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

다국어 초록 (Multilingual 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.

목차 (Table of Contents)

• Ⅰ. Introduction
• 1.1 Research background
• 1.2 Initial motivation
• Ⅱ. Literature Survey
• 2.1 Solution processed phosphorescent OLEDs
• 2.1.2 Device structure and multi-component system
• 2.1.3 Phosphorescent mechanism
• 2.1.4 Phosphorescent materials in OLEDs
• 2.2 Morphological dependence of device properties in
• polymer electronics
• 2.2.1 The effect of processing conditions on film
• morphology
• 2.2.2 Phase separation of polymer blends
• 2.2.3 The effect of thermal annealing on OLEDs
• 2.3 Analysis of structural and compositional characteristic
• 2.3.1 Various analysis techniques for organic material
• 2.3.2 Aberration corrected STEM-EDS
• 2.3.3 Atom probe tomography in soft material
• Ⅲ. Experimental procedures
• 3.1 Fabrication of OLED device
• 3.2 TEM sample preparation
• 3.2.1 FIB lift-out methods
• 3.2.2 Floating methods
• 3.3 In-situ heating experiments in TEM
• 3.4 STEM-EDS
• 3.5 device performance
• 3.6 Atom Probe Tomography
• 3.6.1 Sample preparation for APT analysis
• 3.6.2 APT analysis
• Ⅳ. Result and Discussion
• 4.1 TEM observation of emitting layer of OLEDs
• 4.2 In-situ heating TEM
• 4.3 Composition analysis by STEM-EDS
• 4.4 Correlation between dopant distribution and device
• efficiency
• 4.5 Atomic distribution analysis by atom probe tomography
• 4.5.1 Specimen preparation techniques
• 4.5.2 Mass spectrum identification and 3D reconstruction
• 4.5.3 Laser pulse energy effects on mass spectrum
• Ⅴ. Conclusion
• Ⅵ. Reference
• Summary in Korea