Sunghun Lee

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

High-Efficiency Orange and Tandem White Organic Light-Emitting Diodes Using Phosphorescent Dyes with Horizontally Oriented Emitting Dipoles

Tandem white organic light-emitting diodes (WOLEDs) using horizontally oriented phosphorescent dyes in an exciplex-forming co-host are presented, along with an orange OLED. A high external quantum efficiency of 32% is achieved for the orange OLED at 1000 cd m(-2) and the tandem WOLEDs exhibit a high maximum EQE of 54.3% (PE of 63 lm W(-1)).

Effects of electrolytes variation on formation of oxide layers of 6061 Al alloys by plasma electrolytic oxidation

Plasma electrolytic oxidation(PEO) processes were carried out to produce ceramic layers on 6061 aluminum substrates in four kinds of electrolytes such as silicate and aluminate solution with and without sodium fluorosilicate. The PEO processes were carried out under a hybrid voltage (260 V DC combined with 200 V, 60 Hz AC amplitude) at room temperature for 5 min. The composition, microstructure and element distribution analyses of the PEO-treated layers were carried out by XRD and SEM & EDS. The effect of the electrolyte contents on the growth mechanism, element distribution and properties of oxide layers were studied. It is obvious that the layers generated in aluminate solutions show smoother surfaces than those in silicate solutions. Moreover, an addition of fluorine ion can effectively control the layer porosity; therefore, it can enhance the properties of the layers.

Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells

Advances in flexible optoelectronic devices have led to an increasing need for developing highly efficient, low-cost, flexible transparent conducting electrodes. Copper-based electrodes have been unattainable due to the relatively low optical transmission and poor oxidation resistance of copper. Here, we report the synthesis of a completely continuous, smooth copper ultra-thin film via limited copper oxidation with a trace amount of oxygen. The weakly oxidized copper thin film sandwiched between zinc oxide films exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400–800 nm and a sheet resistance of 9 Ω sq−1) and strong oxidation resistance. These values surpass those previously reported for copper-based electrodes; further, the record power conversion efficiency of 7.5% makes it clear that the use of an oxidized copper-based transparent electrode on a polymer substrate can provide an effective solution for the fabrication of flexible organic solar cells.

Characterization of ternary boron carbon nitride films synthesized by RF magnetron sputtering

Boron carbon nitride (BCxNy) films were deposited on silicon wafers by RF magnetron sputtering of boron and graphite targets. BCN films with different compositions were obtained by varying the sputtering power of the graphite target (60, 180, 240 W). Chemical bonding states, composition and structure of the films were investigated by FTIR, XPS and AES analysis. The BCN films synthesized in this work were found to be BCN hybridized amorphous material, whose microstructure was examined with TEM observation. Mechanical properties of BCN films were compared to find the effect of the carbon sputtering power and resultant compositional changes on this ternary system. A BCN film with hardness of 15 GPa was synthesized at the high power of carbon source. With a higher sputtering power of carbon, hardness, elastic modulus and friction coefficients of a BCN film were increased due to the increase of strong BC, CC and CN bonds in the amorphous BCN film.

A high performance transparent inverted organic light emitting diode with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile as an organic buffer layer

A high performance transparent inverted organic light emitting device with a total maximum luminance efficiency of 67 cd A−1 and power efficiency of 67 lm W−1 was realized using 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) as an organic buffer material. HATCN, a discoid organic molecule, effectively protected the underlying organic emission layers from damage caused by sputter deposition of the indium zinc oxide top electrode, and simultaneously showed good hole injection performance from the transparent top electrode into an organic hole transporting layer. Moreover, transparent inverted OLEDs show an average transmittance of around 81% in the visible range; this value is very close to that of ITO/glass itself.

Silicon nitride films prepared by high-density plasma chemical vapor deposition for solar cell applications

Abstract Silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Buried contact solar cells, passivated by the silicon nitride layer, show efficiency above 17%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of the SiN film. This paper presents the optimal refractive index of SiN for a single layer antireflection (SLAR) coating in solar cell applications.

Reduction of Series Resistance in Organic Photovoltaic Using Low Sheet Resistance of ITO Electrode

The series resistance of organic photovoltaic (OPV) devices was decreased by reducing the sheet resistance (R sh ) of the indium tin oxide (ITO) electrode, which leads to increasing device efficiency. The performance of bulk heterojunction OPVs was critically dependent on R sh of the ITO electrode. Upon reducing R sh of the ITO from 39 to 8.5 Ω/□, the fill factor and power conversion efficiency of OPV was improved (from 0.407 to 0.580 and from 1.63 ± 0.2 to 2.5 ± 0.1%, respectively) under an AM1.5 simulated solar intensity of 100 mW/cm 2 . The dependence of the series resistance on R sh of the ITO suggests the dominance of the bulk resistance of the ITO electrode as a limiting factor in practical cell efficiencies.

Fabrication of a Completely Transparent and Highly Flexible ITO Nanoparticle Electrode at Room Temperature

We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 × 10(-3) Ω cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.

Effects of Hybrid Voltages on Oxide Formation on 6061 Al-alloys During Plasma Electrolytic Oxidation

Abstract Plasma electrolytic oxidation (PEO) is carried out on 6061 Al-alloys in a weak alkaline electrolyte containing NaOH, Na 2 SiO 3 and NaCl. Centered on the correlation of composition and structure, analyses by means of X-ray diffration (XRD), scanning electron microscope (SEM) and energy dispersive spectrometry (EDS) are conducted on the specimens, which have been PEO-treated under hybrid voltages of different direct current (DC) values (140-280 V) with constant alternate current (AC) amplitude (200 V). Attention is paid to the composition, properties and growth mechanism of oxide layers formed with hybrid voltages. Moreover, the main effects of DC value are discussed. Ceramic layers with a double-layer structure which combines hard outer and soft inner layers are found to be consist of α-Al 2 O 3 , γ-Al 2 O 3 and mullite. With the DC values increasing, the growth of the ceramic layers tends to have increasingly obvious three-stage feature.