Long-Term Degradation Mechanism of Organic Light Emitting Devices Based on Small Molecules
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Department of Materials Science & Engineering, McMaster University, Hamilton, Ontario, Canada, L8S 4L7, [email protected] **Xerox Research Centre of Canada, Mississauga, Ontario, Canada, L5K 2L1. *
ABSTRACT The intrinsic degradation of tris(8-hydroxyquinoline) aluminum (AlQ 3)-based organic light emitting devices, that leads to the long-term decrease in the electroluminescence efficiency of the devices operated under constant current conditions, is studied. The injection of holes in A1Q 3 is found to be the main factor responsible for device degradation. OLEDs with dual HTLs in different arrangements are also presented to demonstrate the proposed degradation mechanism. The role of various approaches to increase OLED lifetime, such as, doping the hole transport layer, introducing a buffer layer at the hole-injecting contact , or using a mixed emitting layer of hole and electron transporting molecules, is explained. INTRODUCTION Because of their efficiency [1,2] and their wide range of emission colors [3,4], organic lightemitting devices (OLEDs) have the potential to achieve full-color flat-panel displays. However, their poor stability has, so far, been a major concern, usually limiting the half-lifetime (time elapsed before the luminance of the OLED decreases to half its initial value) of an OLED to few hundred hours [4] . In general degradation in OLEDs occurs through the formation of non-
emissive regions as well as through a long-term "intrinsic" decrease in the electroluminescence efficiency of the device [5]. Although the mechanisms giving rise to the non-emissive have been identified [5,6] and controlled [7], the causes of the intrinsic degradation remain unclear. Doping the hole transport layer (HTL) [8] or introducing a buffer layer at the hole-injecting contact [9] were found to significantly reduce the intrinsic degradation. Half-lifetime exceeding 4000 hours (at an initial luminance of 500 cd/m2 ) have been achieved [9], which can be further increased by doping the emitting layer [10]. As a result of these findings, morphological changes in the HTL or degradation at the hole-injecting contact have been speculated as failure suspects in OLEDs. Nevertheless, the occurrence of such processes in OLEDs or their role in degradation has never been verified. In fact, other reports show no correlation between the morphological stability of the HTL and the lifetime of the OLEDs [11]. In this report, the cause of the long-term degradation of OLEDs based in tris(8hydroxyquinoline) aluminum (A1Q 3), is presented. The injection of holes in A1Q 3 was found to be the main factor responsible for device degradation. Using photoluminescence spectroscopy, we found that the quantum efficiency of A1Q 3 layers, where predominantly holes were transported, decreased upon prolonged current flow. To further demonstrate this degradation mechanism, we present studies on OLEDs with dual HTLs in different arrangements. In view of these findings, the role of various stabilizing agents, such as, doping the HTL [8], introducing a thin
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