Long-Term Degradation Mechanism of Organic Light Emitting Devices Based on Small Molecules

Abstract

The intrinsic degradation of tris(8-hydroxyquinoline) aluminum (AlQ3)-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 A1Q3 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.

Chemical failure modes of AlQ3-based OLEDs: AlQ3 hydrolysis

“…49 Experiments carried out by Aziz et al 16 suggest that for AlQ3-based devices, degradation is related to an imbalance of charge on the AlQ3 side of the heterojunction; resulting in the build-up of cationic AlQ3 species which may be less stable and more reactive than the pristine material. This is consistent with known approaches to extending AlQ3-OLED device lifetime, involving the introduction of charge injection layers to limit carrier transport.…”

“…Additional failure modes have been proposed that are directly related to the chemistry and properties of the AlQ3 layer used in the OLED devices. Popovic and co-workers 16 have attributed the long-term device degradation of AlQ3-based OLEDs to an intrinsic instability of the AlQ3 cation, which is formed due to an imbalance of charge at the heterojunction. A prevalent chemical failure pathway involves the hydrolysis of AlQ3 due to moisture contamination.…”

Improved Device Lifetime In Organic Light Emitting Devices By Using a Solution-Processed Mixing Single Layer Structure

“…The improvement of lifetime in the mixed layer device is associated with several possible reasons. As mostly reported [15,16], the first one is related to the removal of the heterojunction interface in mixed single layer device. For mixed single layer device, the electric field at the abrupt heterojunction is lowered because of the delocalization of the charge recombination zone by mixing the hole transport layer with the electron transport layer and dopant.…”

“…However, clear understanding of degradation and efficient improvement of lifetime in OLEDs are still main issues. Recently, mixed single-layer structure, in which electron-transport material, hole-transport materials, host, and dopant are mixed in one layer, is proposed for improving the device performance by optimizing the mixing ratio and doping concentration [13][14][15][16][17][18][19]. Particularly, it is possible to largely improve the device stability by using a mixed single layer as the emission layer.…”

Performance enhancement of white-electrophosphorescent devices incorporating a mixed-transition layer

“…3 The balance of charge transport is a key feature in the operation of WOLEDs. 4 Holes are normally the main charge carriers ͑as a result of the high hole mobility in organic materials͒. 5 One popular method of improving the charge balance is to incorporate electron-transporting compounds.…”