Catching the sources of organic LED degradation in action

Abstract

Though great improvements have been made in recent years, organic light emitting diodes (OLEDs) still have limited lifetimes. In general, the degradation of OLEDs manifests itself as a decrease in device luminance with time. Three distinct modes of deterioration have been recognized: dark spot generation, catastrophic failure, and intrinsic degradation.1 In the first mode, nonemissive regions appear on the emitting surface. Catastrophic failure occurs when electrical shorts cause large leakage currents. In the third mode, changes in the organic material properties cause long-term decreases in the emissive area brightness. So far, methods for investigating these phenomena have involved chemical and physical techniques, like high-performance liquid chromatography and nuclear magnetic resonance.2 These methods typically destroy the device under investigation in order to extract the byproducts of the degradation. Often, using solvents to extract each chemical specie for analysis requires chemical modification. Using synchrotron radiation-based surface techniques, we have developed a new way to investigate dark spot formation. This approach can be applied to any multilayered structures, such as OLEDs. This analysis method enables physical and chemical investigations of real devices, in some cases, even when they are operating. Moreover, synchrotrons offer surfacesensitive techniques to characterize the topmost molecules, without damaging the underlying layers. We applied a typical surface science approach to determine the properties of a thin-film multilayer structure, similar to a small molecule-based OLED. Because it is possible to tune the impinging beam photon energy on the sample, synchrotron radiation can increase the chemical sensitivity of techniques like x-ray photoemission spectroscopy, x-ray absorption nearedge spectroscopy, x-ray transmission, and x-ray fluorescence. Figure 1. This 64×64μm indium photoemission map captures OLED degradation in action. The central bright spot reveals the complete removal of the organic layers. Movement of indium from substrate to cathode causes the higher intensity near the crater.