Correlation between Small-molecule Dependent Nanomorphology and Device Performance of OLEDs with Ternary Blend Emitting Layer

Abstract

Organic Light emitting diode (OLED) have in the recent, attracted the attention of many since their first introduction into the market. This is due to their unique properties such as flexibility, low weight, full color tenability e.tc., that make them applicable in areas e.g., flat-panel displays and solid state lighting. In order to enhance the efficiency of OLEDs, researchers have continued to employ phosphorescent dopants more particularly, cyclomated iridium complexes such as tris(2-phenylpyridine)iridium (III) Ir(ppy)3 that improve device efficiency through spin-orbit coupling mechanism. Through this approach, OLEDs with internal quantum efficiencies as high as 100% have been realized. Nonetheless, the external quantum efficiency (EQE), that significantly depend on other extrinsic parameters such as fabrication conditions need much improvement. One of the root cause of low efficiency in Ir(ppy)3 based phosphorescent OLEDs is the aggregation of the dopant itself. Numerous researchers have employed various analytical techniques to investigate and correlate the effect of Ir(ppy)3 dopant aggregation with device efficiency. However, most of the work seem to focus either on polymer-polymer or polymer-small molecule binary host system. In this dissertation, we report for the first time, the needle-like aggregation (previously observed in polymer-small molecule) in ternary blend film spin-coated from purely small organic molecules. This has been achieved through a combinatorial approach involving transmission electron microscope (TEM), scanning transmission electron microscope/energy dispersive spectroscopy (STEM-EDS) and atomic force microscopy (AFM). The mechanism that leads to formation of the needle-like aggregates was investigated. The conclusion arrived at is that the needle like aggregations occur at high dopant concentration. This is caused by dipole-dipole interactions that lead to offset - stacking associated with ppy ligand in the cyclometalated iridium complex. Further, an experiment to compare the magnitude of Ir(ppy)3 dopant aggregation in polymer-small molecule versus only small molecule based ternary blend OLED was performed. Tris(4-carbazoyl-9-ylphenyl) amine (TCTA) and polyvinyl carbazole (PVK) as hole transport materials were selected for comparison. Based on the experimental result, the extent of Ir(ppy)3 needle-like aggregates were less pronounced in (TCTA) based films compared to those of (PVK). This is a good indication that small molecule based OLEDs are less exposed to problems associated with dopant aggregation such as triplet-triplet quenching. In addition, a method to eliminate or reduce the needle-like aggregates was demonstrated by blending various solvents with different properties e.g., solvents with different dipole moments. In this dissertation, chlorobenzene, dichlorobenzene and toluene solvents were selected for study. The effect of different solvents and their blending ratio on the needle-like dopant aggregates was further investigated by a combination of conventional (STEM) and atomic force microscopy (AFM). According to the spectroscopic data obtained, the dopant aggregation can easily be eliminated depending on solvent type. Furthermore, by employing mixed solvents with different evaporation rates, the film nanomorphology was improved. Finally, the correlation between device nanomorphology and performance was investigated. According to the experimental result, device luminance efficiency was enhanced in devices with mixed solvents. A variation in Current density and threshold voltages depending on the thickness and morphology of the emitting layer was also demonstrated. The experimental result shows that the overall performance of the solution based OLEDs strongly depends on the morphology of the emitting layer.