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We investigated the optical properties of a dielectric-metal-dielectric multilayer for the transparent top cathode in top-emitting organic light emitting diodes (TOLEDs). The optical transmittance of the metal layer was enhanced by depositing a dielectric material which had a high refraction index n below and above the metal (Ag) layer. Due to multiple reflections and interferences, the Ag layer sandwiched between dielectric materials with a high value of n can show improved transmittance. Because the WO 3 had a high value of n (>2.0), a thin WO 3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO 3 /Ag/WO 3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO 3 to obtain the best transmittance value were determined by theoretical calculation, and they agreed well with the experimental results. The best results were obtained for the thermally evaporated WO 3 (300 Å)/Ag (120 Å)/WO 3 (300 Å) structure, a high transmittance of ∼93.5% and a low sheet resistance about ∼7.22 ohm/sq were obtained. When the top Al cathode was replaced with the WO 3 /Ag/WO 3 multilayer, the maximum luminance value (J = 220 mA/cm 2 ) increased from 8400 to 11700 cd/m 2 , and the power efficiency increased about 26%. To improve the electron injection efficiency at the cathode region, a 20-Å thick Al layer was introduced as an electron injection interlayer between the organic materials and the WO 3 /Ag/WO 3 cathode. Using the Al interlayer decreased the operation voltage at J = 10 mA/cm 2 by 6.9 V. Thus, a WO 3 /Ag/WO 3 with an Al interlayer could promote the transparency of the top cathode and lower the electron injection barrier, enhancing the electroluminescent properties of TOLED.

The effects of O2 inductively coupled plasma (ICP) treatment on the chemical composition and work function of indium-tin-oxide (ITO) surface were investigated. Synchrotron radiation photoemission spectroscopy showed that the O2 ICP treatment resulted in the increase of the ITO work function by 0.8 eV. Incorporation of oxygen atoms near the ITO surface during the ICP treatment induced a peroxidic ITO surface, increasing the work function. The enhanced oxidation of a thin Ni overlayer on the O2-ICP-treated sample suggests that preventing the migration of oxygen atoms into the active region of organic light-emitting diodes is important for improving device lifetime.

We report the change of surface electronic structure of indium–tin–oxide (ITO) as a function of ultraviolet (UV)–ozone treatment time. The voltage of organic light emitting diodes at a current density of 100 mA/cm2 was reduced as the surface treatment time using UV–ozone was lengthened. X-ray photoelectron spectroscopy results showed that the relative concentration of carbon atoms decreased, but oxygen concentration increased relatively with UV–ozone treatment. This led to the increase in the ITO work function via the reduction of operation voltage.

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