N型摻雜有機發光二極體電子結構及界面化學之研究

Abstract

在本論文中，利用紫外光以及X光激發光譜來研究n 型摻雜物在有機發光元件中的影響並探討電子注入機制與介面的化學反應。 首先，在有機發光元件中，探討碳酸銫在熱蒸鍍過程中的特性與電子注入層中所扮演的角色。實驗結果顯示，碳酸銫在熱蒸鍍的過程中是不會解離的。而且，導致有效電子注入機制之原因則與碳酸銫所引起的n型摻雜物之效應有強烈的相關連性，不僅可以降低電子注入能障，並且可增加載子的濃度。 第二，有系統性的比較與研究銫衍生物(碳酸銫、氟化銫、氮酸銫)摻雜在 8-羫基喹晽鋁(Alq3)中之影響。由光激發光譜之實驗結果得知，在這三種n 型摻雜物中，碳酸銫所導致的n 型摻雜效應是最明顯的。並且，以氟化銫或者碳酸銫為電子注入 層的元件，皆可搭配不同的陰極而有良好的元件特性，至於以氮酸銫為電子注入 層的元件則與鋁陰極的搭配效果較佳。 最後，相對於8-羫基喹晽鋁(Alq3)而言，以擁有較佳電子遷移率的4,7-二苯基鄰菲咯啉(Bphen)當作電子傳輸層並搭銣化銫做為電子注入層來分析介面光譜與元件特性之比較。經由電流-電壓的特性曲線可得知，相對於氟化鋰而言，以銣化銫當作電子注入層的元件擁有較佳的結果。並且，由實驗光譜結果得知，元件效應的改進，其主要原因是因為強烈的n型摻雜物之效應以及在最高被占據的分子軌域旁所產生的能態。而由分子模擬結果可得知，所產生的能態是鋁原子與氮原子之間的化學反應所得的產物。

In this dissertation, the mechanisms and effects of n-type dopants in organic light-emitting devices (OLEDs) are discussed in detail. The electronic structures and interfacial chemical reactions are studied via ultraviolet and x-ray photoelectron spectroscopy. First of all, the electron injection mechanisms of the effective cathode structures for organic light-emitting devices incorporating cesium carbonate (Cs2CO3) are investigated. The experimental results show that Cs2CO3 is not decomposed during the evaporation. Moreover, the enhanced electron injection is associated with strong n-doping effects after tris-(8-Hydroxyquinoline)-aluminum (Alq3) molecule doped with Cs2CO3. Furthermore, the properties of thermally evaporated Cs2CO3 and its role as electron-injection layers are also examined. Second, the effectiveness of Cs-derivatives (Cs2CO3, CsF, and CsNO3) as dopants of Alq3 are studied systematically. The n-type doping effect resulted from Cs2CO3 in Alq3 is the strongest. From the view of current efficiency, the performance of OLEDs with CsF in cathode structures is similar to that with Cs2CO3, which can function well with both aluminum and silver in cathodes. As for CsNO3, it is effective only with aluminum cathode due to the reaction between aluminum and CsNO3. Finally, 4,7-diphenyl-1, 10-phenanthroline (Bphen) is introduced as electron-transport layers according to the mobility in Bphen being higher than that in Alq3. Therefore, the electronic properties and chemical interactions of cathode structures using Bphen doped with rubidium carbonate (Rb2CO3) as electron injection layers are investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron-injection efficiency than those with cathode structures of Bphen/LiF/Al. On the basis of experimental results, the n-type doping effects caused by Rb2CO3 and the gap states created by aluminum deposition are both keys to enhance carrier-injection efficiency. In parallel, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.