Se Jong Lee

Electron injection enhancement by diamond-like carbon film in organic electroluminescence devices

Abstract Diamond-like carbon (DLC) is deposited as an electron injection layer between the organic layer and Al cathode electrode in organic electroluminescence devices (OLEDs) by the use of Cs+ ion gun sputter deposition system. The devices consist of; indium tin oxide/PEDOT:PSS/MEH-PPV/DLC/aluminum. Electron injection properties have been investigated by I–V characteristics at various deposition conditions and the mechanism of electron injection enhancement due to a thin DLC layer has been studied. The characteristics of DLC film deposited at various negative carbon ion energies are examined by Raman spectroscopy. Optical microscope is also used to observe the Al surface after the light emission test. It is found that the sp2/sp3 ratio and electron affinity of DLC film are varied as the carbon ion energy increases from 50 to 200 eV, which enhance the electron injection and improve the property of OLEDs. An order magnitude increase in current is obtained from DLC layer inserted devices and the device stability is also improved.

High-k and low-k nanocomposite gate dielectrics for low voltage organic thin film transistors

CeO2–SiO2 nanocomposite films were used as the gate dielectrics in organic thin film transistors (OTFTs) with pentacene active semiconductor. CeO2–SiO2 composite films exhibited a high dielectric capacitance of 57nF∕cm2 with exceptionally low leakage current. Good device characteristics were obtained with saturation at low operating voltages (∼2V) and with a field effect mobility of 0.84cm2V−1s−1, a threshold voltage of ∼0.25V, an on/off current ratio of 103, and a subthreshold slope of 0.3V∕decade, whereas the gate leakage current density is considerably lowered. These results should therefore increase the prospects of using OTFTs in low power applications such as portable devices.

Superhydrophobic modification of gate dielectrics for densely packed pentacene thin film transistors

Pentacene organic thin film transistors (OTFTs) with low-k and high-k hybrid gate dielectrics by CF4 plasma treatment exhibited excellent device performance with field effect mobilities (maximum 1.41cm2∕Vs), a low threshold voltage of +1V, and on/off current ratios of 105 at −5V gate bias. After CF4 plasma treatment, fluorine atoms diffuse into the interior low-k polymer and eliminate ionic impurities which reduce the leakage current density and overall pentacene initial growth on the superhydrophobic surface is significantly improved. It seems apparent that proper surface treatment is desirable for higher quality pentacene film and improving the performance of OTFTs.

Structural and Electrical Properties of Solution-Processed Gallium-Doped Indium Oxide Thin-Film Transistors

We fabricated solution-processed gallium-doped indium oxide (GIO) thin-film transistors (TFTs). The electrical property, crystallinity, and transmittance were investigated as a function of gallium content. Varying the gallium/indium ratio is found to have a significant effect on structural and electrical properties of thin films. The shrinkage of the lattice of a GIO film originates from substitution of Ga on In sites in the In2O3 lattice, which was verified by X-ray diffraction (XRD) analysis. By increasing the gallium ratio of the channel material, the GIO film shows an amorphous phase. The optimized GIO film (Ga/In= 0.35) has an electron mobility of 3.59 cm2 V-1 s-1, a threshold voltage of 0.1 V, an on/off current ratio of 8.2×107, and a subthreshold slope of 0.9 V/decade, and is highly transparent (~92%) in the visible region.

Low-Temperature, Solution-Processed Zinc Tin Oxide Thin-Film Transistors Fabricated by Thermal Annealing and Microwave Irradiation

In this study, we fabricated zinc tin oxide (ZTO) thin-film transistors (TFTs) using a sol–gel solution at a low annealing temperature of 350 °C. We found that the combination of conventional hot plate annealing and microwave irradiation was effective for achieving high performance of ZTO-TFTs. Solution-processed ZTO-TFTs prepared at 350 °C by microwave irradiation showed enhanced device characteristics of 1.84 cm2 V-1 s-1 mobility and a 106 on/off current ratio. X-ray photoelectron spectroscopy analyses confirmed that residual hydroxyls in solution-processed ZTO films can be decomposed at low temperatures by microwave irradiation. Low-temperature microwave-assisted processing makes ZTO TFTs promising for use in flexible, transparent electronics.

