以自組裝單分子層界面修飾技術提升有機發光二極體效率及壽命之研究

Interface engineering with self-assembled monolayer for high efficiency and stable organic light-emitting diodes

余思燕

指導教授 ： 陶雨臺

國立清華大學/理學院/化學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00021

摘要

在有機發光二極體元件的製程應用中，增益元件電荷注入效率及元件使用壽命是相當重要的一環，而電極表面處理對於上述的現象有顯著的效果。利用自組裝單分子薄膜技術，將其修飾於電極表面並應用於有機發光二極體元件，可以藉由改變分子偶極方向及大小來調控電極與有機半導體材料之間的界面能階。在本論文中，我們使用一系列的有機亞磷酸分子吸附於現今常用之電極¬¬-¬銦錫氧化物(ITO)表面，並製作有機發光二極體元件來進行界面對於元件效率影響之研究。所使用的亞磷酸分子包含:不同烷鏈長度亞磷酸分子及末端為三氟甲基取代的烷基亞磷酸；以及分子長度相同但具有相反偶極的兩分子，包含:丁烷基亞磷酸與三氟甲基丙烷亞磷酸，還有對位甲基或對位三氟甲基苯基亞磷酸。
研究結果顯示，藉由修飾不同分子長度的自組裝單分子膜可以改變載子注入的穿隧能障，進而調控載子注入數量。由此研究來觀察元件中載子注入平衡現象。另外，我們也將兩長度相同但偶極相反的兩分子以不同比例混合並修飾於ITO電極表面，藉由兩分子組成比例不同，可將ITO功函數做大幅度調整(5.0~5.75 eV)。接著將混合式自組裝單分子膜修飾後的ITO陽極製作有機發光二極體元件，元件結構為ITO/SAM/HTL/Alq3/MX/Al，其中，電洞傳輸層(HTL)使用NPB或是BPAPF；而電子注入層(MX)使用LiF或是Cs2CO3。實驗結果顯示，儘管使用不同的電洞傳輸層或電子注入層，皆可找到一最匹配的自組裝單分子膜混合比例，平衡元件中載子注入數量，進而得到元件最大的電流值及最佳發光效率。此一系列研究對於載子注入平衡提供了一有系統的分析方法與研究。
此外，針對幾種常見的電極表面處理方式對於元件壽命的影響我們也進一步分析探討。本論文比較了三種電極表面處理方法，包含:氧電漿處理、自組裝單層分子膜修飾，以及高分子PEDOT:PSS塗佈，並製作了紅、藍、綠色三種發光元件。從元件壽命量測結果發現，經由自組裝單分子膜修飾之電極製成的元件，其元件壽命相較於其他兩者增加有三至十倍。壽命量測與操作電壓關係圖顯示，不同的表面修飾是造成元件壽命差異的來源，我們進一步使用AC2以及XPS等儀器進行表面分析以釐清這些現象。
最後，我們嘗試將亞磷酸分子吸附製程縮短並最佳化。經由反射式紅外線光譜及功函數量測分析結果顯示，ITO電極僅需短時間的浸泡再經由高溫烘烤步驟，即可加強分子與基板表面進行脫水吸附反應達飽和吸附。而在溶劑的選用及基板清洗過程中，若修飾後的基板與水接觸，則會造成亞磷酸分子與ITO表面脫附並導致功函數下降

關鍵字

有機磷酸分子自組裝單分子薄膜 ； 有機發光二極體 ； 元件壽命 ； 再結合效率

並列摘要

Pretreatment of electrodes surface is generally adopted to improve the charge injection efficiency and device lifetime in organic light-emitting diodes (OLEDs) fabrication. Through the modification of electrode with self-assembled monolayers (SAMs), the size and direction of the interface dipole can be varied to modulate the energy alignment at the interface. In this study, a series of phosphonic acid molecules were used, including n-alkylphophonic acids of different chain length and their trifluoromethyl-terminated analogues; binary mixtures with opposite dipoles of 1-butylphosphonic acid and the trifluoromethyl-terminated analogue, p-methyl- and trifluoromethyl-phenyl phosphonic acid, to modify ITO anode through the adsorbed SAMs and applied in the fabrication of OLED devices.
The device performance as function of the tunneling barrier for hole injection, which can be modulated by the chain length of the SAM-forming molecules, provides insight to the charge balance situation in the device. Moreover, the binary mixtures of two molecules with the same chain length but opposite dipole were formed on ITO surfaces to tune the work function of ITO over a range from 5.0 to 5.75 eV by varying the mixing ratio of the two adsorbents. The mixed SAM-modified ITO surfaces were used as the anode in the fabrication of OLED devices with a configuration of ITO/SAM/HTL/Alq3/MX/Al, where HTL was the NPB or BPAPF hole transporting layer and MX was the LiF or Cs2CO3 injection layer. It was shown that, depending on the HTL or MX used, the maximum device current and the maximum luminance efficiency occurred with anodes of different modifications because of a shift in the point of hole/electron carrier balance. This provides information on the charge balance in the device and points to the direction to improve the performance.
Besides, for the common approaches of surface treatment such as oxygen plasma treatment, self-assembled monolayer adsorption and PEDOT:PSS coating, different effects on the device lifetime were observed. In the device lifetime measurement, a three- to ten-fold lifetime enhancement was observed for devices with SAM modification compared to the other two. A correlation of the lifetime and the driving voltage change during device operation was proposed and suggestion was made to the effect of different surface modification and the cause of device failure. The surface analyses by AC2 and XPS provide support to the suggestion.
Finally, we try to optimize the adsorption process of phosphonic acid SAM on ITO anode. Through the surface analysis of ref-IR spectrum and work function, the results indicated that only a brief immersion of ITO substrate followed by high-temperature baking is enough to achieve a saturated monolayer adsorption. However, the phosphonic acid SAM tends to desorb from ITO surface as long as it had been in contact with water, during adsorption step or washing step.

並列關鍵字

phosphonic acid self-assembled monolayer ； Organic light-emitting diodes ； Half-lifetime ； luminance efficiency

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