Effects of Annealing Temperature on the Dopant Distribution and Efficiency of Phosphorescent OLEDs

Abstract

Even though Organic Light-Emitting Diodes(OLEDs) have good technological aspects such as low cost, the possibility of realizing flexible or large-area displays, their low external quantum efficiency and short life time should be improved. It is generally known that iridium complexes as a phosphorescent dopant in emitting layer (EML) have great ability to enable nearly 100% internal quantum efficiency. In the present study, the effect of annealing condition on the dopant distribution and device efficiency was investigated by using in-situ heating transmission electron microscopy (TEM) and high angle annular dark field (HAADF) mode of scanning transmission electron microscopy(STEM) as well as atom probe tomography(APT). Emitting layer including tris(2-phenypyridine) iridium(Ⅲ) (Ir(ppy)3) was fabricated by solution process. It was observed that the power efficiency was enhanced dramatically upon an annealing temperature at 180℃. It might be related to the change of dopant distribution. We applied STEM-HAADF to observe the distribution of iridium complexes. The bright spot regarded as the iridium complexes in HAADF image distributed more uniformly after annealing, which means that aggregation of guest molecule is decreased. When the Ir(ppy)3 is distributed well, exciton generated in active region(host) could easily transfer to the Ir(ppy)3 which is emitting spot. Composition analysis by STEM-EDS demonstrates that the molecule aggregation reduced after annealing. Furthermore, 3D atom map by APT directly shows the dopant distribution depending on the annealing conditions (RT, 180℃, 300℃). With these advanced analysis technique, it was found that the efficiency of the device was enhanced by suppressing the aggregation of guest molecule.