| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 102 === Phosphorescent OLEDs (PhOLEDs), known for their 100% exciton utilization efficiency, are the mainstream of the current OLED development. The proper host materials and finely tuned device structure have envisaged the theoretical maximum quantum efficiencies of 20-30% for PhOLEDs. However, nowadays blue PhOLED hosts are still deficient due to the below average device performances and operational longevities. Therefore, the device improvement for blue devices has become the most pivotal issue for OLEDs before commercialization. In this regard, thermally activated delayed fluorescent OLEDs (TADF OLEDs), possessing equally high emitting capability and similar device configuration in comparison with PhOLEDs, have become the latest novel research topic in organic optoelectronic fields. The use of pure aromatic compounds with small singlet-triplet energy gap (&;#8710;EST) as emitters exempts the dependence on noble metals and therefore reduce the production cost, which is especially attractive for academia and industries.
The main attribute in this dissertation are the design, synthesis, and characterization of novel bipolar pure aromatic materials comprising of common electron-donating group &;#8722; carbazole and various electron-deficient groups as emitting layer materials for 2nd and/or 3rd generation OLEDs. Their physical properties and device applications were fully investigated to explore their structure-property- performance relationship. The essence of each chapter is briefly outlined as follows. The 1st chapter provides an overview of modern development of OLEDs, emitting mechanisms, and design criteria of host materials. The 2nd chapter focuses on the carbazole (D)/1,3,4-oxadiazole (A) ratio manipulation of bipolar hosts for PhOLEDs to finely tune and balance their physical properties rendering the devices with better performances. The 3rd chapter introduces three groups of highly twisted bipolar hosts featuring different conjugation length and/or structural topology. The reduced electronic coupling between D and A via twisted conformation gives each of the compounds unique optical characters and balanced charge transporting ability and thus, PhOLEDs based on such materials exhibit remarkable EL performance. The 4th chapter commences with the recent TADF literature survey and, later on, redeems the potential TADF character of the highly twisted molecule introduced in Chapter 3. Last, but not the least, the novel bipolar molecules based on carbazole and 1,3,5-trazine/CN moieties were redesigned, synthesized and characterized. From the preliminary physicochemical investigations and device applications of these materials, their roles as efficient TADF materials is going to unambiguously disclose.
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