Structural Dependence and Dynamics of White Luminescence in Multilayer Structures for White Organic Light Emitting Devic
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Structural Dependence and Dynamics of White Luminescence in Multilayer Structures for White Organic Light Emitting Devices Shizuo Fujita1,2, Ken-ichi Uesaka2, and Shigeo Fujita2 International Innovation Center, Kyoto University, Kyoto 606-8501, Japan 2 Department of Electronic Science and Engineering, Kyoto University, Kyoto 606-8501, Japan 1
ABSTRACT White electroluminescence (EL) spectra are carefully investigated in terms of the multilayer structures and a rule for designing the white luminescence is discussed. For a simple structure which consists of tris(8-hydroxyquinoline)-aluminum (Alq) and 1,2,3,4,5pentaphenyl-1,3-cyclopendadiene (PPCP) emitting green and blue luminescence, respectively, as luminescent and electron transport layers, white luminescence appeared if the thickness of each layer was appropriately chosen. The luminescence efficiency was higher in PPCP, and therefore one of the structures effectively emitting white luminescence was designed as PPCP(10 nm)/Alq(40 nm). The 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was then added to the PPCP/Alq structure intending to enhance red luminescence. We propose the structure where red luminescence is from the DCM as photoluminescence excited by the green and blue luminescence from Alq and PPCP, which was effective, compared to the PPCP/Alq/DCM stacking structure, in avoiding increase of the operating voltage. The importance of designing structures of organic white EL devices based on fundamental optical properties of the constituent layers and on luminescence dynamics is highlighted. INTRODUCTION Excellent optical functions in organic materials are now strongly expected to be utilized as white electroluminescence (EL) devices, which are capable of being widely applied for displays and lightings. Great progress, as a result, has been brought in this area for enhanced efficiency and improved chromaticity. One of the advantages of organic EL devices lies in their wide and continuous emission spectra which makes white luminescence as a more "natural color" compared to GaN or ZnSe-based semiconductor white light emitting diodes (LEDs) containing sharp bandedge blue luminescence lines. The general idea to obtain white luminescence and to improve chromaticity is to stack several layers emitting different colors, in the case of EL devices with lower molecular weight materials[1-4], or to dope several different dyes in the case of polymer EL devices[5-7]. Concentrating on the former, which possesses an advantage of lower operating voltage compared to the latter, recent investigations tend to be directed toward complicated multilayer structures in order for improving chromaticity with overlapping of more colors emitted from a larger number of different layers. However, it does not seem that this development has been carried out by a systematic scheme for the structures (materials combination and thicknesses of constituent layers) based on understanding of the luminescence and transport mechanisms. The motivation of this research is to sys
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