| 摘要: | 有機發光二極體中，陽極與其上的有機材料層介面間，由於存在了能階障礙 (Schottky energy barrier，φe)，影響了電荷的注入，以及有機發光二極體元件效率及壽命。藉由具有不同偶極矩大小及方向的有機分子薄膜來修飾電極表面，可調控電極的功函數。降低電極與有機材料層間的能障，從而有助於載子的注入，進而提高有機發光二極體元件效率。
本篇論文是以不同偶極矩大小的有機亞磷酸分子(PyOPA、CF3SO2BnPA、CF3BnPA和CF3C3PA)以不同吸附方式(氣相與液相)來製備自組裝單分子薄膜修飾於ITO電極表面上及後續元件製程。由之前的文獻中[1],分子偶極矩的大小及方向對於ITO電極功函數的增減有重要的影響。一般使用氧電漿處理過後的ITO表面上功函數約為5.8電子伏特，而當所用自組裝單分子薄膜的尾端為一拉電子基團時，偶極矩的方向為指離ITO表面，能提升ITO電極的功函數；推電子基團則相反。
然而，由我的實驗中，分子偶極矩的增加並非完全能表現在功函數的增加，因為分子吸附在ITO表面上時，分子是否緊密堆疊，垂直站立在ITO電極上有重要的影響，由反射式紅外線光譜儀測量，當所使用分子吸附於ITO上時，會偏離ITO表面的法線方向，使得有效偶極矩減少，功函數無法如期增加，造成有機發光二極體元件效率無顯著提高。
在汽相吸附的實驗中，由XPS光譜圖中發現，在同樣吸附時間下，測到的P 2p和F 1s對In 3d3/2的強度都比在溶液中浸泡吸附來的大，顯示可能不只單層存在。多層的存在。使得載子不易穿隧到有機層中，造成元件電流密度和發光強度較低。而以乙醇震洗過後的結果亦同。然而由光學顯微鏡中看到，當吸附完後未經乙醇震洗的基板所製成元件，其上殘留較多黑點；當使用乙醇震洗和科技泡棉輕刷表面後，黑點數目下降許多，雖然震洗完後，無法完全把表面多餘的有機分子帶走，但在元件發光表面上，可以除去不少汙染物，因此我們認為當今天以氣相修飾過後的基板，可能還是要以溶劑來去除其表面上的汙點，使其不會影響到元件的品質。
;In organic light emitting diode, there exists an energy barrier(Schottky energy barrier,φe) between the anode and the organic layer, affecting the charge injection and thus the efficiency and lifetime. By using organic molecules having different size and orientation of the dipole moment to modify the anode surface, one can regulate the interfacial barrier between the electrode and the semiconductor, favoring charge carrier injection and increase efficiency in OLED device.
In this study, organic phosphonic acid molecules(PyOPA, CF3SO2BnPA, CF3BnPA and CF3C3PA) with different sizes of dipole moment were used to prepare self-assembled monolayers on the ITO surface by different methods(vapor phase and solution phase) for subsequent device fabrication. According to previous literatures, the magnitude and direction of the molecular dipole moment of the modifying molecule has an important influence on the work function of ITO electrode. In general, ITO surface treated by oxygen plasma has a work function near 5.8 eV .For ITO modified with a SAM carrying an electron-withdrawing end group(ex trifluoromethyl), the direction of the dipole moment is pointing away from the ITO surface, the work function of the ITO electrode will be increased. The reverse is true for electron-donating group.
However, it was found from my experiment that, further increasing the dipole moment in the molecule does not necessarily increase the work function proportionally. The packing and orientation of the molecules adsorbed on the ITO surface, plays an important role in modulating the work function. By the reflection absorption infrared spectroscopy measurement, it is suggested that the strongly polar molecule adsorbed on the ITO may orient away, from the ITO surface normals, leading to a smaller effective dipole along the surface normal and the work function did not increase as expected so that the device efficiency did not increase as expected.
In gas adsorption experiments, the XPS spectrum suggest that after the same adsorption time, the P 2p and F 1s intensity versus of In 3d3 / 2 is stronger than that for the sample prepared by immersion in the solution, suggesting more than monolayer was obtained. For devices, lower current density and luminance were obtained ,similar result were obtained after sonication of the substrate by ethanol. However, by optical microscope, it was observed that for sample without sonication of the substrate after gas phase adsorption, there are many dark spots in the device. After rinsing the substrate with ethanol and rub gently the surface of substrates by a foam pad, the number of dark spots decreased. Although sonication in ethanol did not completely remove the excessive organic molecules remaining on surface of substrate, quality of the pixel improved. So, it is suggested that solvent rinse is necessary after gas phase adsorption in order to maintain the quality of the device. |
