| 研究生: |
王振道 Wang, Chen Tao |
|---|---|
| 論文名稱: |
軟性透明陽極之製備及表面處理應用於有機發光元件 The Fabrication of Flexible Transparent Electrodes and Surface Treatment for Organic Light-Emitting Devices |
| 指導教授: |
朱聖緣
Chu, Sheng-Yuan |
| 學位類別: |
博士 Doctor |
| 系所名稱: |
電機資訊學院 - 電機工程學系 Department of Electrical Engineering |
| 論文出版年: | 2019 |
| 畢業學年度: | 107 |
| 語文別: | 英文 |
| 論文頁數: | 100 |
| 中文關鍵詞: | 三明治電極 、有機發光二極體 、表面能 、軟性電極 、活化能 、陽極緩衝層 、介面粗糙度 、表面處理 |
| 外文關鍵詞: | OLEDs, Interface Roughness, Transmittance, Flexible, Surface Treatment, Anode Buffer Layer, Transparent Conducting Electrodes |
| 相關次數: | 點閱:262 下載:15 |
| 分享至: |
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有機發光二極體(OLEDs)因為有著製程簡易、高反應速度、高對比、輕薄及應用便利等優點,因此被認為是現今小尺寸最具競爭優勢的顯示器,而隨著光電元件步入穿戴式元件的時代,OLED元件的可撓取特性引起了許多關注,現今工業上普遍使用ITO作為OLED元件的透明陽極,但其不耐撓曲以及銦離子擴散等特性,使得OLED元件的應用受到嚴重限制,因此開發新型的替代電極是現今備受關注的議題,本論文利用銀網狀電極(silver grids)來做為OLED的透明陽極,並分為三部分進行討論。
第一部分將探討銀網狀電極之表面特性,首先,我們利用黃光製程及熱蒸鍍法來製備六角形的銀網狀電極,先利用品質因子(Figure of Merit, FOM)來確定電極之最佳參數,並利用表面能 (surface energy)及能量色散X-射線光譜 (EDS) 來進行表面分析,發現銀網狀電極會因為空氣中的硫分子產生硫化,進而使得其親水性提高,但單純的銀網狀電極其表面粗糙度仍然非常高,因此在第二部分將會對粗糙度的部分來做進一步的改善。
第二部分將會利用銀網狀電極製備出MoO3/Ag grids/MoO3的三明治軟性電極來改善傳統的MoO3/Ag film/MoO3電極之穿透度及機械性質,並利用AFM來探討電及表面之粗糙度變化,其中MoO3作為透明電極之平整層來降低銀網狀電極之表面粗糙度,但MoO3在可見光區有明顯的衰減,我們為了改善此一現象,使用ZnO來取代MoO3並製備成ZnO/Ag grids/ZnO的透明電極,並改變銀網狀電極的厚度,發現銀網狀電極厚度會對會對ZnO的結晶產生影響,並進而影響透明電極載子遷移率。
第三部分將會探討表面處理對陽極緩衝層及銀網狀電極的影響,由於MoO3作為平整層以及陽極緩衝層,所以MoO3和電洞傳輸層(Hole Transport Layer, HTL)之間的介面特性及機制探討尤為重要,我們使用正己基磷酸 (hexylphosphonic acid, HPA)以及UV-ozone 對陽極緩衝層進行表面處理,再利用過往使用在記憶體量測活化能的變溫I-V量測方法,結合阿瑞尼士圖來計算出無機之陽極緩衝層和HTL之間的能障差距,並將使用HPA、UV-ozone處理後之MoO3陽極緩衝層製備成元件進行機制探討。
此外,在本論文中使用Ag grids/MoO3製備OLED元件,但Ag grids和MoO3平整層之間存在一明顯的能障差距,因此我們利用UV-ozone進行表面處理,使Ag grids表面形成銀氧化物(Ag2O或AgOx)的緩衝層來減少Ag grids和MoO3平整層之間的功函數差距,並利用XPS來證明銀氧化物緩衝層的組成成分,最後將其製備成OLED元件,並利用元件特性來加以驗證我們的論述。
Organic Light Emitting Diodes (OLEDs) have been developed as a future display technology due to many advantages such as fast response time, high contrast and application to be incorporated in lightweight display. Indium tin oxide (ITO) is the most commonly used material for transparent conducting electrodes (TCEs) for OLEDs due to its excellent optical transmittance (typically 85% in the visible wavelength region) and low sheet resistance (as low as 10Ω/square). However, ITO has a high cost due to the scarcity of indium and its fragility is a significant drawback for use in OLEDs. In addition, ITO requires a high processing temperature and the migration of indium limits the lifetime of OLEDs. These drawbacks limit the application of the ITO TCEs. Alternative TCEs are thus needed to replace ITO electrodes. In this research, we choose silver grids based TCEs as anode in OLEDs. The thesis are divided into three parts.
