摘要:有機電激發光元件具備可製作於任意基版上及適用滾動製程之優點,有潛力於大尺寸可撓式基版上,製作低成本之顯示及照明元件。然而,由於其效率及元件壽命仍有限制,目前尚無法廣泛之應用。本計畫之目的,即為提升元件特性,增加其應用範圍。由於高效率為長壽命元件之必要條件,因此本計劃之主要目標係為延長元件壽命及探討其老化機制。
本計畫之執行方式,首先係以數種技術提升元件壽命。本實驗室目前已開發三種獨立之技術:(1)於電子傳輸層中摻雜鹼金族金屬,可同時降低驅動電壓及增長壽命,元件壽命可增長三倍;(2)使用電子傳輸材料及電洞傳輸材料混合成發光層之主體材料,可增長六倍之元件壽命;(3)利用貼上一增光膜增加外部量子效率,可增加出光量50%,亦即相同元件亮度下,壽命可延長50%。整合以上三種已開發之技術,壽命估計可增長為原先之二十倍以上。在此同時,本實驗室亦將開發新技術,如:串連式元件及激子獲得技術等,設法進一步增長元件壽命。
其次,利用許多之量測技術,如:變溫電性量測、外部量子效率量測、暫態電激發光及飛行時間量測等,探討其影響元件壽命之種種因素,並將其定量化,最重要的一點為:如何在元件製作出來時,即可以若干
Abstract: Organic light emitting devices (OLEDs) have recently attracted much attention because of the advantages of high brightness, high contrast, and low cost. Such a device can be fabricated on a flexible substrate and is suitable for the roll-to-roll process. Hence it can be potentially used for flexible display or lighting applications with a large size. However, since the efficiency is low and lifetime is short, the applications of OLED are limited to the small displays of the consumer electronics, such as a mobile phone or a digital camera. The objectives of this proposal are to increase the efficiency and to elongate the lifetime of an OLED those make the indoor lighting and large panel display applications possible. Typically, a higher efficiency device has a longer device lifetime since the generated heat is fewer, which is one of the reasons accounts for the device degradation. That means, a long lifetime OLED must be one with high efficiency. Hence, we will include the efficiency into the lifetime issues in this project.
First, we will use three techniques developed by our research group to improve the OLED lifetime: (1) metal-doped electron transport layer (MD-ETL): doping the alkali metal into the ETL layer increases the electron concentration and decreases the driving voltage. Not only voltage reduction, a 3.14 times improvement in operation lifetime was obtained at the same time by our novel structure; (2) mixed-host emitting layer (MH-EML): co-evaporating the hole-transport layer (HTL) and the ETL as the host of the EML blurs the organic/organic interface, reduces the carrier accumulation, broadens the emission zone and hence elongate the lifetime. A lifetime four times longer than that of a conventional OLED can be obtained; and (3) film attachment to increase the external quantum efficiency (EQE): a 50% increase of EQE was obtained in our group. That means, the driving current can be 50% lower with the film and lifetime can be 50% longer at the same luminance level. By integrating the three existing techniques, we can roughly estimate the lifetime has a 20 times improvement. At the same time, our group will develop new techniques, such as tandem and exciton-harvesting devices to further elongate the OLED lifetime.
Then, we will develop several measurement techniques, such as temperature-dependent current-voltage, EQE, transient electroluminescence and time-of-flight measurement to investigate quantitatively the root causes affecting the OLED lifetime and to establish the physical models of degradation mechanisms which come from the organic materials, device structures, fabrication method and environments. One of the most important subjects is: how to predict degradation behaviors of an OLED with a non-destructive measurement as the device is just fabricated? By using the theoretical models developed, we can determine the physical limitation of the lifetime of an OLED. Then, we will design a new device to further increase the OLED life