Optical Properties of WO3/Ag/WO3 Multi layer As Transparent Cathode in Top-Emitting Organic Light Emitting Diodes

Abstract

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 WO3 had a high value of n (>2.0), a thin WO3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO3/Ag/WO3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO3 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 WO3 (300 angstrom)/Ag (120 angstrom)/WO3 (300 angstrom) structure, a high transmittance of similar to 93.5% and a low sheet resistance about similar to 7.22 ohm/sq were obtained. When the top Al cathode was replaced with the WO3/Ag/WO3 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-angstrom thick Al layer was introduced as an electron injection interlayer between the organic materials and the WO3/Ag/WO3 cathode. Using the Al interlayer decreased the operation voltage at J = 10 mA/cm(2) by 6.9 V. Thus, a WO3/Ag/WO3 with an Al interlayer could promote the transparency of the top cathode and lower the electron injection barrier, enhancing the electroluminescent properties of TOLED.