Enhancement of the OLED driving stability by introducing an LiF-mixed α-NPD hole-transport layer

Abstract

ABSTRACT 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 therma l 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 (.-NPD) as a HTL in the OLED having tris(8-hydroxyquinoline) aluminum (Alq 3 ) as a light-emitting and electron-transport la yer. Compared with the reference device with the Þ -NPD HTL, the device having a double layer of LiF-mixed Þ -NPD and Þ -NPD as a HTL showed an increased thermal stability up to 170 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 attri buted to the enhanced thermal stability of the LiF-mixed .-NPD layer, which could be estimated from the result that the film morphology of LiF-mixed .-NPD film was nearly unchanged after heated above the glass transition temperature of .-NPD while that of .-NPD film was significantly changed. Keywords: Organic Light-emitting-diode (OLED), thermal stability, LiF-mixed HTL