Polymeric tandem organic light-emitting diodes using a self-organized interfacial layer

The authors have demonstrated efficient polymeric tandem organic light-emitting diodes (OLEDs) with a self-organized interfacial layer, which was formed by differences in chemical surface energy. Hydrophilic poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) was spin coated onto the hydrophobic poly(9,9-dyoctilfluorene) (PFO) surface and a PEDOT:PSS bubble or dome was built as an interfacial layer. The barrier heights of PEDOT:PSS and PFO in the two-unit tandem OLED induced a charge accumulation at the interface in the heterojunction and thereby created exciton recombination at a much higher level than in the one-unit reference. This effect was confirmed in both the hole only and the electron only devices.

Influence of the Oxygen Concentration of YSZ Gate Dielectric Layer on the Low Voltage Operating Pentacene Thin Film Transistor

We report on the influence of oxygen concentration on structural and electrical properties of the amorphous yttria-stabilized zirconia (YSZ) thin films which are the potential high-k gate dielectric material of organic thin film transistors (OTFTs). The films were prepared by the E-beam evaporation process. To investigate the influence of the oxygen flow rate on the structural and electrical properties of the YSZ films, X-ray diffraction, X-ray photoelectron spectroscopy, current density-electric field, current-voltage were carried out in this work. Oxygen vacancies are expected to be the most predominant type of defect in metal-oxide dielectrics. The leakage current density decreased mainly because of the reduction of oxygen vacancies with increasing oxygen flow rate.

Homogeneous liquid crystal alignment on inorganic–organic hybrid silica thin films derived by the sol–gel method

We developed an inorganic–organic hybrid thin film via the sol–gel method for a new liquid crystal alignment layer and investigated the influence of an organic species on the alignment characteristics of the liquid crystals (LCs). A thin film of methyl-doped amorphous silicon oxide (a-SiOx:CH3) was fabricated from the hydrolysis and condensation reaction of the initial precursors of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) at a proper ratio. A low-energy ion beam (IB) treatment gives rise to the homogeneous alignment of LC in an IB condition on a-SiOx:CH3 thin film; however, it is difficult to control the LC alignment on a-SiOx thin film derived only from TEOS as a precursor. The LC alignment depending on the chemical structure of the silica thin film was investigated and analyzed in terms of the sensitivity of the axis-selective destruction of the chemical bonding on the surface of the thin silica film.

Low-Temperature, Aqueous-Solution-Processed Zinc Tin Oxide Thin Film Transistor

We fabricate solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs). The solution used is prepared by precipitating metal hydroxide using NaOH and dissolving it using NH4OH. The X-ray diffraction (XRD) data of the spin-coated ZTO film demonstrates an amorphous phase, and the atomic force microscopy (AFM) image shows a smooth surface. The device performance of solution-processed TFTs was analyzed as a function of annealing temperature. The fabricated TFTs were operated in the enhancement mode, and exhibited a carrier mobility of 3.03 cm2 V-1 s-1, a threshold voltage of 10.2 V, an on/off current ratio of 1.23×107, a subthreshold slope of 0.78 V/decade, and high transparency (with ~90% transmittance) at a low annealing temperature of 300 °C.

Observation of the Hysteresis Behavior of Pentacene Thin-Film Transistors in I–V and C–V Measurements

Current-voltage and capacitance-voltage measurements of the organic thin-film transistors (OTFTs) were performed to observe the change of the surface state of gate dielectrics using various surface treatments. The hexamethyldisilazane (HMDS)-treated OTFTs, which has few OH groups on the dielectric surface, showed little hysteresis behavior than that of other surface-treated OTFTs. In particular, when the positive gate bias (for a depletion state) was applied, more hysteresis was observed, indicating that OH groups on a gate dielectric are responsible for the electron trapping in the channel. However, when the surface is terminated with (CH 3 ) 3 -Si, eliminating OH groups by HMDS treatment, much less hysteresis was observed in the positive direction. Therefore, it seems apparent that an OH-free surface is desirable for a more stable operation of OTFTs.