Intrinsic degradation mechanism in tris(8-hydroxyquinolato) aluminum-based organic light emitting devices

Abstract

<p>Intrinsic degradation, which leads to the long-term decrease in the electroluminescence efficiency, has been a major limitation facing the new technology of organic light emitting devices (OLED). Traditionally, degradation has been speculated to be caused by morphological instability of the organic layers, especially the less stable hole transport layer (HTL), or by the formation of deep traps at the hole-injecting contact. These speculations were based on experimental observations showing that doping the organic layers or introducing a buffer layer at the hole-injecting contact can dramatically improve device stability. However, the real causes of OLED degradation remained uncertain. In this study, the cause of the long-term degradation of OLEDs based on tris(8-hydroxyquinolato) aluminum (A1Q3), the most widely used electroluminescent molecule, is investigated. OLEDs with various structures are studied. Results reveal that the injection of holes into the AlQ3 layer is the dominant factor responsible for device degradation. Cationic AlQ3 species are found to be unstable and their degradation products are fluorescence quenchers that lower the electroluminescence efficiency of OLEDs. In view of these findings, the effectiveness of stabilizing agents, such as, doping the HTL, introducing a buffer layer at the hole-injecting contact, or using mixed layers of hole and electron transporting molecules, is explained in terms of their role in slowing down the injection of holes into the AlQ 3 , which results in a higher electron density and thus a more rapid electron-hole recombination. Therefore, the lifetime of the unstable cationic AlQ3 species is reduced leading to a significant decrease in AlQ 3 degradation and consequently increases device stability. Other earlier observations pertaining to OLED degradation are also addressed. The degradation mechanism is further demonstrated on OLEDs with dual-layer HTL made of materials with different ionization potentials. The important features of a theoretical framework to model OLED degradation are also discussed.</p>