Nano-Structure Control of Cathode Buffer-Layers and Luminescent Properties of Organic Electroluminescent Device
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NANO-STRUCTURE CONTROL OF CATHODE BUFFER-LAYERS AND LUMINESCENT PROPERTIES OF ORGANIC ELECTROLUMINESCENT DEVICE K. SHINBO, E. SAKAI, F. KANEKO, K. KATO, T. KAWAKAMI, T. TADOKORO1, S. OHTA1 AND R. C. ADVINCULA2 Dept of Electrical and Electronic Engineering, Niigata Univ, Ikarashi 2-8050, Niigata, Japan. 1 R&D Center, Nippon Seiki Co., Ltd., Hujihashi 1-190-1, Nagaoka 940-2141, Japan. 2 Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294-1240, U.S.A. ABSTRACT Organic light emitting diodes (OLEDs) containing cathode buffer layers of nanometer thickness were fabricated and their electrical and emitting properties were investigated. The OLEDs have an indium tin oxide (ITO) anode/ copper phthalocyanine (CuPc) / N, N’-dephenyl-N,N’-bis(3-methylphenyl)-1,1’-diphenyl-4,4’-diamine (TPD) / 8- hydroxyquinoline aluminum (Alq3) / buffer layer / Al cathode structure with the buffer layers made from alternating thin films of Alq3 and Al with nanometer thickness. Improvement of driving voltage and the efficiency for the devices were observed by insertion of the buffer layer. It was estimated that some modulations of the Schottky barrier at the Alq3 and the Al cathode interface were induced due to the insertion of the buffer layer and it caused an enhancement of electron injection from the Al cathode. A model of the band structure at the buffer layer was proposed. INTRODUCTION OLEDs have been greatly investigated for constructing new optical devices such as full-color displays, because of the many advantages in efficiency, flexibility, cost and so on [1-4]. It is well known that buffer layers, that is electron or hole injection layers, are very useful for improving driving voltage and efficiency of OLEDs [5-9]. Several phthalocyanine derivatives have been used as anode buffer layers, because the band structures are appropriate for reducing the carrier injection barrier at the interface between ITO anode and hole transporting layers [5, 6]. Furthermore, LiF is also often used for the buffer layer at the cathode [7, 8]. Although the mechanism for enhancement of the electron injection has not been fully clarified, it is considered to be due to the low work function induced by mixing of Al and LiF [7] or a tunneling effect [8]. Co-evaporated films of the Al and Alq3 layer have also been reported for the cathode buffer [9]. In this study, OLEDs having an ITO anode / CuPc / TPD / Alq3 / cathode buffer-layer / Al cathode structure were fabricated using a vacuum evaporation method. The cathode buffer layers were made from nanostructured alternating films of Alq3 and Al. The electrical and luminescence properties have been investigated for the OLEDs containing the cathode buffer layers. EXPERIMENTAL DETAILS The device structure fabricated in this study is shown in Figure 1. Alq3 (Aldrich Co.) is a well-known emitting and electron-transporting material. TPD (Chemipro Kasei Co., Japan) is transparent in the visible wavelength region and is a hole-transporting material. CuPc (Tokyo Kasei Kogyo Co., Japan) is used f
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