Optical Properties of WO₃/Ag/WO₃ Multilayer 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 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.

Nitrogen incorporation and composition facilitated tailoring of the optical constants and dispersion energy parameters of tungsten oxynitride films

“…These factors, as well as an increase of oxygen content seen in the same range as the lower values for the mechanical properties (Fig. 13), lead to an increasingly amorphous structure, lowering hardness and Young's modulus values [48]. Thus, the decreasing trend observed (Fig.…”

“…Using the chemical composition found through XPS and RBS analyses, the density and index of refraction (n b ) values for the bulk materials were calculated using the fraction of W-O and W-N bonds within WO x N y . The fractions of each bond was then used to estimate the new value of the bulk material by assuming the index of refraction for bulk WO 3 and WN, at~630 nm, to be 2.0 [48] and 3.8 [49,50], respectively, and their bulk densities to be 7.16 g/cm 3 [51] and 17.7 g/cm 3 [52,53], respectively. The measured values n at~630 nm from SE for W-O-N films were then used to calculate the approximate values of the density for the deposited W-O-N thin films as a function of nitrogen flow rate.…”

Transparent electrode of nanoscale metal film for optoelectronic devices

“…Metal/ dielectric and dielectric/metal/dielectric structures combine the characteristics of high electrical conductivity of metals and good mechanical ductility of thin films. [32][33][34][35][36][37] However, these structures have relatively low OT ∼80%. Ultrathin Ag electrodes are promising for this function because the OT and R s of these electrodes are comparable to or better than those of ITO electrodes.…”

Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes

“…Dielectric/metal/dielectric multilayer films are considered an efficient and facile transparent electrode scheme for the development of a variety of optoelectronic applications, including solar cell devices [1,2], light-emitting diodes [3,4], and general colorselective light absorbers [5e7], due to their superior optical (transmission > 90% [8]) and electrical properties (sheet resistance < 10 U/sq [8]). In addition, a dielectric/metal/dielectric multilayer film that is composed of relatively thin (<a few tens of nm) metal and dielectric layers is suitable for use in flexible optoelectronic devices [9,10].…”