts in the threshold and turn-on voltages were explained by the energy level diagrams.

Organic light emitting diodes using NaCl:N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine composite as a hole injection buffer layer

Composite buffer layers of N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and NaCl at the anode/organic interface were found to be very effective on the hole injection enhancement from an indium tin oxide anode to the hole-transport layer (HTL) of NPB. Two maxima of significant current injection with respect to compositional variation were observed, implying multiple injection mechanisms of the tunneling effect and other interfacial effects. From a longer operation lifetime, the enhanced device stability was also confirmed as compared with a standard device with copper phthalocyanine as the hole injection layer. Those results are partly attributed to the better mechanical contact between anode and HTL via the composite buffer, observed from atomic force microscopy measurement.

Deposition sequence dependent variation in interfacial chemical reactions between 8-hydroxyquinolatolithium and Al

The chemical reactions between 8-hydroxyquinolatolithium (Liq) and Al were investigated by using high resolution synchrotron radiation photoelectron spectroscopy. Unlike the LiF/Al case, two opposite deposition sequences (Al/Liq versus Liq/Al) give different interface reactions. When Al is deposited on a Liq layer, there occurs a strong reaction between Liq and Al, which accounts for a clear peak shift in the Li 1s core level. On the other hand, an interface-localized charge transfer without Li 1s splitting occurs with the reversed deposition sequence. The former strong interface reaction can generate ionic Li as a dopant material in Liq layer, causing band bending.

Interface formation between tris(8-hydroxyquinoline) aluminum and ZnO nanowires and film

The energy level alignments at the interface between tris(8-hydroxyquinoline) aluminum (Alq3) and ZnO nanowires and thin film were studied with in situ x-ray and ultraviolet photoemission spectroscopy. The changes of work functions, highest occupied molecular orbitals, and core levels were measured with step-by-step deposition of Alq3 on each ZnO substrate. Although both substrates show similar electronic structures, a larger interface dipole is induced at the interface between Alq3 and ZnO nanowires. This results in the reduction of the electron injection barrier at the interface of Alq3/ZnO nanowires. Thus, the ZnO nanowire substrate is expected to show better performance than that of ZnO film when used as a cathode. We discussed the different interface dipole formation at each interface.