Simple Structure and High Efficiency Phosphorescent OLEDs Using Narrow Band-gap Bipolar Host Material
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Simple Structure and High Efficiency Phosphorescent OLEDs Using Narrow Band-gap Bipolar Host Material Young Hoon Son and Jang Hyuk Kwon* Dept. of Inform. Display, Kyung Hee Univerisity, Dongdaemoon-ku, Seoul 130-701, Korea. ABSTRACT We report high-efficiency phosphorescent blue OLEDs with an organic three stacked structure. Using a high-triplet-energy-hole transporting material of TAPC and a high-tripletenergy-electron transporting material of TmPyPB, the organic three stacked structure has been realized with three new narrow band-gap blue host materials. These host materials have bipolar characteristics and high triplet energy of >2.8 eV. Very low onset voltages of 2.8~3.0 V and driving voltages of 4.2~4.6 V to obtain a brightness of 1000 cd/m2 are achieved in this three stacked device configuration. Maximum external quantum efficiency above 20% is reported. INTRODUCTION Organic light-emitting diodes (OLEDs) have been accepted as a technology for mobile and television (TV) displays and for potential future use in the lighting industry. In particular, phosphorescent OLEDs (PHOLEDs) have been of considerable interest in recent years as singlet and triplet excitons contribute to the emission of photons.[1] Highly efficient OLEDs with over 20% external quantum efficiency (EQE), an almost ideal value, have been reported using this electrophosphorescent technology.[2] During the last couple of years, blue PHOLEDs with over 20% EQE have also been reported by several groups.[3] However, the driving voltage is relatively high compared with those of red and green PHOLEDs. Therefore, high driving voltage and short lifetime issues have to be addressed for meaningful application of blue PHOLEDs. In order to make highly efficient PHOLEDs, the triplet energy of the host should be higher than that of the phosphorescent guest to facilitate the exothermic energy transfer from the host to the guest. Thereby, reverse energy transfer from the guest back to the host is prohibited effectively and triplet excitons are confined on guest molecules. However, the host materials with wide band-gaps often cause an increase in driving voltages of the PHOLEDs. Therefore, the selection of suitable host materials for PHOLEDs is very imperative to achieve the high efficiency. In this paper, we report high efficiency and low driving voltage blue PHOLEDs with a simple organic triple stacked structure. Narrow band-gap characteristics of our bipolar transporting host materials are a key parameter to give such results. EXPERIMENT Blue PHOLEDs were fabricated using TAPC as a hole transport material and TmPyPB as an electron transport material. FIrPic were used as a blue phosphorescent dopant and mCP, KHU-BH-1, KHU-BH-2, and KHU-BH-3 were used as phosphorescent blue host materials. Figure 1 shows each molecular structure of these host materials.
Fig. 1. Molecular structures of blue phosphorescent host materials. The HOMO levels of new three host materials were obtained from the cyclic voltammetry measurements. The LUMO levels and the optical band-gaps of
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