應用高折射率材料增進有機發光元件效率之研究

Investigating Use of High Refractive Index Materials for Enhancing Efficiencies of Organic Light-Emitting Diodes

王柏凱(Bo-Kai Wang)

指導教授 ： 吳忠幟

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2021年)

https://doi.org/10.6342/NTU202100308

摘要

隨著有機發光二極體之發展，目前已被視為重要的顯示及照明技術，而目前發展的主要目標著重於兩大方向，第一是開發新材料，透過提升發光效率或載子傳輸特性，來使元件的效率得到提升，就現今的OLED而言，在許多研究學者的努力下，材料面已有重大之突破。目前已有內部量子效率達接近100%之熱激活化延遲螢光材料以及磷光系統的出現。第二則是提升光萃取技術，如何將更多的光從有機發光二極體之中萃取出來是一個很重要的課題。
儘管高折射率基板和水平偶極矩發光材料均已被證實可以用於增強有機發光元件的光萃取效率，但它們的組合對有機發光元件之光學耦合效率的好處及潛力尚未充分的被研究。在本論文第一部份研究中，模擬結果指出，同時使用高折射率基板（折射率> 1.8-1.9）和高水平偶極矩發光材料（水平偶極矩比例> 85%），並一起搭配上低折射率或折射率匹配的載子傳輸層，並優化各有機層和透明電極的厚度，可以達到非常高的光耦合出光效率約90%（即有機發光元件外部量子效率約90%）。運用這些適當的元件設計條件，可以有效地抑制元件中所有的波導模態和大部分表面電漿模態，以達到最佳化的光輸出耦合效率。在元件實驗中，結合高折射率n〜1.78的藍寶石基板以及最新開發的有機發光元件之放光材料，其水平偶極矩比例高達87%，搭配上簡單的外部光萃取透鏡，成功實現了外部量子效率超過80%的有機發光元件 。
此外，近年來已經有許多透明有機發光元件及其可能應用之報導，例如透明顯示器、透明照明面板、頭戴式顯示器、智能窗戶等，因此，透明有機發光元件之出光效率以及穿透度是研究所關注之議題。本論文研究的第二部份中，運用高折射率摻鈮之二氧化鈦(TNO)透明電極（折射率約為2.4）與高水平偶極矩發光材料（水平偶極比率為87%）之組合，增強透明有機發光元件之微共振腔效應與有效地抑制侷限模態，進而顯著提高透明有機發光元件的整體出光耦合效率，達到不利用外部出光結構即可具有33.5%之總外部量子效率且峰值穿透率高達73%。除此之外，藉由調整元件厚度來達到不同穿透反射頻譜的峰值位置，使元件在未點亮前，因不同結構下呈現不同的顏色變化，使得透明元件更具有特色，也具有更多有趣的應用，例如彩色教堂和車用抗藍光擋風玻璃顯示器或照明面板等。

關鍵字

有機發光元件 ； 透明有機發光元件 ； 熱激活化延遲螢光 ； 水平發光偶極矩 ； 高折射率基板 ； 高折射率透明電極 ； 微共振腔

並列摘要

With the development of organic light-emitting diodes (OLEDs), it has been regarded as an important display and lighting technology. The main goals of the current development focus on two major directions. The first is the development of new materials, that can improve efficiency or carrier transport characteristics. With the efforts of many researchers, many high-performance materials have been developed. At present, thermally activated delayed fluorescent materials and phosphorescent materials giving internal quantum efficiency close to ideal 100% have been developed. The second is to improve the light extraction of OLEDs. How to extract more light from the organic light emitting diode to air is an important issue.
Although both high-index substrates and horizontal-dipole emitters have been shown to be facile approaches for enhancing OLEDs light extraction, the full benefits and potential of their combination for OLEDs optical out-coupling have not been thoroughly studied and explored. In the first part of this thesis study, simulation studies indicate that very high optical coupling efficiency into substrates ϕsub (and perhaps similarly high OLEDs external quantum efficiencies) of ~90% can be possibly obtained with both high-index substrates (refractive index >1.8-1.9) and highly horizontal-dipole emitters (horizontal dipole ratio >85%), together with adoption of low-index or index-matching carrier transport layers and optimization of organic layer and transparent electrode thicknesses. With these judicious device design conditions, all waveguided modes and surface plasmon modes in devices can be effectively suppressed for optimal optical out-coupling. Finally, combining the sapphire substrate having high index of n~1.78, the recently developed OLEDs emitters having high horizontal emitting dipole ratio of up to 87%, and simple external extraction lens, OLEDs devices having external quantum efficiency of over 80% were successfully realized.
In recent years, possible applications of transparent organic light-emitting devices (TOLEDs) have been proposed, such as transparent displays, transparent lighting panels, head-mounted displays, and smart windows etc. As such, the efficiency and transparency of TOLEDs have been extensively studied. In the second part of this thesis, with the adoption of the high-index transparent electrode and the highly horizontal dipole emitters in transparent organic light-emitting devices (TOLEDs), the guided modes can be efficiently suppressed. As a result, overall coupling efficiencies of TOLEDs can be significantly enhanced. By using blue thermally activated delayed fluorescence (TADF) emitters with a highly horizontal dipole ratio of 87% and high-index niobium-doped titanium oxide (TNO) transparent electrode (with refractive index of 2.4), TOLEDs achieving a high external quantum efficiency (EQE) of up to 33.5% and a high peak transmittance of up to 73% was demonstrated. Furthermore, we could also adjusted the thicknesses of carrier transport layers to realize color-tunable transmissive/reflective hues in the off-state of such TOLEDs may have some interesting applications, such as stained color windows for churches and stylish smart windows etc.

並列關鍵字

OLEDs ； transparent OLEDs ； thermally activated delayed fluorescence ； horizontal emitting dipole ； high-index substrate ； high-index transparent electrode ； microcavity

參考文獻

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