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

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• 作者：Kihyon Hong ; Kisoo Kim ; Sungjun Kim ; Illhwan Lee ; Hyunsu Cho ; Seunghyup Yoo ; Ho Won Choi ; Nam-Yang Lee ; Yoon-Heung Tak ; Jong-Lam Lee
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：March 3, 2011
• 年：2011
• 卷：115
• 期：8
• 页码：3453-3459
• 全文大小：999K
• 年卷期：v.115,no.8(March 3, 2011)
• ISSN：1932-7455

文摘

We investigated the optical properties of a dielec-tric鈭抦etal鈭抎ielectric 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 脜)/Ag (120 脜)/WO₃ (300 脜) 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 WO₃/Ag/WO₃ multilayer, the maximum luminance value (J = 220mA/cm²) increased from 8400 to 11700 cd/m², and the power efficiency increased about 26%. To improve the electron injection efficiency at the cathode region, a 20-脜 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/cm² 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.