由於有許多的優點(如: 反應速度、亮度、視角、質輕、軟性基版以及成本低)，有機發光二極體在近幾年內逐漸在顯示器市場上的受到重視。但由於目前所採用較具穩定性的電子傳輸層與發光層材料—Alq3，因其電子親和力低，需要採用低功函數的陰極材料以降低其電子注入能障，常見的陰極材料多為鎂:銀(10:1)合金，在柯達(Kodak)提出以氟化鋰鋁為雙層陰極後，對於電子注入有較佳的效果，此後便有許多學術機構投入這方面的研究。
我們以NPB為電洞傳輸層以及Alq3為電子傳輸和發光層，在作為陰極的鋁與有機層間加入一層約10Å的薄氟化鋰可以大幅降低元件的操作電壓並可提升發光亮度約三倍左右。其中氟化鋰的厚度對於元件效能有高度的影響性，在我們的實驗機台中，最佳厚度約為8Å。由原子力顯微鏡觀測到當氟化鋰的厚度到達20Å時，有機層的表面並未完全被覆蓋，並且在元件的表面零散的佈滿小氣泡，而這些小氣泡在元件操作下會逐漸被下層的有機層(Alq3)所吸收，因此推斷氟化鋰鋁電極具有降低操作電壓的作用不只是因為其間的化學反應所可能釋出的低功函數鋰原子降低了陰極與有機層間的能障，更由於有機層吸收了釋出的氣體而對整體元件的發光效率有影響。

Because of many advantages of Organic Light-Emitting Diode (OLED), for example : fast responsibility、high brightness、large view angle、light weight、flexible substrate and low cost, OLEDs have attracted increasing attention in display market in recent years. But the popular materials used as electron-transport layer and emitting material is Alq3 because of its high stability, it is needed to take low work-function metal as cathode to lower the barrier of electron-injection from cathode into the low electron affinity material, Alq3. The most common cathode is made of Mg0.9:Ag0.1 which has lower barrier height than Al. After the report of using LiFAl layer as an cathode by Kodak, there are several research reports have been proposed to find out the reasons that improve carrier injection from LiFAl cathode.
In our structure, we took NPB as HTL and Alq3 as ETL and light emitting material. Inserting a thin layer of LiF (about 10Å) between the emission layer and the metal cathode could reduce the operation voltage and enhance light-emitting efficiency to about 3 times. The thickness of LiF would take an important influence in device performance. In our system, the optimized thickness is 8Å. By atomic force microscope, the surface of organic layer was not covered completely with LiF up to 20 Å, and there were several fine bubbles loosely appearing on the surface of LiFAl. These bubbles would be adsorbed by the underlying Alq3 after biasing.We deduces that the ability of LiFAl cathode to reduce operation voltage doesn’t only come from the Li atoms released from the chemical reaction between LiF and Al, but also come form the gas-adsorption by Alq3, which gas released form the chemical reaction of LiF and Al.

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