Organic Light-Emitting Diodes Using Triphenylamine Based Hole Transporting Materials
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ORGANIC LIGHT-EMITTING DIODES USING TRIPHENYLAMINE BASED HOLE TRANSPORTING MATERIALS Hisayoshi Fujikawa, Masahiko Ishii, Shizuo Tokito, and Yasunori Taga TOYOTA Central Research and Development Labs., Inc. Nagakute, Aichi, 480-1192, JAPAN. ABSTRACT The durability of the tris(8-quinolinolato) aluminum based light-emitting diode (LED) is related to the thermal stability of the hole transport layer. Several linear linkage triphenylamine oligomers were used for the hole transport layer. The thermal stability was clearly seen to depend on a glass transition temperature (Tg) of the hole transporting material, and a linear relationship between the Tg and the thermal stability was found. A lowering of "turn-on voltage" for light emission and an increase of luminous efficiency with increasing temperature was also observed. Excellent durability of the organic LED with a tetramer of triphenylamine was achieved at a high temperature of 120°C. Our results indicate that the linear linkage of triphenylamine leads to a high Tg and high device performance at high temperatures.
INTRODUCTION Light-emitting diodes (LEDs) based on organic materials are attracting much attention as candidates for flat-panel displays and backlights for liquid crystal displays[1-6]. Over the past few years a considerable number of studies has been made on the improvements of the durability of the devices[7-9]. A half-decay lifetime of more than 10000 h from an initial brightness has been already realized at the present time. More recently, a practical multi-color display has been commercialized in small molecular organic materials. However, further research and development are required to improve the lifetime and the color tunability, especially materials developments are significantly important. For the applications for automobile, good reliability at a high temperature is necessary. The most typical structure of the organic LED comprises two organic layers of a hole-transporting layer and an emitting layer. The thermal stability of the device characteristics is primarily limited by the morphological stability of the organic layers, especially, the hole-transporting layer[10,11]. For the hole-transporting layer aromatic amines are commonly used, because they have the excellent hole-transporting ability and good film-forming ability. By vacuum deposition, the aromatic amine forms a uniform amorphous film on any substrate. Therefore, the hole-transporting material based on aromatic amine structure which has a high glass transition temperature (Tg) is one of promising candidates to realize the high durability at a high temperature and at high driving current. In our previous study, it was shown that the organic LEDs with a tetramer of triphenylamine can be operated in a continuous operation up to 130 °C without breakdown[12]. Furthermore, a systematic study of the temperature dependences of EL characteristics has been carried out in organic LEDs fabricated using trimer, tetramer and pentamer of triphenylamine in addition to the well-known dimer for the hole-
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