二代與三代有機發光二極體材料之設計、合成、鑑定及其元件應用

Abstract

磷光有機發光二極體(Phosphorescent OLEDs, PhOLEDs)因其具有100%激子利用效率的潛力，是近年OLED研究領域中的主流。利用適當的主體材料以及妥切的元件設計，不同光色的PhOLED元件效率已逼近其理論最大值。然而，當前報導的藍光主體材料其元件效率以及操作壽命仍具許多改善空間，成為PhOLEDs在正式商業化前一個關鍵課題。熱激活化延遲螢光有機發光二極體(Thermal activated delayed fluorescent OLEDs, TADF OLEDs)與PhOLEDs在具有相同最大理論放光效能及相似元件結構下，前者利用調降具有高放光特質的單純有機結構其單重態-三重態能隙取代了後者對有機金屬發光體的依賴，而成為近期OLED研究領域中最新的研究對象。 在本論文中，個人藉由設計與合成一系列具有雙極性特色的有機芳香材料，在引入共同電子予體−咔唑下與各式不同的電子受體進行結合，以期待實際應用於二代(PhOLEDs)或三代(TADF OLEDs)有機發光二極體的發光層中。透過各式物理分析以及元件應用，進而詳細探討與剖析分子結構−物質特性−元件效能之間系統性的關係。各章節內容簡要如下: 第一章中，主要概括OLED的近代發展、發光原理與機制及其主體材料的設計要點；第二章論述磷光有機發光二極體的主體分子在具有相同電子予體−咔唑和電子受體−噁二唑的情況下，如何透過彼此比例間的微調，逐步達到性質與元件效能的最佳化；第三章介紹一系列具有高扭性的雙極性主體材料，如何透過分子共軛及結構拓撲的操縱，進而改變其物理性質並平衡載子傳輸能力，最後成功應用於多種不同色光的磷光有機發光二極體元件之中；第四章始於簡介當前TADF材料的發展與設計，並探討第三章中符合TADF分子設計概念的潛力分子於新領域的應用性。接著，透過新分子架構進行電子予體和受體的組合，並藉由探討其各項物理性質以及元件上的應用，探求新材料於TADF OLEDs中應用的可能性。

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 (∆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 − 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.