臺灣博碩士論文加值系統

摘 要
　　在有機發光元件中，提升元件光電特性及效能的方式有二。 其一，為藉由元件結構的設計及處理來平衡在有機物中的載子傳輸數量，使電子和電洞數目趨於一致，進而改善元件效率；其二，則為尋找並研發新的有機材料，以改善電子的傳輸和材料的發光效率。在本論文中，我們採用第一種方式，針對有機發光元件結構的部份來做改善，增進其發光效能。大致上分為四部份：1.陽極表面的電漿處理 2.陽極與有機層間的阻隔層 3.陽極與有機層間的雙緩衝層 4.電洞阻隔層之機制與改善。

　　首先我們以ITO/TPD/Alq3/Al結構的有機發光二極體作為研究的基礎，利用氬氣及氧氣電漿去處理ITO透明電極的表面；我們發現元件在最佳的處理條件下電性有顯著的提升。其次，由原子力顯微鏡 (AFM)去偵測ITO表面形態及粗糙度，我們發現經由電漿處理過的陽極，其表面均方根粗糙度 (RMS Roughness) 及片電阻可以有效的降低，並且經顯微鏡照相的觀察，我們發現元件暗點數目大幅的減少。這重要結果對於我們元件的穩定性及壽命的增進有當相大的助益。

　　在本研究的第二部份，我們將具高介電常數的絕緣層Ta2O5使用於陽極和有機層的界面當電洞阻隔層。很明顯地，加入絕緣層阻隔了部分電洞的注入，使注入電子電洞數目較平衡，亦使元件的起始電壓上升；因此元件的亮度及效率大幅提升。在Ta2O5最佳厚度為5Å時可得到最佳的元件效能。

　　在本論文的第三部份，我們以 ITO/CuPc/MTDATA/NPB/Alq3/AL 結構，來探討雙層緩衝層[ ITO/CuPc/MTDATA ] 對有機發光二極體之光電特性的變化和其機制。利用改變CuPc 和 MTDATA 厚度的比例，來研究不同厚度比例對元件光電特性的影響。在對於CuPc 和 MTDATA的探討中，可以觀察到 MTDATA比CuPc 適合作為緩衝層的材料，並且適當的雙層緩衝層能較為有效的減少超額電洞的注入，提高復合的效率，產生較佳的光特性。

　　最後，在本篇論文中，我們使用最常見的電洞傳輸材料NPB加上電洞阻隔層BCP來製作藍色有機電激發光元件。我們元件的架構為ITO/MTDATA/NPB/BCP/AlQ3/LiF/Al。我們觀察到當元件在直流電壓操作時，若持續增加偏壓，電洞將會聚積在NPB和BCP的界面上，最後過量的電洞聚積將會使BCP層崩潰。為了減少聚積在界面上的電洞並增加元件的效率，我們使用分層電洞阻隔層的架構來分散界面上的聚積電洞。我們發現雙電洞阻隔層不但在發光強度上，並且在光譜的半高寬(FWHM)上都有相當程度的改善。

Abstract

　　There are two ways to improve the opto-electric performance and efficiency of the organic light-emitting device. The first one is to balance the numbers of transported carriers by means of the improving structures and treatments of the device. The second one is to search and synthesis new organic materials to increase the electron mobility and emission efficiency of the material. In this study, we have revolved around the structure improvements in organic light-emitting devices. This work deals with four topics in organic light-emitting devices. The first is related to indium-tin-oxide (ITO) anode cleaning with plasma treatment. The second topic regards the insulating layer on ITO anode. The third topic concerning hole injecting with double buffer layers. And the last part of this work is the mechanism of hole-blocking layer and its improvement.

　　In the first part of this work, the Ar and O2 plasmas were employed on ITO anode surface, respectively. The atomic force microscopy (AFM) was then used to measure the surface morphology and do roughness analyses. It can be found that both the root-mean-square roughness and the sheet resistance of the plasma-treated ITO anodes have reduced. Furthermore, with a microscopy inspection, there were less dark spots in our devices after plasma treatments. This shows that the stability and lifetime of our devices can be greatly improved using plasma treatments.

　　The second part of this study is inserting an inorganic, high dielectric constant, insulating Ta2O5 layer to separate the ITO anode from the hole transporting layer (HTL). The Ta2O5 insulating layer can effectively block out partial hole-injections from the anode. It was observed that the enhancements in brightness and EL efficiency in our devices due to an improved balance numbers of the holes and the electrons. The optimum thickness of Ta2O5 layer in our devices is 5Å.

　　The third part of this work is using double buffer layer [ITO/CuPc/MTDATA] in our devices to study their emission characteristics and conducting mechanism. In the studies of CuPc and MTDATA, MTDATA is found to be a better buffer material than CuPc. By changing the thickness ratio of CuPc to MTDATA, an optimum double buffer layer can be reached. The double buffer layer structure reduces the excess hole injection and increases recombination efficiency.

　　Finally, we focus our study on blue organic light-emitting device with the commonly NPB/BCP structure. It was observed that with increasing applying voltage, holes will accumulated in the NPB/BCP interface and BCP layer will finally breakdown due to the excess field. A double hole-blocking structure NPB/BCP/NPB/BCP was proposed to distribute the excess hole accumulation and increase the stability of device. With the double hole-blocking structure, the devices largely improve the brightness and reduce the full width at hall maximum (FWHM).

Content
Abstract (in Chinese) i
Abstract (in English) iii
Acknowledge v
Content vi
Table Captions ix
Figure Captions x

Chapter 1 Introduction 1
1-1 Brief History of Organic Electroluminescence 1
1-2 Optical phenomena in organic molecules 2
1-3 The operation theory of OLEDs 3
1-4 Comparison of OLED and other flat panel displays 5
1-5 Thesis outline 5
Chapter 2 ITO substrate cleaning-plasma treatment 15
2-1 Abstract 15
2-2 Introduction 15
2-3 Experiment 16
2-4 Results and discussion 17
2-5 Conclusions 18
Chapter 3 Improvement of carrier balance by Ta2O5 buffer layer on ITO 26
3-1 Abstract 26
3-2 Introduction 26
3-3 Experiment 27
3-4 Results and discussion 28
3-5 Conclusions 29
Chapter 4 Improvement of hole injection by stepwise energy level alignment 36
4-1 Abstract 36
4-2 Introduction 36
4-3 Experiment 37
4-4 Results and discussion 38
4-5 Conclusions 39
Chapter 5 Luminance improvement by double hole-blocking structure 47
5-1 Abstract 47
5-2 Introduction 47
5-3 Experiment 48
5-4 Results and discussion 50
5-5 Conclusions 52
Chapter 6 Conclusions and Future Prospects 63
6-1 Conclusions 63
6-2 Future Prospects 64

References 67

Appendix 72
A. Calculation of Efficiency 72
B. Publication List 74

Resume 76

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