Seung Yoon Ryu

Transparent organic light-emitting diodes consisting of a metal oxide multilayer cathode

The authors have developed a semitransparent, multilayered cathode of indium tin oxide (ITO)/Ag/tungsten oxide (WO3) for transparent organic light-emitting diodes. The device showed a weak negative differential resistance (NDR), until the operating voltage of 8V was reached. NDR was due to the resonant tunneling by both the quantum barrier and quantum well. The silver oxide (Ag2O) on the Ag metal was confirmed by x-ray photoelectron spectroscopy, and the energy levels of Ag2O were quantized due to the quantum size effect and this produced the resonant tunneling channels. The device using ITO∕Ag∕WO3 with a LiF∕Al bilayer was superior to those devices which only used ITO or WO3, mainly because the out coupling was enhanced by employing a WO3 material, which is much more transparent than ITO.

High Efficiency Inorganic/Organic Hybrid Tandem Solar Cells

Hybrid tandem solar cells comprising an inorganic bottom cell and an organic top cell have been designed and fabricated. The interlayer combination and thickness matching were optimized in order to increase the overall photovoltaic conversion efficiency. A maximum power conversion efficiency of 5.72% was achieved along with a V(oc) of 1.42 V, reaching as high as 92% of the sum of the subcell V(oc) values.

Indium Oxide Thin-Film Transistors Fabricated by RF Sputtering at Room Temperature

Thin-film transistors (TFTs) were fabricated using an indium oxide (In₂O₃) thin film as the n-channel active layer by RF sputtering at room temperature. The TFTs showed a thickness-dependent performance in the range of 48-8 nm, which is ascribed to the total carrier number in the active layer. Optimum device performance at 8-nm-thick In₂O₃ TFTs had a field-effect mobility of 15.3 cm² · V^-1 · s^-1, a threshold voltage of 3.1 V, an ON-OFF current ratio of 2.2 × 10⁸, a subthreshold gate voltage swing of 0.25 V · decade^-1, and, most importantly, a normally OFF characteristic. These results suggest that sputter-deposited In₂O₃ is a promising candidate for high-performance TFTs for transparent and flexible electronics.

Self-assembled monolayer as an interfacial modification material for highly efficient and air-stable inverted organic solar cells

Organic solar cells with inverted structures can greatly improve photovoltaic stability. This paper reports a method to lower the work function of indium tin oxide (ITO) in inverted organic solar cells by modification with ultrathin 3-aminopropyltriethoxysilane (APTES) monolayers. The device studies showed that the resulting photovoltaic efficiencies were significantly increased from 0.64% to 4.83% with the use of the APTES monolayer, which could be attributed to the dramatic enhancement in the open-circuit voltage and fill factor. The effective electron selectivity in the case of the APTES-modified ITO could be attributed to the reduction of the work function of ITO as a result of the electron-donating nature of the amine groups in the APTES monolayer. The power conversion efficiency of the unencapsulated inverted organic solar cells with APTES-modified ITO remained above 80% of their original values even after storage in air for thirty days. Our results provide a promising approach to improve the performance of highly efficient and air-stable inverted organic solar cells.

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.

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.

Characterization and luminescence properties of Alq3 films grown by ionized-cluster-beam deposition, neutral-cluster-beam deposition and thermal evaporation

Tris-(8-hydroxyquinoline) aluminum (Alq3) films have been grown on silicon substrates by several techniques: neutral-cluster-beam deposition; thermal evaporation; and ionized-cluster-beam deposition technique. The films were characterized by low angle X-ray reflectivity, atomic force microscopy, Fourier transformed infrared (FTIR) spectroscopy and photoluminescence. According to the FTIR spectroscopy measurement, the spectra of all the Alq3 samples show almost the same signals of atomic binding regardless of the process conditions. However, the photoluminescence intensities of the films are different. When all the films are adjusted to the same thickness, neutral-cluster-beam deposition films show more intense photoluminescence than the thermal-evaporated ones, while ionized-cluster-beam deposition samples are found to be inferior in intensity. Since the photoluminescence intensity of the 8-hydroxyquinoline aluminum layers is one of the important factors for the performance of organic light emitting devices, the neutral-cluster-beam deposition seems to be a promising method for the film deposition of organic electroluminescence materials.

Increase in indium diffusion by tetrafluoromethane plasma treatment and its effects on the device performance of polymer light-emitting diodes

The effects of tetrafluoromethane (CF4) plasma treatment of indium-tin-oxide (ITO) anode on indium diffusion into a poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS) layer were studied. Auger electron spectroscopy (AES) depth profile showed that 0.2at.% indium was present in the PEDOT:PSS layer when ITO was not plasma treated. The plasma treatment of ITO increased the indium concentration to ∼6at.%. The increase in indium can be explained by an oxygen deficiency in the CF4 plasma treated ITO. The presence of indium in the PEDOT:PSS layer showed a correlation with performance degradation of polymer light-emitting diodes.

Effects of Gold-Nanoparticle Surface and Vertical Coverage by Conducting Polymer between Indium Tin Oxide and the Hole Transport Layer on Organic Light-Emitting Diodes

UNLABELLED The effect of varying degrees of surface and vertical coverage of gold nanoparticles (Au-NPs) by poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) ( PEDOT PSS), which was used as a capping layer between indium tin oxide (ITO) and a hole transport layer (HTL) on small-molecule fluorescent organic light-emitting diodes (OLEDs), was systemically investigated. With respect to the Au-NP loading amount and size, the resultant current densities influenced the charge balance and, therefore, the OLED device performance. When the capping layer consisted of ITO/Au-NPs/ PEDOT PSS+Au-NPs, superior device performance was obtained with 10-nm Au-NPs through increased surface coverage in comparison to other Au-NP PEDOT:PSS coverage conditions. Furthermore, the Au-NP size determined the vertical coverage of the capping layer. The current densities of OLEDs containing small Au-NPs (less than 30 nm, small vertical coverage) covered by PEDOT PSS decreased because of the suppression of the hole carriers by the Au-NP trapping sites. However, the current densities of the devices with large Au-NPs (over 30 nm, large vertical coverage) increased. The increased electromagnetic fields observed around relatively large Au-NPs under electrical bias were attributed to increased current densities in the OLEDs, as confirmed by the finite-difference time-domain simulation. These results show that the coverage conditions of the Au-NPs by the PEDOT PSS clearly influenced the OLED current density and efficiency.

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.