液態製程刮刀塗佈小分子有機發光二極體效率及壽命改善

Abstract

本論文研究主要以刮刀塗佈製備全液態製程有機發光二極體，透過改善其製程條件進一步得到高效率元件以及強化元件操作壽命。 第一部分回顧有機發光二極體發展歷史，接著介紹液態製程有機發光二極體的發展以及本研究所使用之刮刀塗佈製程。 第二部分，系統性的針對全液態製程有機發光二極體之材料溶解性，薄膜形態，元件表現等特性進行研究。 從第二章到第五章，我們使用本實驗是開發之刮刀塗佈製程製作全液態有機發光二極體，其利用下方加熱器以特定溫度均勻地加熱及上方均勻之熱風裝置快速將溼膜形成乾膜的特性，此製程可用於液態製程有機半導體元件的製做。在第二章及第五章，我們使用為蒸鍍製程設計開發之未改質小分子材料利用刮刀塗佈製程製作全液態有機發光二極體，並且在相同元件架構下比較熱蒸鍍與刮刀塗佈製程之效率表現，本研究成果顯示，數個相同元件架構下以刮刀塗佈方式製作之有機發光二極體幾乎可以達到熱蒸鍍製程製作之元件的效率。 第三部分，研究如何提升刮刀塗佈製程製作有機發光二極體元件壽命，全液態製程可以藉由摻雜多種材料於溶液中進行塗佈，解決了真空蒸鍍製程中多種材料共蒸鍍的難題，即可達到調控及維持載子注入的平衡以改善元件的效率與壽命等各項表現。本研究利用同質接面元件(homojunction)、雙主體結構(co-host)、單層結構(single layer) 等方法改善元件壽命。透過加熱來達到超過材料的玻璃轉移溫度，使有機材料的小型鍵結略微移動、排列更加穩定，促使元件提升壽命。此外，多數液態製程有機發光二極體會使用PEDOT：PSS作為電洞注入層材料，但PEDOT：PSS屬於酸性物質具有侵蝕性，直接接觸發光層長時間操作可能會對其造成傷害，導致元件特性表現下降，因此我們引入一層堅固結構之傳輸層以避免發光層受到酸性的侵蝕，同時也可以防止一些薄膜上會產生漏電的缺陷，再加上其電洞傳輸的特性，可以幫助載子的注入，以達到注入平衡，進而提升元的特性與壽命。

In this thesis, I focus on the device engineering and the improvement of operation-lifetime of all-solution-processed organic light-emitting diodes (OLEDs) through adopting a blade coating process. In the first part of the thesis, I reviewed the background of OLEDs, the literature review on OLEDs development, the solution-process OLEDs, and the means of improving OLEDs. In the second part, I systematically investigate the properties of all-solution-processed OLEDs, such as the material solubility, thin-film morphology, and device performance. From chapter 2 to chapter 5, blade coating with substrate heating and hot wind was demonstrated to be a general approach to the multi-layer deposition of unmodified small-molecule organic semiconductors. The optimized OLEDs structure developed for vacuum deposition can therefore be exactly copied through a low cost blade coating method involving a solution process. A general method for characterizing the interface dissolution in multi-layer OLEDs by blade coating was developed on the basis of fluorescence microscopy. Tris (8-hydroxyquinolinato) aluminum (Alq3) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq3 through a solution process is difficult because of its extremely low solubility in organic solvents, which is due to its symmetrical molecular structure. In this study, Alq3 was successfully deposited through blade coating at an extremely low concentration below 0.1wt. %, and applied to the fluorescent materials used in OLEDs. The third part of this thesis, described an effective method for fabricating all-solution-processed phosphorescent OLEDs and fluorescent OLEDs with a long operation-lifetime.