In the first part of this thesis, we fabricate the silver grids TCEs via the thermal deposition method. The proposed grid shows low sheet resistance and a good figure of merit. The sheet resistance decreased from 688 to 3.37Ω/square when the thickness was increased from 30 to 70 nm. The samples are characterized in terms of the contact angle to calculate the surface energy and polarity. The surface energy and polarity of the samples increased from 8.15 to 58.029 mJ/m2 and 0.024 to 0.067, respectively, when the sulfur content was increased from 6.67 to 9.26% (thickness increased from 50 to 70 nm). The fabricated Ag grid transparent conducting films show good optical and electrical characteristics and have potential for application in optoelectronics.
Secondly, we fabricate the MoO3/Ag grids/MoO3 (MAM) flexible TCEs to smooth the surface morphology of silver grids. The proposed structure also improves transparency compared with that of the traditional tri-layer electrode (dielectric/metal film/dielectric) by using metallic grid patterns (dielectric/metal grids/dielectric). The MoO3 layer will decrease the transmittance although it smoothes the surface roughness of the silver grids. Therefore, we replaces the MoO3 layer with zinc oxide (ZnO) to fabricate the ZnO/Ag grids/ZnO (ZAZ) structure via thermal deposition. We find the crystallization and electrical, optical, and mechanical characteristics of ZAZ TCEs are compared with those of MAM and ZnO/Ag film/ZnO TCEs. It is found that the improvement in electrical characteristics is due to the crystallization of ZnO film.
In the third part of this thesis, we discuss the surface treatment on MoO3 layer and the silver grids. When the silver grids with an UV ozone treatment duration of 15 s, the Ag2O thin films do not grow completely and current-voltage characteristics are poor. However, a 30 s UV-ozone treatment yielded good-quality Ag2O thin films. The Ag2O thin films were reconverted into the AgOx phase with further increases in UV-ozone exposure time. The Ag2O work function is nearly 5.0 eV, which decreases the injection barrier of the silver grids (~4.7 eV) and MoO3 (~5.3 eV). Nevertheless, excessive treatment time leads to the production of AgOx thin films and an increase in the work function to 5.3 eV, the same as the highest occupied molecular orbital energy of MoO3, which causes a work function mismatch. The work function mismatch between the Ag grids and the MoO3 layer results in a high injection barrier, decreasing OLED performance. The electrical properties of the electrodes and devices apparently depend on the composition of the silver oxide buffer layer, as determined using X-ray photoelectron spectroscopy. The surface and optical properties of the TCEs were also investigated. The results show that the OLED devices with the proposed TCEs have better roll off and current efficiency compared to traditional ITO-based devices. We also demonstrate the performance of OLEDs with hexylphosphonic acid (HPA) or UV-ozone treatment on their MoO3 anode buffer layers. The OLEDs with a PA treated (5 mM @1 h) MoO3 layer have lower turn-on voltage and low current efficiency roll-off under high operating current. The hole-only device (ITO/ MoO3 with PA or UV-ozone treatment/NPB/Al) was fabricated to calculate the active energy via temperature dependent I-V measurement. When the devices were operated at high temperatures, the activation energy of the UV-ozone treated, and untreated hole-only devices became nonlinear. However, the activation energy of the PA treated devices had a more stable performance at high temperatures. The interfacial resistance of the untreated hole-only devices and the PA and UV-ozone treated devices were calculated by Admittance Spectroscopy (AS).
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