Abstract
In this thesis, I focus on the organic light emitting applications utilizing advanced light emitting mechanisms and novel flexible transparent electrodes. The above-mentioned advanced light emitting mechanisms include exciplex emission, thermally activated delay fluorescence (TADF) and aggregation induced emission (AIE). First, I briefly introduce the history of organic light emitting diodes (OLEDs) and their applications in both solid-state lighting and displays. Then I discuss the working mechanism and the manufacturing processes of OLEDs. In chapter two, I use spin coating process to fabricate solution process based OLEDs. A red phosphorescent emitter PR-02 supplied by Industrial Technology Research institute (ITRI), Taiwan is used and the solution-processed PR-02 devices achieve decent external quantum efficiencies. I also use PR-02 as a red emitter in white OLEDs. Furthermore, exciplex emission is discussed in this chapter. I use photoluminescence spectrum analysis and photoluminescence quantum yield measurement (PLQY) to investigate the efficiency of the exciplex formation pairs. In chapter three, I do a study on TADF, which is one of the very promising emission mechanisms in OLEDs recently. The advantage of TADF is that it can harvest triplet excitons, thus it has the internal quantum efficiency four times higher than traditional fluorescent emitters. Two kinds of devices utilizing TADF emitters PyC-TAZ and CLY-01 are demonstrated. In the fourth part of the thesis I develop a new type of flexible transparent electrode. Continuous thin Ag layers are achieved by the assistance of nucleation-inducing seed layers, making the thin metal electrodes highly transparent with low sheet resistances. In chapter five, I focus on the emission dipole orientation of the emitting layers in OLEDs. The measuring methods, instrument set-ups and emission theory are discussed. The orientations of emission dipoles usually have a large influence on out-coupling efficiency. I fabricate two kinds of OLEDs with same device structure but different emitters which are Ir(ppy)3 and Ir(ppy)2acac. Variable angle PL intensity measurement is executed to analyze the emission dipoles in these systems. By the assistance of variable angle PL intensity measurement I prove that the different external quantum efficiencies of these two devices result from their emission dipole orientations. In the sixth part of the thesis I investigate the novel emission called aggregation induced emission (AIE). Traditionally, organic molecules are preferred to be isolated to have a high PLQY, however AIE molecules show high PLQY from the aggregate and ordered crystal states. With preferred molecule alignment, orientation of emitting dipoles can also be manipulated and achieve higher out-coupling efficiency. I use AIE molecules K093, K094 and K095 to fabricate devices and they show high external quantum efficiencies of 23.5%, 20.9% and 23.7%, respectively. It should be noted that device fabricated utilizing K093 has an emission peak of 736 nm, which is by far the most efficient infrared OLEDs nowadays. Finally I summarize all the works above and give some future prospects.