Improved stability of interfaces in organic light emitting diodes with high Tg materials and self-assembled monolayers

In order to improve the thermal stability of organic light emitting diodes (OLED), films made of classical hole transporters (TPD and NPB) and of new blue emitters (based on carbazole dimers) have been deposited on bare ITO substrates, and on ITO grafted with three different self-assembled molecules. These materials have low (60-70 degrees C) and high (90-100 degrees C) glass transition temperatures (T-g). The surface properties like desorption, surface diffusion and wettability, have been studied by observing the morphology of the films with scanning electron microscopy. High T-g films show higher stability and reduced desorption. A similar effect is obtained with the grafted substrates. These tendencies can be explained by considering the polarizability of the molecules. This points towards possible improvements in the running temperature of OLED devices. (C) 1999 Elsevier Science S.A. All rights reserved.