A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are fabricated with particular organic molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors. Most basic OLEDs comprised a single organic layer, for instance the first light-emitting polymer device synthesized by Burroughs et al. involved a single layer of poly (p-phenylene vinylene). Multilayer OLEDs can have more than two layers to improve device efficiency. For good conductive properties, layers may be chosen to help charge injection at electrodes by providing a more proper energy level, or block charge carriers from reaching the opposite electrode and being wasted. It was recognized that the quantum efficiency of OLEDs depends on the status of carrier injection, the mobility of charge carriers, and the balance of the holes and electrons [34, 35].
Above all, light emission is the consequence of the recombination of holes and electrons injected from the electrodes to the organic emissive layer. Such carrier recombination generates excited molecules, which eventually emit light. Thus, the device efficiency is highly dependent on both carrier recombination efficiency of the emissive material. It is widely recognized that unbalanced charge carriers due to higher hole mobility in the hole transport layer (HTL) and slower electron mobility in the electron-transport layer (ETL) leads to reduced efficiency of OLEDs. Thus, it is important to balance the injected charges to improve device performance.
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The organic films for OLED can separate from several layers with particular functions. The general device can contain substrate, anode, hole injection layer (HIL), buffer layer, hole transporting layer (HTL), emission layer (EML), hole blocking layer (HBL), electron transporting layer (ETL), electron injection layer (EIL), and cathode. The detail of function and characteristic for each layer are discussed as follows:
The organic films must be fabricated on a substrate. The common substrates are glass and plastics. The plastics substrate can be applied to flexible OLED.
As a good anode material, several characteristics must be considered. The characteristics contain high conductivity, high work function, high transparency in visible light, and good morphological stability. Indium tin oxide (ITO) is commonly used as the anode material [36]. It is transparent to visible light (better than 90％) and has a high work function (4.5 ~ 4.8 eV) [37]
which promotes injection of holes into the polymer layer. The methods of ITO thin film depositions are sputter [38, 39], chemical vapor deposition (CVD) [40], and spray pyrolysis [41].
However, ITO thin film is grown on general glass substrate with high temperature (great than 215 ℃). The high temperature is not suitable for plastic substrate, which result in buckle and deformation for plastic substrate. The difficult is so important for flexible OLED to overcome. In order to achieve good injection efficiency for hole, the treatment in ITO surface attracts much attention. It is believed that the work function of ITO without treatment is about 4.5~ 4.8 eV. In addition, the contaminant in the ITO surface also decreases the wok function [42]. The work
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function of ITO increases to above 5.0 eV by O2 plasma and UV ozone [43-45] treatments. The treatments also improve the characteristics of interface with organic layer to increase hole injection, decrease operating voltage, and increase stability of OLED. Organic materials are deposited on the ITO-coated glass substrate. Because the organic thin films contact with ITO directly, ITO surface characteristics deeply influence the performance of OLED. The O2 plasma treatment increase the wok function, removing its surface contaminant, and enhance the hole injection. However, there are several handicaps in the O2 plasma treatment at low pressure, such as vacuum system is expensive and the size of ITO substrate is limited by the size of vacuum chamber [46]. Another problem for ITO anode is the diffusion of indium into the organic layer as device functions. The diffusion result in the decay of device performance [47]. Besides, the spikes in the ITO surface cause low uniformity of ITO surface, which procures current leakage.
Even if the work function of ITO anode increases after O2 plasma treatment or UV ozone treatment, the wok function of ITO is still lower than the highest occupied molecular orbital for common hole transporting layer (about 0.4 eV). Thus, it is beneficial to improve hole injection between anode and hole transporting layer as inserting hole injection layer (HIL) in the interface between node and hole transporting layer. The ideal HIL material should have the characteristics as following: matching ITO work function, morphological stability, good thermal stability, and adhesion promotion. Proper HIL can not only improve efficiency but also increase the longevity for OLEDs. Organic hole injection layer materials usually have ability of hole transporting. The
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familiar HIL materials contain copper phthalocyanine (CuPc) [48], polyaniline [49] in small molecules, and poly ethylenedioxy thiophene (PEDOT) in polymer material [50]. PEDOT owns many advantages such as smoothing the surface of ITO, decreasing threshold voltage, and extending the longevity for OLEDs [51].
Most of hole transporting materials are tri-arylamine that applied to xerography former.
They all have high hole mobility, which are about 10-3 ~10-4 cm/Vs. For hole transporting layer (HTL) in OLEDs, they need characteristics such as easy to inject hole, efficient hole mobility, good thermal stability, and easy synthesis. Ideal materials of HTL must be deposited as thin films without pinholes. If the HTL material with high glass transition temperature enables to form stable and amorphous morphology, they will unchangeably generate pinholes in thin film. The large use of HTL is NPB. NPB has advantages such as easy synthesis and simply purify.
