OLED中LiF薄层的作用机理主要是哪方面呢？越详细越好！谢谢！

好专业啊，我也是百度来的信息，供你参考：
Optimization of OLEDs involves more efficient and balanced injection of carriers from the electrodes, higher mobility of both electron and hole into the recombination regions, and they must perform a radiative decay of exciton. The brightness and efficiency of OLEDs depend on the number density of electrons and holes in the emission layer, so that effective charge injection into the organic materials is critical for optimum device performance. Efficient electron-injecting contact is usually a low-work function material such as magnesium, calcium, or lithium. But the use of these low work function metals results in unreliable OLEDs, mainly due to the reactive nature of these materials, especially in air atmosphere. More stable materials are preferable as cathodes. However, OLEDs based on a cathode made from these materials are inefficient, and their light output is very low compared to OLEDs with a reactive metal cathode. To achieve better performance, a thin film of an insulator, such as lithium fluoride (LiF), deposited between the organic layer and the Al cathode have been used. Recently, Jabbour et al. found that for OLEDs, the use of a buffer layer of LiF is not the only route to enhance OLED’s performance and proposed a novel cathode structure for the fabrication of more efficient and bright OLEDs. This cathode is achieved by codeposition of an inorganic insulator and a metal (referred to as composite cathode). Because tunneling through an insulator is very sensitive to thickness variations, devices using a buffer layer of LiF might be difficult to fabricate with a large active area. In contrast, the use of a composite cathode eliminates the need for a thin insulating layer in OLED fabrication, thus leading to better field distribution over the device area. We will investigate the influence on efficiency of inserting the LiF layer and doping LiF into the Ca cathode.
Results and Discussions
A. Insertion effect of LiF layer
Current-voltage, luminance-current and power efficiency-voltage characteristics of the devices having LiF/Ca cathodes with different thicknesses of the LiF layer were shown in Fig. 1, respectively. For comparison, the properties of a device having a pure Ca cathode were also presented. As shown in Fig. 1, more currents flowed in the device with LiF/Ca cathode than that with a pure Ca cathode and light output was also enhanced at the same voltage. With increasing the thickness of the LiF layer, up to 0.7 nm, operating voltage was gradually lowered, and the light output was enhanced. But beyond 0.7 nm, I-V curve shifted to the right and light output decreased compared to that of the device of which the thickness of the LiF layer is 0.7 nm.
The electron injection process has been mostly in terms of tunneling and Schottky emission mechanisms. One possible mechanism for the above enhancements in our devices can be obtained from tunneling theory. LiF is a superior insulating material because it has the highest band gap energy of about 12 eV among oxides and fluorides. The presence of the LiF layer allows for a large voltage drop, across it and thus moves the Fermi level of metal cathode to the point where it is aligned with the lowest unoccupied molecular orbital (LUMO) of adjacent organic material. The effect of the potential barrier that is present in the devices without the LiF layer is thus eliminated via tunneling. This results in more electrons being injected through the insulating layer directly into the LUMO of the organic layer leading to an increase in electroluminescence. The increase in efficiency indicates a more balanced injection of both types of carriers. From Campbell’s observation, we can suppose that the electron Schottky barrier between BCzVBi and Ca would be larger than that expected by the ideal Schottky model. Therefore, we can expect similar effects as in the device with LiF/Al cathode to be shown in the device with LiF/Ca cathode.
From Fig. 2, I-V characteristics, L-I characteristics and power efficiency vs. applied voltage for the device with the Ca/LiF/Ca cathode were shown respectively. The multilayer Ca/LiF/Ca cathode was fabricated by depositing a layer of Ca (the blocking layer) followed by depositing a 0.5 nm thick LiF layer, and then capping the device with about a 100 nm Ca layer without breaking the vacuum. The thicknesses of the Ca blocking layer were 0.5, 1.0, 2.0, 5.0 and 10.0 nm. The reference device is that with a pure Ca cathode. When compared with reference, the device with Ca/LiF/Ca cathode showed lowering of the operating voltage and the enhancement of luminance, which results in the enhancement of power efficiency and emission efficiency. The efficiency of the device with a 0.5 nm thick Ca blocking layer was the highest of all. As the thickness of a Ca blocking layer increases, efficiency of the device decreased gradually close to that of the reference. From this result, it could be concluded that only the presence of LiF in the BCzVBi/Ca interface does not necessarily enhance efficiencies of the device.

B. Doping effect of LiF
To investigate doping effects of LiF we fabricated devices with Ca:LiF composite cathode. The device structure is as follows: ITO/TPD(50nm)/BCzVBi(60nm)/Ca:LiF/Ca. In this experiment, the LiF/Ca ratio and the thickness of Ca:LiF composite cathode layer were used as manipulating variables.
We investigated the effects of Ca:LiF composite cathode thickness variation on efficiencies of the device. From above result, we obtained the optimum Ca:LiF composite cathode thickness, 30 nm. It is thought that the LiF/Ca ratio of Ca:LiF composite cathode affects the efficiency more than the thickness. Therefore, we concentrated on finding the optimum of the LiF/Ca ratio. We investigated the effect on efficiencies of LiF/Ca ratio for the ratios of 0.5 %, 1.0 %, 2.0 %, 4.0 %, 8.0 %, 10 %, 14%, and 20 %. As shown in the Fig. 3, the optimum LiF/Ca ratio was ca. 2 %.
It can be inferred that the barrier height is reduced and the electron injection is increased from I-V and L-I characteristics. But, in contrast to previous devices inserting a thin LiF layer between the cathode and the organic layer, the increased electron injection in devices with Ca:LiF composite cathode cannot be explained using the tunneling through an insulator argument. To understand the mechanism resulting in improved performance, it is important to know the nature of the interfaces, as well as the composite cathode chemical contents by using time-of-flight secondary ion mass spectroscopy, angle-resolved high-resolution x-ray photoelectron spectroscopy, and so forth.

LiF超薄层的引入较好地修饰了ITO表面，减少了阳极和有机层界面缺陷态的形成，增强了器件的稳定性。实验结果表明： LiF层有效地阻挡空穴注入，增强载流子注入平衡，提高了器件的亮度和效率，含有1 nm厚LiF空穴缓冲层器件的性能最好，效率较不含缓冲层器件提高了近1.5倍。
将厚度为0.5 nm的LiF薄层引入到双层有机电致发光器件(OLEDs)的Alq3发光/电子传输层中作为空穴阻挡/激子限制层,研究其位置对器件光电性能的影响。发现LiF薄层在不同位置均明显提高器件的发光效率,当LiF薄膜距离TPD/Alq3界面20-40 nm时,OLEDs的最大发光效率约为4.5 cd/A,是对比器件(没有LiF薄层)的1.8倍。OLEDs的电流密度随着减小LiF薄层与阴极的距离而增大。研究表明,这是因为LiF薄层可有效阻挡进入复合发光区域未复合的过剩空穴并导致其积累,空穴积累可提高电子传输区域中的电场,提高其中电子的传输和从阴极的注入,从而提高复合发光区域中的载流子平衡及其复合几率;LiF薄层可将激子限制在复合发光区域,减少激子被阴极淬灭的几率。

LiF的作用应该主要是与Al作用生成Li自由基，增强了器件的电子注入性能，也有人提出是因为LiF的隧穿效应，增加了电子的注入能力，目前比较偏向于前一种说法。

LiF是电子注入层，Mason等认为，LiF的注入机理是界面处与Alq和Al的三元协同反应，从而释放出活泼金属Li。