Kyu-Sung Lee

Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic light-emitting diodes

We report on plasma damage-free sputtering of an indium zinc oxide (IZO) top cathode layer for top-emitting organic light-emitting diodes (TOLEDs) by using a box cathode sputtering (BCS) technique. A sheet resistance of 42.6Ω∕cm and average transmittance above 88% in visible range were obtained even in IZO layers deposited by BCS at room temperature. The TOLED with the IZO top cathode layer shows electrical characteristics and lifetime comparable to a TOLED with only thermally evaporated Mg–Ag cathode. In particular, it is shown that the TOLED with the IZO top cathode film shows very low leakage current density of 1×10−5mAcm2 at reverse bias of −6V. This suggests that there is no plasma damage caused by the bombardment of energetic particles during IZO sputtering using the BCS system.

Direct Al cathode layer sputtering on LiF∕Alq3 using facing target sputtering with a mixture of Ar and Kr

Using facing target sputtering (FTS) with a mixture of Ar and Kr, direct Al cathode sputtering on LiF∕Alq3 layers was accomplished without the need for a protective layer against plasma damage. Organic light emitting diodes (OLEDs) with a directly sputtered Al cathode in a mixture of Ar and Kr showed a much lower leakage current density (∼1×10−5mA∕cm2 at −6V) than those (∼1×10−1mA∕cm2 at −6V) of OLEDs with an Al cathode prepared by FTS or dc sputtering in a pure Ar ambient. This indicates that the bombardment of energetic particles is effectively restricted by mixing a heavy noble gas. Based on the current-voltage curve for the OLED, a possible mechanism is proposed to explain the effect of a heavy noble gas mixture on electrical properties of OLEDs for direct Al cathode sputtering by FTS.

Characteristics of Indium Zinc Oxide Top Cathode Layers Grown by Box Cathode Sputtering for Top-Emitting Organic Light-Emitting Diodes

Electrical, optical, and structural properties of indium zinc oxide (IZO) films grown by a box cathode sputtering (BCS) were investigated as a function of oxygen flow ratio. A sheet resistance of 42.6 Ω/□, average transmittance above 88% in visible range, and root-mean-square roughness of 2.7 A were obtained even in the IZO layers grown at room temperature. The top-emitting organic light-emitting diodes (TOLEDs) with the IZO top cathode layer were prepared and their electrical properties compared with TOLEDs with Mg-Ag/indium tin oxide (ITO) cathode layer and TOLEDs with only thermally evaporated Mg-Ag cathode layer. In addition, the effects of oxygen flow ratio in IZO films are investigated, based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet/visible (UV/VIS) spectrometer, scanning electron microscopy (SEM), and atomic force microscopy (AFM) analysis results.

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

Transparent conductive electrodes of mixed TiO2−x–indium tin oxide for organic photovoltaics

A transparent conductive electrode of mixed titanium dioxide (TiO2−x)–indium tin oxide (ITO) with an overall reduction in the use of indium metal is demonstrated. When used in organic photovoltaic devices based on bulk heterojunction photoactive layer of poly (3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester, a power conversion efficiency of 3.67% was obtained, a value comparable to devices having sputtered ITO electrode. Surface roughness and optical efficiency are improved when using the mixed TiO2−x–ITO electrode. The consumption of less indium allows for lower fabrication cost of such mixed thin film electrode.

Effect of Introducing H2O Vapor on Properties of RF Sputter-Grown ITO Anode Layer for OLEDs

We report the effects of introducing H 2 O vapor on the structural, surface, and electrical properties of radio-frequency (rf) sputtered indium tin oxide (ITO) anode layer for organic light-emitting diodes (OLEDs). By introducing H 2 O vapor during rf sputtering, we obtained an ITO anode film with lower resistivity, higher transmittance, and a smoother surface relative to the conventional rf-sputter-grown ITO anode even though it was grown at a substrate temperature of 200°C. Secondary ion mass spectroscopy analysis clearly shows that OH content in the rf-sputter-grown ITO anode film was significantly increased by adding H 2 O vapor. In addition, the current density-voltage-luminance result of an OLED fabricated on an H 2 O-vapor-incorporated ITO anode showed a lower turn-on voltage and higher luminescence than those of OLEDs fabricated on the reference ITO anode. This suggests that the introduction of H 2 O vapor during rf sputtering is an effective technique employed to improve the ITO anode layer for both top-and bottom-emitting OLEDs.

Molten-droplet synthesis of composite CdSe hollow nanoparticles.

Many colloidal synthesis routes are not scalable to high production rates, especially for nanoparticles of complex shape or composition, due to precursor expense and hazards, low yields, and the large number of processing steps. The present work describes a strategy to synthesize hollow nanoparticles (HNPs) out of metal chalcogenides, based on the slow heating of a low-melting-point metal salt, an elemental chalcogen, and an alkylammonium surfactant in octadecene solvent. The synthesis and characterization of CdSe HNPs with an outer diameter of 15.6 ± 3.5 nm and a shell thickness of 5.4 ± 0.9 nm are specifically detailed here. The HNP synthesis is proposed to proceed with the formation of alkylammonium-stabilized nano-sized droplets of molten cadmium salt, which then come into contact with dissolved selenium species to form a CdSe shell at the droplet surface. In a reaction-diffusion mechanism similar to the nanoscale Kirkendall effect it is speculated that the cadmium migrates outwardly through this shell to react with more selenium, causing the CdSe shell to thicken. The proposed CdSe HNP structure comprises a polycrystalline CdSe shell coated with a thin layer of amorphous selenium. Photovoltaic device characterization indicates that HNPs have improved electron transport characteristics compared to standard CdSe quantum dots, possibly due to this selenium layer. The HNPs are colloidally stable in organic solvents even though carboxylate, phosphine, and amine ligands are absent; stability is attributed to octadecene-selenide species bound to the particle surface. This scalable synthesis method presents opportunities to generate hollow nanoparticles with increased structural and compositional variety.

Enhanced performance of hybrid solar cells using longer arms of quantum cadmium selenide tetrapods

We demonstrate that enhanced device performance of hybrid solar cells based on tetrapod (TP)-shaped cadmium selenide (CdSe) nanoparticles and conjugated polymer of poly (3-hexylthiophene) (P3HT) can be obtained by using longer armed tetrapods which aids in better spatial connectivity, thus decreasing charge hopping events which lead to better charge transport. Longer tetrapods with 10u2009nm arm length lead to improved power conversion efficiency of 1.12% compared to 0.80% of device having 5u2009nm short-armed tetrapods:P3HT photoactive blends.