Intrinsic degradation, which leads to the long-term decrease in the electroluminescence (EL) efficiency of the devices, has been a major limitation facing the new technology of organic light emitting devices (OLED). Recently we investigated degradation in OLEDs based on tris(8-hydroxyquinoline) aluminum (AlQ3), a widely used electroluminescent small molecule. These studies showed that injection of holes in AlQ3 is the main factor responsible for device degradation. Devices in which predominantly holes were transported through a 5 nm thick AlQ3 layer showed a significant decrease in the photoluminescence (PL) efficiency upon prolonged current flow. In the present work we will also show that similar devices constructed in such a way to allow only electron flow through AlQ3 did not show any decrease in PL efficiency under similar current driving conditions. This demonstrates that AlQ3 cations are unstable and their degradation products lead to device degradation, while AlQ3 anions are stable. Further studies of time resolved fluorescence show that the AlQ3 degradation products are fluorescence quenchers, which cause decrease in device efficiency during prolonged operation. Comparison between fluorescence and electroluminescence decay of AlQ3 shows that PL decay is consistently smaller than EL decay indicating that decrease of PL efficiency is not the only mechanism leading to EL degradation of AlQ3.
Zoran D. Popovic, Hany Aziz, Nan-Xing Hu, Paulo N. M. dos Anjos, Andronique Ioannidis, "Long-term Degradation Mechanism In Small Molecule Based OLEDs" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP16), 2000, pp 350 - 353, https://doi.org/10.2352/ISSN.2169-4451.2000.16.1.art00092_1