Study on Electron Injection Effect of Low Molecular Organic Light-emitting Diodes Utilizing LiF/Al and Li/Al Cathode
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0965-S03-09
Study on Electron Injection Effect of Low Molecular Organic Light-Emitting Diodes Utilizing LiF/Al and Li/Al Cathode Zenken Kin, Hirotake Kajii, and Yutaka Ohmori Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita, 5650871, Japan
ABSTRACT The electron injection effects of organic light-emitting diodes (OLED) from the cathode interface between LiF or Li and Al on the tris-(8-hydroxyquinolinato) aluminum (Alq3) emissive layer have been investigated. Efficient electron injection was achieved in the case of thin layer of LiF or Li deposited on Alq3 emissive layer at the thickness of 0.5 nm and 2 nm, respectively. The results indicate that the coexistence of LiF and Al layer or the Li and Al layer on the Alq3 emissive layer result in the efficient electron injection. Electron injection effect in the case of LiF/Al and Li/Al electrode configurations and also the effect of air explosion during the electrode formation have been discussed. In the case of the device with Li, the diffused Li in the Al layer acts as the efficient carrier injection process. INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted much attention because of their potential for applications for full-color flat-panel displays or lighting applications. For OLEDs as lighting applications, highly carrier injection is one of the key issues for achieving high device performances. In order to achieve high device performances electrode configuration is strongly influence the device performance, especially, the contact between cathode electrodes and electron transport layer. Indium-tin-oxide (ITO) is commonly used as the hole injection electrode, while metal or metal alloy with a low work function normally forms an effective electron injection electrode. The low work function metals have a demerit of easily oxidized in air and not stable in air. For practical use the low chemical reactive metals such as Al are widely used as cathodes. Tris-(8-hydroxyquinolinato) aluminum (Alq3) is a typical light-emitting material, [1] and is widely used as an emissive material. It has been shown that the insertion of a thin LiF layer between Al cathode and Alq3 emissive layer leads to a significant enhancement of the electron injection into the emissive layer and improve the electroluminescence (EL) efficiency. Some mechanisms were proposed for the enhancement of the electron injection. They can be classified into two groups; (I) a lowering of the overall height of the electron injection barrier, (II) chemical interaction at the interface between metal and the fluorinated material. For group (I), several reasons for the decrease of the barrier height for electron injection process has been proposed; the decrease of the interface work function of the anode, the formation of the interface dipole, the band bending in the LiF layer, and the polarization of the LiF layer. [2, 3] For group (II), the formation of Alq3 anion by doping with metallic Li or doping with dissociated Li from LiF/Al interface is proposed. [4] Howeve
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