Kihyon Hong

Enhanced Light Out‐Coupling of Organic Light‐Emitting Diodes: Spontaneously Formed Nanofacet‐Structured MgO as a Refractive Index Modulation Layer

Organic light-emitting diodes (OLEDs) have attracted attention owing to their potential applications in full color fl at panel displays, as a back-lighting source for liquid-crystal displays. in fl exible display devices, and in solid-state lighting. [ 1–3 ] Typical bottom-emitting OLEDs are composed of a glass substrate, a transparent indium-tin-oxide (ITO) anode, thin organic multilayers, and a refl ective metal cathode. [ 4 ] The internal quantum effi ciency and lifetime have seen dramatic improvements since reports of organic fl uorescent OLEDs, and devices using phosphorescent emitting materials are operating at nearly 100% internal quantum effi ciencies. [ 5 ] However, the majority of the light generated in the organic material is confi ned in the ITO anode and glass substrate due to the large difference in the refractive indices n between their layers ( n ITO = 1.9, n glass = 1.5). [ 2 ] As explained by classical ray optics theory (i.e., Snell’s law), this results in out-coupling effi ciencies ( η out ) – expressed as the ratio of surface emission to all emitted light – of only around 20%. [ 6,7 ] The remaining 80% of the photons are trapped in the organic and substrate layers. This low out-coupling effi ciency has become one of the main limitations to highly effi cient OLEDs. Hence, the greatest potential for a substantial increase in external quantum effi ciency and power effi ciency is to enhance the light out-coupling effi ciency of OLEDs. Many techniques, such as microlenses on the glass substrate, photonic crystals, high refractive index substrates, low-index grids, and low-index-silica aerogels, have been studied to enhance η out . [ 6 , 8–12 ] Although these methods can increase the η out , they have several limitations, such as a shifted output spectra, changes in the electrical properties, complex processing techniques, and high-cost fabrication procedures. [ 13 , 14 ]

Enhancement of electrical property by oxygen doping to copper phthalocyanine in inverted top emitting organic light emitting diodes

We reported the evidence of oxygen doping to copper-phthalocyanine (CuPc) by O2-plasma treatment to inverted top-emitting organic light-emitting diodes (ITOLEDs). In situ synchrotron-radiation photoelectron spectroscopy results showed that a new Cu–O bond appeared and the energy difference between the highest-occupied molecular orbital and EF is lowered by 0.15 eV after plasma treatment. The oxygen ions chemically interacted with Cu atoms and transferred charges to the CuPc. Thus the hole injection barrier was lowered, enhancing the electroluminescent property of ITOLEDs.

Effect of magnesium oxide buffer layer on performance of inverted top-emitting organic light-emitting diodes

The effect of magnesium oxide (MgO) buffer layer between cathode and emitting materials on performance of inverted top-emitting organic light-emitting diodes (ITOLEDs) was investigated. The operation voltage at the current density of 100mA∕cm2 decreased from 14.9to9.7V for ITOLEDs with 1nm thick MgO buffer layers. The maximum luminance value increased about 78% in ITOLEDs using MgO buffer layer, which is 1000cd∕m2 at the current density of 191mA∕cm2. Synchrotron radiation photoelectron spectroscopy results revealed that the atomic concentration of Al–O bond increased after deposition of MgO on Al, indicating the oxidation of Al surface. Secondary electron emission spectra showed that the work function increased about 0.8eV by inserting the insulating MgO buffer layer. Therefore, the enhancement of device performance results from the decrease of the energy barrier for electron injection based on the tunneling model.

Effect of N2, Ar, and O2 plasma treatments on surface properties of metals

We report the effect of N2, Ar, and O2 plasma treatments on the surface properties of metals. The carbon atoms reduced more in O2 and Ar plasma than in N2 plasma due to a chemical reaction with O2 plasma and large plasma density in Ar plasma. A water contact angle decreased after the plasma treatment regardless of the kinds of plasma gas, showing the increase in the hydrophilicity in surfaces. Synchrotron radiation photoemission spectroscopy data showed that the work function increased after N2, Ar, and O2 plasma treatments in sequence. This is due to the reduction of carbon atoms and the formation of O-rich surface in O2 plasma case.

Inverted Top-Emitting Organic Light-Emitting Diodes Using Transparent Silver Oxide Anode Formed by Oxygen Plasma

We demonstrate an inverted top-emitting organic light-emitting diode using a transparent silver oxide (AgO x ) anode formed by oxygen plasma treatment. The turn-on voltage decreased from 22 to 12 V, and the luminance increased from 175 to 630 cd/m 2 at 150 mA/cm 2 as the Ag anode was treated by O 2 plasma. The measurement of synchrotron radiation photoelectron spectroscopy and ultraviolet photoelectron spectroscopy revealed that the Ag changed AgO x : and the work function was increased by 0.45 eV during O 2 plasma treatment. The AgO x lowered the injection barrier for holes, enhancing the electroluminescent properties.

Enhancement of hole injection in pentacene organic thin-film transistor of O2 plasma-treated Au electrodes

The effect of oxygen plasma treatment on enhancement of hole mobility was demonstrated using pentacene organic thin-film transistors (OTFTs) with bottom-contact Au electrodes. Linear field-effect mobility increased from 3.2×10−2to7.4×10−2cm2∕Vs as the Au electrodes were treated with O2 plasma. Secondary electron emission spectra revealed that the work function of oxygen plasma-treated Au is 0.5eV higher than that of untreated Au. This led to a reduced hole injection barrier at the interface of Au with pentacene, increasing the field-effect mobility of OTFTs.

Self-Supporting Ion Gels for Electrochemiluminescent Sticker-Type Optoelectronic Devices

Nowadays, there has been an increasing demand to develop low-cost, disposable or reusable display devices to meet and maximize short-term user convenience. However, the disposable device has unfortunately not materialized yet due to the light-emitting materials and fabrication process issues. Here, we report sticker-type electrochemiluminescent (ECL) device using self-supporting, light-emitting gel electrolytes. The self-supporting ion gels were formulated by mixing a network-forming polymer, ionic liquid, and metal complex luminophore. The resulting ion gels exhibit excellent mechanical strength to form free-standing rubbery light-emitting electrolyte films, which enables the fabrication of sticker-type display by simple transfer and lamination processes on various substrates. The sticker-type ECL devices can be operated under an AC bias and exhibit a low operating voltage of 4 V (peak-to-peak voltage) with a maximum luminance of 90 cd/m2. It is notable that the result is the first work to realize sticker displays based on electrochemical light emitting devices and can open up new possibilities for flexible or disposal display.

Reduced Graphite Oxide-Indium Tin Oxide Hybrid Materials for use as a Transparent Electrode

for use as a Transparent Electrode Kyoung Soon Choi, Yensil Park, Ki-Chang Kwon, Jooheon Kim, Chang Keun Kim, Soo Young Kim, Kihyon Hong and Jong-Lam Lee b,z School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul, 156-756, Korea Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk, 790-784, Korea

Change of interface dipole energy with interfacial layer thickness and O2 plasma treatment in metal/organic interface

The authors determined the interface dipole energies between interfacial layers with different thicknesses coated on indium tin oxides (ITOs) and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl using ultraviolet and synchrotron radiation photoemission spectroscopy. The interface dipole energy increased as a function of interfacial layer thickness up to 4nm. After O2 plasma treatment on thick-metal (>4nm) coated ITO, the work function and interface dipole energy increased. In thin-metal (<2nm) coated ITO, no change in the interface dipole energy was found though the work function increased. Thus, the O2 plasma treated thin (<2nm) interfacial layer reduced the hole injection barrier.

Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.