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.

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.

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Highly transparent and efficient red phosphorescent inverted top-emitting organic light emitting diodes were investigated by using a surface-modified tungsten oxide ͑WO 3 ͒/silver/WO 3 ͑WAW͒ anode. A thin buckminsterfullerene ͑C60͒ dipole layer was introduced for the surface treatment of the WAW anode, and the thickness of the surface-modified WO 3 was controlled to optimize the WAW anode for hole injection. The optimum thickness of the surface-modified WO 3 was 5 nm, and the C60 dipole layer further improved the hole injection from the WAW anode to the hole transport layer.In recent years, organic electronic devices have been actively studied because of the merits of a large printing area, 1 a low cost fabrication process, 2 and flexibility. 3 In particular, there have been many researches about organic light emitting diodes ͑OLEDs͒ 4,5 for applications as a backlight for liquid crystal displays and flexible displays. Since the discovery of high efficiency OLEDs during the 1980s and 1990s, 4,6 many studies have been conducted to improve the electroluminescent properties of the OLEDs.In the past decade, inverted top-emitting organic light emitting diodes ͑ITOLEDs͒ 7-10 with reflective bottom cathodes have been developed. Common top-emitting OLEDs are compatible with p-type transistors, while the ITOLEDs can be combined with n-type thin film transistors. 7-11 In general, ITOLEDs require a high quality transparent anode, and commonly gold ͑Au͒ semitransparent anodes have been used because of their adequate hole injection. 7,11-13 However, the thickness of the Au film is limited in practical usage because of low transmittance. Several types of hole injection mechanisms have been proposed. Chen et al. 14 observed that the ITOLED with a surface-modified silver ͑Ag͒ anode treated with a UV/ozone exhibited better device performance than a bottom-emitting device. Silver oxide ͑Ag 2 O͒ was generated during the UV/ozone treatment, and it played a role of the interfacial layer. Thin tungsten oxide ͑WO 3 ͒ ͑ϳ5 nm͒ 11 was also used as the interfacial layer to reduce the potential barrier between the organic layer. Meyer et al. 12 showed that a thin ͑5 nm͒ WO 3 layer enhanced the device performance through an adequate band alignment between the organic layers and the anode. Recently, Leo et al. 9,15 also demonstrated the top-emitting OLEDs with a reasonable efficiency using Ag top electrodes.In this work, the surface morphology of the WO 3 /Ag/WO 3 ͑WAW͒ multianode was modified by reducing the thickness of WO 3 in contact with the organic layer from 40 to 5 nm. This thickness reduction affected the band alignment between the organic layer and the WAW anode. It was demonstrated that holes were directly injected from the Ag layer to the organic layer through the thin WO 3 layer ͑5 nm͒ in the WAW multianode.Multielectrodes 10,16-18 are another alternative for the electrodes in transparent OLEDs. Indium zinc oxide ͑IZO͒/Ag/IZO, indium tin oxide ͑ITO͒/Ag/ITO, and WAW have been used as highly transparent multielectrodes with low she...