Ki-Beom Kim

Criteria for ITO (indium―tin―oxide) thin film as the bottom electrode of an organic light emitting diode

Abstract TCO (transparent conductive oxide) thin films such as ITO (indium tin oxide) have important roles in fabrication of OLED (organic light emitting diode) displays. Although ITO is the most widely used transparent conductor in the field of FPD (flat panel display) industry, there are some unique requirements from the viewpoint of OLED, which is different from other display devices such as LCD (liquid crystal display) and PDP (plasma display panel). Especially surface roughness and work function are very important to enhance the stability and efficiency of OLED. Because all functional organic layers which act as injection, transportation and emission layers are deposited on ITO, surface morphology of ITO is directly transferred to them and uneven interface is not desirable for the efficiency and stability of OLED. On the other hand, there are several different terminologies—Ra (average roughness), Rr.m.s. (root-mean-square roughness) and Rpv (peak to valley roughness)—which are used to interpret the surface roughness, are now all used altogether. In this work, we tried to define which parameter is the most suitable to characterize ITO for OLED and to explain the relationship between surface and I–V characteristics of OLED. To characterize the effect of surface morphology of the ITO electrode, light emitting diodes were fabricated using various types of ITO films which have different surface morphology and their dynamic characteristics were compared.

Mechanism for the increase of indium-tin-oxide work function by O2 inductively coupled plasma treatment

The effects of O2 inductively coupled plasma (ICP) treatment on the chemical composition and work function of indium-tin-oxide (ITO) surface were investigated. Synchrotron radiation photoemission spectroscopy showed that the O2 ICP treatment resulted in the increase of the ITO work function by 0.8 eV. Incorporation of oxygen atoms near the ITO surface during the ICP treatment induced a peroxidic ITO surface, increasing the work function. The enhanced oxidation of a thin Ni overlayer on the O2-ICP-treated sample suggests that preventing the migration of oxygen atoms into the active region of organic light-emitting diodes is important for improving device lifetime.

Effect of ultraviolet–ozone treatment of indium–tin–oxide on electrical properties of organic light emitting diodes

We report the change of surface electronic structure of indium–tin–oxide (ITO) as a function of ultraviolet (UV)–ozone treatment time. The voltage of organic light emitting diodes at a current density of 100u2009mA/cm2 was reduced as the surface treatment time using UV–ozone was lengthened. X-ray photoelectron spectroscopy results showed that the relative concentration of carbon atoms decreased, but oxygen concentration increased relatively with UV–ozone treatment. This led to the increase in the ITO work function via the reduction of operation voltage.

Enhancement of hole injection using O2 plasma-treated Ag anode for top-emitting organic light-emitting diodes

We report the enhancement of hole injection using AgOx layer between Ag anode and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in top-emitting organic light-emitting diode (OLED). The turn-on voltage of OLEDs decreased from 17 to 7V as Ag changed to AgOx by the surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased about 0.4eV by the O2 plasma treatment. This led to the decrease of the energy barrier for hole injection, reducing the turn-on voltage of OLEDs.

Relationship between Surface Roughness of Indium Tin Oxide and Leakage Current of Organic Light-Emitting Diode

The relationship between surface roughness of indium tin oxide (ITO) and leakage current of organic light-emitting diode (OLED) was investigated. The surface roughness of ITO was measured with atomic force microscope (AFM) before the device fabrication by using these substrates, OLEDs were fabricated and their electrical properties were measured. leakage current of OLEDs depending on the surface roughness of the substrate. In this letter, we report that leakage currents of OLEDs are highly dependent on the peak-to-valley roughness (Rpv) of ITO.

Band bending of LiF/Alq3 interface in organic light-emitting diodes

The insertion of LiF for an interlayer material between the Al cathode and tris-(8-hydroxyquinoline) aluminum (Alq3) in the organic light-emitting diodes (OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq3 layer.

Enhancement of hole injection using iridium-oxide-coated indium tin oxide anodes in organic light-emitting diodes

We report the enhancement of hole injection using an IrOx layer between indium tin oxide anodes and 4,4’-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in organic light-emitting diodes (OLEDs). The turn-on voltage of OLEDs decreased from 7V to 4V and the maximum luminescence value increased from 1200cd∕m2 to 1800cd∕m2 as the Ir layer changed to IrOx by surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.6eV as the Ir layer transformed into IrOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of OLEDs.

Novel patterning method using Nd:YAG and Nd:YVO4 lasers for organic light emitting diodes

Abstract A laser ablation method has been applied to the patterning of the cathode of organic light emitting diodes (OLEDs). Second harmonic (532xa0nm) from the diode pumped by solid state Nd:YAG and Nd:YVO4 lasers were used. The devices fabricated show the similar characteristics those made by using cathode separators. By using the “laser patterning method”, one step of photolithography process that required to making barrier rib can be removed. And this technique has less possibility of contamination of ITO/organic interface and superiority in the view point of pixel shrinkage, so that longer lifetime of device is expected.

Development of Asymmetric Twin Objective Lens Actuator Using Integrated Design Method

We propose an asymmetric twin objective lens actuator for a slim Blu-ray (BD) drive, which is compatible with a compact disc (CD) and a digital versatile disc (DVD). A special electromagnetic (EM) circuit was designed to generate high driving forces in the three-axis direction and an integrated design method was applied to perform multiphysics modeling and multiphysics analysis simultaneously. The design of experiment (DOE) and optimization were carried out to improve dynamic characteristics of the proposed actuator. Consequently, the proposed actuator was evaluated and verified by experiment.

Surface characteristics of indium-tin oxide cleaned by remote plasma

We investigated the remote oxygen and hydrogen plasma cleaning of indium-tin oxide and its surface electronic properties. Samples cleaned by hydrogen plasma after oxygen plasma cleaning showed the complete absence of surface contaminants while samples cleaned by only hydrogen or oxygen plasma showed some residual contaminants. Work function is mainly affected by oxygen plasma treatments while sheet resistance is more closely related to the removal of surface carbon contaminants. This study revealed that surface dipoles due to the O- ions are believed to have a more significant contribution to the change in work function than the reduction of Sn4+.