Approaches to Advanced Organic Light Emitting Diodes: Materials and Devices
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Approaches to Advanced Organic Light Emitting Diodes: Materials and Devices Masamichi Ikai and Yasunori Taga TOYOTA Central Research & Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ABSTRACT We present our recent findings on the development of organic light emitting diodes (OLEDs). One of the keys to highly efficient phosphorescent emission in organic light-emitting diodes is to confine triplet excitons generated within the emitting layer. To confine triplet excitons, we employ perfluorinated phenylene dendrimers (C60F42) as a both hole- and exciton-block layer, and a hole-transport material (4,4’,4”-tris(N-carbazolyl) triphenylamine [TCTA]) as a host for the phosphorescent dopant, Ir(ppy)3, in the emitting layer. The maximum external quantum efficiency reaches to 19.2%, and is over 15% even at high injection current densities of 10 to 20 mA/cm2, where the brightness of the device reaches to approximately 10,000 cd/m2. INTRODUCTION The organic light-emitting diodes (OLEDs) are one of the most promising next-generation low-cost, full-color, flat panel display alternatives to liquid crystal-based ones. Recently phosphorescent OLEDs have received considerable attention due to their highly efficient emission compared with conventional fluorescent OLEDs [1-5]. However, the external quantum efficiency (photons extracted in the forward direction per electron injected) [6,7] is not enough to reach the theoretical limit of about 20% derived from simple classical optics; although very recently there were a few reports concerning the external quantum efficiencies of close to or greater than 20% only at low injection current densities below 0.1 mA/cm2 [8,9]. In simple-matrix (dot-matrix) OLEDs, the devices instantaneously require high luminance for the high duty drive. Consequently, high injection current is required to generate high luminance. If we employ the phosphorescent OLEDs for such displays, we can reduce the injection current compared with fluorescent OLEDs, preventing degradation of the devices. Unfortunately, as quantum efficiency of phosphorescent OLEDs significantly decreases due to triplet-triplet annihilation at high current densities [10], and it is generally difficult to obtain high quantum efficiency at high injection current densities. Here we present the highly efficient phosphorescent emission from the OLEDs with both a hole-transport material as a host of a phosphorescent dopant dye and novel “starburst” perfluorinated phenylene compounds (C60F42) as a hole- and exciton-block layer even at high C4.3.1
injection current densities of 10 to 20 mA/cm2 where the brightness of the device reaches to approximately 10,000 cd/m2. EXPERIMENTAL DETAILS Glass substrates coated with indium tin oxide (ITO, Sanyo Vacuum, thickness: 150 nm, sheet resistance: 10 Ω/square) were cleaned ultrasonically in methanol and isopropyl alcohol, rinsed in deionized water. Prior to the organic deposition, the substrates were treated with UV-ozone and O2/Ar plasma, and immediately transferred into the evaporation cham
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