寡聚芴化物有機發光二極體之電子結構及界面化學

Abstract

利用高解析度同步輻射光源之光電子發射能譜，本論文將有系統地研究寡聚芴化物之電子結構、界面化學以及變溫特性。首先，我們觀察並歸納出寡聚芴化物的能階結構和許多重要的特性，包括游離能、界面電偶極和發光能隙，同時我們也在這一系列寡聚芴化物中發現到一些規則性的趨勢。而根據本篇論文的研究，寡聚芴化物的支鏈取代物將會大幅地影響到其電子和光學特性，寡聚芴化物之元件操作性能，也會受到支鏈取代物的影響。為了讓寡聚芴化物元件之電子注入能障降低，並且提升電子在陰極界面的注入效率，氟化鋰和氟化銫等鹼金屬氟化物將被用作為寡聚芴化物之n型摻雜物，這兩樣n型摻雜物都能夠與寡聚芴化物反應並造成n型摻雜的結果，但卻不需要鋁的存在，這使得寡聚芴化物元件在陰極金屬的使用上有了更多地選擇。然而想要獲得高效率之元件陰極結構，只有n型摻雜是不夠地，透過研究我們發現，n型摻雜和能隙能階的產生都是獲得高效率元件陰極結構的必要條件。另外，經由價電帶電子能譜的證明，我們證實了碳酸銫對寡聚芴化物也是有效的n型摻雜物，而氧化鉬等過渡金屬氧化物則是有效的p型摻雜物，還有四氟-四氰基代對二亞甲基苯則是有機物之p型摻雜物。最後我們更探討了溫度變化對寡聚芴化物能階和構形的影響，在不同溫度下蒸鍍之寡聚芴化物將會改變其分子內部不同單體單元之相對角度，在低溫時角度大約是將近垂直的狀態，但在室溫的穩定狀態下則會呈現41°的角度。而原本鹼金屬氟化物和寡聚芴化物之間會產生n型摻雜效果的反應，在低溫時將不會出現，這是因為在低溫時沒有足夠之熱能來驅使反應發生，但一旦溫度回升到室溫時，鹼金屬氟化物和寡聚芴化物之間的n型摻雜反應將會再度發生。

The electronic structures, interfacial chemistry, and temperature dependences of oligofluorenes are investigated via high resolution synchrotron radiation photoemission. The energy structures and physical properties of pristine oligofluorenes are reported and their regular tendencies are observed. Substitutions of the pristine oligofluorenes will significantly influence the electronic and optical characteristics and the operation of oligofluorene-based devices are also substitution dependent. In order to lower the electron injection barrier and enhance the electron injection efficiency at the cathode, the alkali metal fluorides as LiF and CsF are used as n-type dopants for the oligofluorenes. Both of them react with oligofluorenes without the existence of Al and offer us more choices for cathode metals. Our results indicate that to obtain an effective cathode structure, not only n-type doping effect, but also gap state appearance is crucial. In addition, Cs2CO3 as n-type dopant and transition metal oxide, such as MoOx, and F4-TCNQ as p-type dopants are verified. Influences of temperature and inter-unit angle to the electronic structures of oligofluorenes are also studied. Deposition at different temperatures alters the electronic structures due to the inter-unit angles in oligofluorenes molecules. To be specific, the fluorene-units at low temperature are nearly perpendicular to each other and the inter unit angle becomes 41° in the stable state at room temperature. In addition, the low temperature will soften the n-type doping reaction between alkali metal fluoride and oligofluorenes, since the thermal energy is too low to drive this reaction. Upon the sample warming back to the room temperature, however the reaction between these two layers is completed.