Organic light-emitting diode (OLED) technology has found multitudinous applications in the development of solid-state lighting, flat-panel displays and flexible screens. Nowadays, the phosphorescent OLEDs based on metallophosphors can reach sufficiently high efficiencies for practical application. Recently, red, green and blue (RGB) platforms of highly efficient phosphorescent emitters have been achieved and OLEDs TV is now commercialized in the marketplace. However, the design and synthesis of innovative emitter materials play an important role in commercialization of the OLED technology. The basic concept of OLED is herein discussed in chapter 1 putting main focus on phosphorescent iridium(III) complexes. In chapter 2, a series of cyclometalated iridium(III) complexes containing 2-(4-benzylphenyl)pyridine have been synthesized and different electron-donating and electron-withdrawing substituents were attached on the pyridyl ring in the ligand. The device D6 doped with 8 wt% B4 gave the excellent OLED performance with a peak of external quantum efficiency (ext) of 21.4%, power efficiency (P) of 51 lm/W and current efficiencyL) of 76.3 cd/A, which is much higher than that of commercial available fac-Ir(ppy)3 under the same operation condition. These findings draw our attention to the fact that a weak electron-donating benzyl group could alleviate intermolecular aggregation in the solid state, thus improving the device performance. The bulky moiety introduced on 2-phenylpyridine through a CH2 spacer in the ligands could suppress the triplet-triplet annihilation in their metal complexes. Cylcometalating ligands and their respective metal complexes have been fully characterized by 1H and 13C NMR spectroscopy and matrix-assisted laser desorption inoization-time of flight (MALDI-TOF) mass spectrometry. In chapter 3, a series of thiazole-based iridium(III) complexes have been synthesized and characterized. It is considered that the thiazole moiety is infrequently used for organic semiconducting materials. To have a better understanding on this functional unit, different hole-transporting groups (eg, carbazole or fluorene) are attached to the thiazole ring in the cyclometalating ligands in order to tune the HOMO and LUMO levels of the complexes. Device D29 doped with 8 wt% T2 gave the highest L of 35.8 cd/A and ext of 11.1%. This result implied that thiazole moiety is an alternative option to afford a new class of cyclometalating ligands for OLED research. In chapter 4, a series of cationic iridium(III) complexes bearing diimine ligand have been synthesized and characterized. The diimine ligands were decorated with the sterically bulky groups. As self-aggregation could deteriorate the device efficiency, this molecular design strategy can diminish the aggregation-caused quenching problems, which has been supported by the aggregation-induced emission enhancement present in complexes E2 and E3. In Chapter 5, a series of bis-tridentate iridium(III) complexes have been synthesized and characterized. Our challenging is to design two types of tridentate chelates (ie. monoanionic and dianionic ligands) for balancing the charge on the metal center. Besides, these chelates should be a good cyclometallate to coordinate with the iridium metal. Four compounds with different dianionic tridentate chelates were designed to achieve distinct color emission. Compound K4 exhibited extremely high quantum yield of 85.5%. This finding revealed that the metal complex featuring two tridentate chelates is a promising phosphorescent dye in OLED. Lastly, the concluding remarks and the experimental details of all the compounds in the previous chapters were included in Chapters 6 and 7.
|Date of Award||24 Aug 2015|
|Supervisor||Wai Yeung WONG (Supervisor)|
- Light emitting diodes