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標題: | 可用於增進有機發光元件效率之光學結構研究 Investigation of Optical Structures for Enhancing Efficiencies of Organic Light-Emitting Devices |
作者: | Min Jiao 焦閔 |
指導教授: | 吳忠幟(Chung-Chih Wu) |
關鍵字: | 有機發光二極體,微共振腔效應,出光技術, OLED,Micro-cavity effect,out-coupling, |
出版年 : | 2017 |
學位: | 博士 |
摘要: | 隨著有機發光二極體的發展,現今有機發光二極體幾乎具有100%的內部量子效率,對於達到高外部量子效率,光萃取技術更顯重要。而氧化銦錫具有高透明以及高導電度,因此現今廣泛應用於各種光電元件之透明導體。但由於地殼中稀土元素銦的稀少,導致銦的價格較高,又銦的易脆性以及製程溫度高,限制了現今軟性光電元件的應用。因此近年來廣泛尋找能取代氧化銦錫之透明導體,在其中導電高分子PEDOT:PSS因為具有優異的機械可饒性、高穿透度、高導電性、可溶液製程、低成本,最重要的是具有低折射率(n~1.5)之特性。利用PEDOT:PSS之優勢,本論文第一部分展示出應用低折射率PEDOT:PSS來取代ITO電極之有機發光元件。其中,PEDOT:PSS低折射率特性可以抑制波導模態,使更多的光進入到基板,但因在基板之場型能量分布大多集中在較大角度,反而使直接出光效率降低。因此利用在PEDOT:PSS電極下成長高折射率之二氧化鈦薄膜,可以增強有機發光二極體之微共振腔效應,此元件結構不但具有低折射率PEDOT:PSS電極抑制波導模態之特性,也可利用微共振腔效應改變光在基板之能量分布,使更多光能夠直接出到空氣,展示出達到外部量子效率39%之平面有機發光元件。
穿透式有機發光元件具有許多有趣之應用,為近年來快速發展之有機發光元件技術,應用層面從穿透式顯示器、穿透式照明技術、頭戴式裝置、智慧櫥窗與建築藝術到車用裝置.....等等。因此,本論文第二部分展示利用高折射率摻鈮之二氧化鈦(TNO)電極與低折射率PEDOT:PSS電洞注入層之組合,增強有機發光元件之微共振腔效應與出光效率,達到透明有機發光元件不利用外部出光結構即可具有30.8%之總外部量子效率(對應到功率效率132.6 lm/W以及電流效率105.8 cd/A),以及高峰值穿透率74%。除此之外,由於此元件具有較強之微共振腔效應,藉由調整兩反射面之強度,展示出雙面發光比例平衡之穿透式有機發光元件,總外部量子效率達到27.5% (下發光外部量子效率13.8%與上發光外部量子效率13.7%)。 Although current organic light-emitting devices (OLEDs) can offer ≈100% internal quantum efficiencies (IQEs), light extraction of internally generated emission remains a critical issue for achieving high external quantum efficiencies (EQEs) for display and lighting technologies. In addition, indium tin oxide (ITO), with its high conductivity and transparency, has been the most commonly used transparent conductors for transparent electrodes of optoelectronics, including organic optoelectronics such as organic light-emitting devices (OLEDs). However, scarcity of the indium element renders ITO a high cost material. Brittleness and usual high processing temperatures also make it incompatible with flexible devices and applications. Alternative (ITO-free) transparent conductors/electrodes for replacing ITO have thus been intensively pursued in recent years. Among them, the conducting polymer PEDOT:PSS has been an attractive candidate due to its excellent mechanical flexibility, good transmittance and conductivity, solution processing capability, low cost, and low refractive index (n~1.5). Therefore, we adopted conducting polymer PEDOT:PSS as transparent electrode in planar ITO-free OLED. Although the low index of PEDOT:PSS electrode helps to suppress waveguided modes and couple more light into the substrate, more radiation is dispatched outside the escape cone and cannot be out-coupled directly. Such a dilemma can be removed by simply inserting a high-index layer between the PEDOT:PSS electrode and the substrate to induce an appropriate microcavity effect. As such, most of optical benefits of the low-index PEDOT:PSS electrode (suppressed waveguided modes, enhanced optical coupling into substrates) are retained, and yet the radiation into the substrate is now more convergent within the escape cone for direct optical out coupling. As a result, a simple planar ITO-free OLED having significantly enhanced optical out coupling and EQE (nearly 39%) is demonstrated without adopting any external out-coupling technique. Transparent organic light-emitting devices (OLEDs) are also one of the most fascinating developments of OLED technologies in recent years due to their various possible interesting applications. Possible applications of transparent OLEDs could range from transparent (see-through) displays, transparent (see-through) lighting panels, head-mounted displays, smart windows for architectural or advertising purposes, to navigation/warning displays on car windshields. In this thesis, we adopted combination of high refractive index TNO electrode and low refractive index PEDOT:PSS hole-conducting layer to enhance microcavity effect and out-coupling efficiencies of transparent OLEDs. A simple planar transparent niobium-doped titanium oxide (TNO)-based green phosphorescent OLED having significantly enhanced total optical out-coupling and EQE of up to 30.8% (corresponding to a total current efficiency of 105.8 cd/A and total luminous efficiency of 132.6 lm/W) and a rather high peak transmittance of up to 74% is demonstrated without adopting any external out-coupling technique. Besides, due to stronger microcavity effect of TNO-based device, we also demonstrated a TNO-based transparent OLED exhibiting a highly balanced ratio of bottom to top emission ≈1 and a rather high total EQE of up to 27.5% (bottom emission 13.8% and top emission 13.7%). |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/77820 |
DOI: | 10.6342/NTU201800798 |
全文授權: | 有償授權 |
電子全文公開日期: | 2023-06-07 |
顯示於系所單位: | 電子工程學研究所 |
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