Taehyoung Zyung

Circuit-Model-Based Analysis of a Wireless Energy-Transfer System via Coupled Magnetic Resonances

A simple equivalent-circuit model is developed for a wireless energy-transfer system via coupled magnetic resonances, and a practical design method is also provided. Node equations for the resonance system are built with the method, expanding on the equations for a transformer, and the optimum distances of the coils in the system are derived analytically for optimum coupling coefficients for high transfer efficiency. In order to calculate the frequency characteristics for a lossy system, the equivalent model is established at an electric-design automation tool. The model parameters of the actual system are extracted, and the modeling results are compared with measurements. Through the developed model, it is seen that the system can transfer power over a midrange of a few meters and that impedance matching is important to achieve high efficiency.

Hysteresis of pentacene thin-film transistors and inverters with cross-linked poly(4-vinylphenol) gate dielectrics

The authors report the effects of hydroxyl groups (OH bonds) on the electrical reliabilities of pentacene organic thin-film transistors (OTFTs) with poly-4-vinylphenol (PVP) gate dielectrics. PVP gate dielectric films mixed with different concentrations of methylated poly(melamine-co-formaldehyde) (MMF) were fabricated, and experiments on the hysteresis behavior of the OTFT device were conducted. Pentacene TFTs with the PVP (MMF 0wt.%) exhibited a large hysteresis, while in the PVP (MMF 125wt.%), nearly no hysteresis was observed. Large hysteresis observed in OTFT devices was confirmed to be strongly related to the hydroxyl groups existing inside of the polymeric dielectrics and could reduced by the decrease of OH group.

Deep-level defect characteristics in pentacene organic thin films

Organic thin-film transistors using the pentacene as an active electronic material have shown the mobility of 0.8 cm2/V s and the grains larger than 1 μm. To study the characteristics of electronic charge concentrations and the interface traps of the pentacene thin films, the capacitance properties were measured in the metal/insulator/organic semiconductor structure device by employing the capacitance–voltage and deep-level transient spectroscopy (DLTS) measurements. Based on the DLTS measurements, the concentrations and the energy levels of hole and electron traps in the obtained pentacene films were formed to be approximately 4.2×1015 cm−3 at Ev+0.24 eV, 9.6×1014 cm−3 at Ev+1.08 eV, 6.5×1015  cm−3 at Ev+0.31 eV and 2.6×1014 cm−3 at Ec−0.69 eV.

Photodegradation of poly(p ‐ phenylenevinylene) by laser light at the peak wavelength of electroluminescence

We report the photodegradation of poly(p‐phenylenevinylene) polymer when the film was irradiated with laser light at a wavelength corresponding to the peak wavelength of electroluminescence. Degradation in photoluminescent properties was significant in an air environment but not under vacuum. This indicates that the oxygen in air aids photodegradation and this hypothesis was confirmed by optical spectroscopy. This degradation may be associated with long‐term stability of the electroluminescence device.

Impedance spectroscopy of single- and double-layer polymer light-emitting diode

ac electrical properties of the heterostructured polymer light-emitting diode consisting of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH–PPV) and sodium sulfonated polystyrene (SSPS) ionomer are studied. It is found that the carrier in SSPS is not the ion but the electron, and the conduction mechanism seems to be a space-charge limited current with exponential trap distribution. Based on the results of the impedance analysis, we propose a microscopic model to explain the enhancement of the electroluminescence efficiency of the indium tin oxide/MEH–PPV/SSPS/Al structure.

Polymer light-emitting devices using ionomers as an electron injecting and hole blocking layer

The effect of ion concentration, neutralization level and counterions in ionomers was systematically studied to obtain the optimal electroluminescent (EL) characteristics in polymer light-emitting diodes using poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) for the emissive layer and sulfonated polystyrene (SPS) ionomers for the electron-injecting layer. The optimum ion concentration of NaSPS was determined to be at 6.7 mol %. Ionomers with a higher neutralization level make the EL device more efficient, with the highest efficiency being at 200% overneutralization. The ionomer with a smaller metal counter ion greatly enhances the efficiency of EL devices with the indium–tin–oxide/MEH-PPV/LiSPS/Al device having the highest EL quantum efficiency, 1.18% photons/electron. The dominant factor in enhancing the luminance is the number of ionic dipoles near the cathode irrespective of the type of metal counterions, while the hole blocking mostly depends on the restriction of chain segmental ...

Optimization of wireless power transmission through resonant coupling

We investigate the vaious experimental setups to find the optimum condition for the wireless power transmission through resonant coupling. We can transmit the power with 20% efficiency up to 2 m distance and light the LED lamp by transmitting 30 watt.

Tris-(8-hydroxyquinoline)aluminum-based organic light-emitting devices with Al/CaF2 cathode: Performance enhancement and interface electronic structures

The device characteristics and the interface electronic structures of organic light-emitting devices based on tris-(8-hydroxyquinoline)aluminum were investigated with Al/CaF2, Al/LiF, and Al-only cathodes. Similar to the Al/LiF cathode, the Al/CaF2 cathode greatly improved the performance of the device over the Al-only cathode. However, a photoelectron spectroscopy study revealed that despite the performance improvement, the evolution of the new peaks during the Al/CaF2 cathode formation closely resembled those of the Al-only cathode rather than the Al/LiF cathode.

Electroluminescence of polymer blend composed of carbazole group contained PMA and MEH-PPV

Abstract Alkylstyryl carbazole group containing poly(methacrylate) (Cz-PMA) was prepared and blended with poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). The blends were characterized by UV-Vis, photoluminescence (PL), and electroluminescence (EL) studies. The polymer blends showed two isolated PL emission peaks at 440 and 560 nm corresponding to their component polymers, but the only one EL emission peak of the MEH-PPV/Cz-PMA blended light-emitting diode (LED) appeared near 580 nm which corresponds to that of MEH-PPV. This result indicates that the energy of excited Cz-PMA was transferred into MEH-PPV, resulting in the enhancement of EL intensity.

Use of ionomer as an electron injecting and hole blocking material for polymer light-emitting diode

Light-emitting diodes of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEHPPV) are fabricated using sodium sulfonated polystyrene (SSPS) ionomer containing 10 mol % ionic groups as an electron injecting and hole blocking material. When an electric field stronger than 2.7×107 V/m is applied, SSPS in the indium–tin oxide ITO/MEH-PPV/SSPS/Al system causes a bridging effect between sodium ions and the Al cathode at the SSPS/Al interface, leading to excellent electron injection. Furthermore, the ionomer has a high band gap energy of ∼5 eV, resulting in hole blocking. The operating voltage for the ITO/MEH-PPV/SSPS/Al is reduced by ∼60% and the relative quantum efficiency is enhanced by three orders of magnitude compared with those of the corresponding single-layer MEH-PPV device.