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5 December 2006 Enhancement of the OLED driving stability by introducing an LiF-mixed α-NPD hole-transport layer
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The stability of the organic light-emitting diode (OLED) at the high temperature is important for their applications to automotive displays or various lighting applications which are more susceptible to Joule heating problems. In addition, it is known that the OLED lifetime is limited by the poor thermal stability of the hole-transport layer (HTL) material. Thus, the improvement of the thermal stability of the HTL layer is essential for enhancing both thermal stability and the operation lifetime. Here, we report that the thermal stability of OLED device can be significantly enhanced by introducing an LiF-mixed N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) as a HTL in the OLED having tris(8- hydroxyquinoline) aluminum (Alq3) as a light-emitting and electron-transport layer. Compared with the reference device with the α-NPD HTL, the device having a double layer of LiF-mixed α-NPD and α-NPD as a HTL showed an increased thermal stability up to 170°C without degrading the quantum efficiency of the electroluminescence. In addition, the driving voltage variation over time (less than 3 V) was significantly suppressed while the reference device shows a variation over 6 V. The improved device stability is attributed to the enhanced thermal stability of the LiF-mixed α-NPD layer, which could be estimated from the result that the film morphology of LiF-mixed α-NPD film was nearly unchanged after heated above the glass transition temperature of α-NPD while that of α-NPD film was significantly changed.
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Heume-Il Baek, Hyun-Ku Lee, and Changhee Lee "Enhancement of the OLED driving stability by introducing an LiF-mixed α-NPD hole-transport layer", Proc. SPIE 6333, Organic Light Emitting Materials and Devices X, 63331B (5 December 2006);

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