Loss Process in Efficiency of Blue Organic Electroluminescent Devices
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Loss Process in Efficiency of Blue Organic Electroluminescent Devices
Yasuo Enatsu, Chimed Ganzorig and Masamichi Fujihira Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
ABSTRACT We report EL characteristics of blue organic electroluminescent (EL) devices with a hole transport layer (HTL) as an emitter i.e. ITO/NPD/BCP/Alq3/Cs(0.5 nm)/Al. Here, ITO, NPD, BCP, and Alq3 are abbreviations for indium-tin-oxide, 4,4`-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, and tris(8-hydroxyquinoline) aluminum, respectively. Three different chemically modified ITO electrodes were used. As a hole-injecting buffer layer, copper phthalocyanine (CuPc) or 4,4`,4``-tris-(3-methylphenylphenylamino)triphenylamine (MTDATA) was used in this study. We found that the luminance increased less than linearly with an increase in current for all EL devices studied even when hole and electron injection were enhanced. The luminance loss in the device is attributed to quenching . singlet excited states (1NPD*) by large excess radical cations (NPD+ ) accumulated in the emission zone due to large overlap between a fluorescence spectrum of 1NPD* and an absorption . . spectrum of NPD+ . In addition, 1NPD* can be also quenched by radical anions (NPD- ) when excess electrons are injected.
INTRODUCTION Since the first report of a two-layer efficient organic EL device by Tang and Van Slyke in 1987 [1], a great deal of effort has been made to improve their performance. Compared with green organic EL devices, EL characteristics of blue and red organic EL devices need to be improved particularly in terms of efficiency and color purity. At present, development of high performance organic EL devices with blue emission is not satisfactory. However, there are many studies focused on blue organic EL devices, and variety of blue luminescent materials has been developed [2-7]. Only few blue luminescent materials [4-7] appeared to be more significant, where blue emission is over 10,000 cd m-2. Thus, more efficient blue organic EL devices still remain to be developed. The hole-blocking layer is essentially needed to develop a new type of blue organic EL devices. There have been a few reports on hole-blocking organic materials [811]. Among them, phenanthroline derivatives are well known as hole-blocking organic materials. Electron mobility of phenanthroline derivatives was studied using a time-of-flight technique [12]. Blue light-emitting organic materials have lower efficiencies than green or red light-emitting materials because of their larger energy band gaps, which may deter charge injection from electrodes. Enhanced hole and electron injection is greatly important to improve power and EL efficiencies [13-18]. More recently, we reported that the luminance increased more than linearly with an increase in current in Alq3-based EL devices when hole and electron injection was well balanced. [19]. In order to maximize the EL efficiency, we ha
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