Effects of Surface Morphology of ITO Thin Film on the Instability of Organic Light Emitting Diodes
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Effects of Surface Morphology of ITO Thin Film on the Instability of Organic Light Emitting Diodes Ki-Beom Kim, Yoon-Heung Tak , Hyoung-Guen Park, Kwang-Ho Lee1 and Jong-Ram Lee1 OEL Development Gr., PDP Division, LG Electronics 191-1 Kongdandong, Kumi, 730-030, Korea 1 Dept. of Material Science and Engineering, POSTECH San 31 Hyojadong, Pohang, 790-784, Korea ABSTRACT Surface roughness of ITO (Indium Tin Oxide) was measured by AFM (Atomic Force Microscope) and the existence of spikes was verified. Then OLED (Organic Light Emitting Diode) devices made from those substrates were fabricated and their electrical properties were measured with an I-V measurement system. There is considerable difference in the reverse leakage current of OLEDs depending on the surface roughness of the substrate. We observed a good correlation of reverse leakage current with Rpv (Peak-to-Valley roughness), but the correlations between reverse leakage current and Ra (Average roughness) and Rrms (Root-MeanSquare roughness) are not as good. INTRODUCTION OLEDs are very attractive candidates as display devices and for various applications. Since Tang et al’s report on organic electroluminescence devices [1], OLEDs have been widely studied by many researchers and developed to commercial products. Because OLEDs have advantages for use in mobile displays, such as low power consumption, light weight and thinness, much effort has been made to develop robust mobile displays with OLEDs. In addition, OLED displays have excellent features as follows: high luminescence (Over than 100,000 cd/m2); high light emission efficiency (Over 10 lm/W); low voltage DC driving is possible; fast response time (~micro second); possibility of full color display by light emitting material itself. ITO film is the most widely used material as the transparent electrode of OLEDs [2,3] and also in many other display devices like LCDs(Liquid Crystal Displays) or PDPs(Plasma Display Panels). However, there remain some problems to be solved to enhance the stability and efficiency of OLEDs. The control of surface morphology of ITO is one of the most serious problems. To fabricate OLEDs with long lifetime, very smooth surface morphology of ITO is BB3.24.1
needed. Especially, the uneven surface of ITO is the crucial reason for defect generation. Because all the functional organic thin films are deposited on ITO, the surface morphology of ITO is directly transferred to the morphology of the organic layers. In general, organic layers have a thickness of about 1000Å. Therefore, uneven surface of the hole injecting electrode can produce negative effects on device performance. So, ITO works not only as the hole injection electrode, but also the bottom electrode of OLEDs. R. Castro-Rodriguez et al [4] studied the effect of ITO substrates on the morphology, structural and optical properties of CdS thin films. In their result, the residual strain of CdS thin films were increased with the increase in the surface roughness of ITO. L.M. Do et al [5] suggested that instability of the organic/
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