Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Abstract

Controlling the alignment of emitter molecules in the active layer of organic light-emitting diodes has become a main approach to maximize the device efficiency when emitter molecules with luminescence quantum yields approaching 100% are used. In order to guarantee stable device performance, the initial molecular orientation should not change over time. In this work, we study this property for a time frame of 1.5 years and storage temperatures up to 80 °C which may be reached in displays exposed to direct sun light. For the studied material systems, this temperature is close to the glass transition at which drastic morphological changes occur and a randomization of the molecule arrangement is expected. We compare two different phosphorescent emitter molecules and, additionally, investigate the impact of the substrate temperature during evaporation. Concluding this long-term study, we prove experimentally that the emitter orientation remains unchanged under those device-critical storage conditions. On the contrary, the fatal potential of heat-induced reorientation is revealed by post-annealing experiments that show a strong change of the emitter orientation at about 20 K above the glass transition temperature.