However, its glass transition temperature (about 98℃) is low. The new HTL materials are emphasized that the characteristics of the high glass transition temperature and stable thin film morphology. Besides, it is also important to search the optimum control of hole injection and hole transporting.
The major function of electron injection layer (EIL) is promotion of electron injection from cathode to the electron transporting layer (ETL). The common use of HIL is LiF. With LiF as HIL, the applying voltages of OLED decrease, and the phenomenon is attributed to reason that thin LiF film avoid directly contact between Al and Alq3, which effectively decrease interface
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barrier [52]. But, up to now, the mechanisms of LiF layer in OLED is indefinite. The exact mechanisms of LiF layer need to demonstrate and discuss.
The conditions for good electron transporting layer (ETL) contain efficient electron mobility, good thermal stability, easy synthesis, formation of thin film with amorphous morphology, ability to block holes, and easy to inject electrons. The ideal HTL material must have proper values of HOMO and LUMO. The suitable value of LUMO (low LUMO) with ETL can effectively inject electrons from cathode into ETL. The suitable value of HOMO (high HOMO) with ETL can effectively block holes in emission layer and improve the probability of charge carrier recombination. It is perfect for uniform mobility of holes and electrons. However, the mobility of electrons is far smaller than the mobility of holes actually in organic materials. Thus, it is crucial to ETL with high electron mobility, and it makes the recombination zone far away the cathode to increase excitons generation. The ETL with high glass transition temperature and good thermal stability is important to avoid producing heat accumulation in high current density.
Besides, the thin film of ETL must be uniform and there are not pinholes which formed by thermal evaporation or spin coating. In 1987, Tang and Van Slyke utilize Alq3 to emit high efficiency electroluminescence. Besides, Alq3 owns some advantages such as good thermal stability and easy to deposit thin film without pinholes [53]. Thus, Alq3 is generally used as emission layer or HTL in OLEDs.
Proper work functions of cathode and anode are important for effective injection of holes
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and electrons into organic layers [54-56]. Due to most organic materials have LUMO value (2.5~3.5 eV) and HOMO value (5~6 eV). Cathode must be a low work function metal and anode must be a high work function metal, so it is possible to produce the lowest injection barriers. The good cathode has characteristics like Ohmic electron contact, adhesion to ETL, and low work function. For effective electrons injection, low work function metals, like calcium (Ca) [57], magnesium (Mg) [58], are used as cathodes. In addition, some stable metals such as aluminum (Al) and silver (Ag) are common use of cathodes in OLEDs. Some composite cathodes such as Mg:Ag and Li:Al also become cathodes in OLEDs. To add silver into magnesium is not only improving the stability of cathode but also increasing the ability of adhesion with Alq3 [59].
It is believed that the mobility of holes in HTL is faster than the mobility of electrons in ETL. The fact of unbalanced mobility for charge carriers results in bad recombination condition and reduces efficiency of organic light emitting devices. The difficulty can be solved in two ways.
One way is the improvement of electrons injection from cathode to organic layers, and increasing probability of recombination for charge carriers. This section contains proper cathode with low work function to increase electrons injection and good ETL materials with high mobility of electrons to transport electrons fast. The other way is inserting a buffer layer between anode and HTL to improve the hole injection and decrease the apply voltages. Choosing suitable buffer layer and depositing thin film of ideal thicknesses for buffer layer are beneficial to increase efficiency of OLEDs. The common buffer layers contain TiO2, SiO2 [60], and LiF [61]. CuPc is
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inserting as buffer layer between anode and HTL to slow down the transport of holes and balancing the transport for charge carriers [62, 63]. Thus, the bad injection of hole with CuPc layer improves the recombination of charge carriers and promotes the efficiency of OLEDs.
Besides, the hole blocking layers (HBL) have characteristic that it can limit holes in the emission layer to increase the probability of recombination for charge carriers. The general HBL materials are such as BCP, and BPhen.
The materials as emission layer (EML) in OLEDs require the characteristics that contain whose light emission must be in the range of visible light and high photoluminescence (PL) quantum efficiency and good thermal stability. Besides, the combination of host material with excellent transporting and emission characteristics and the guest material whose emission characteristic is good can effectively produce all kinds of colors emission.
High internal quantum efficiency is composed of good carrier injection (proper work function for electrode; suitable HOMO and LUMO for organic materials), charge carriers balance (better carrier transporting ability; ideal recombination zone in particular organic layer), and emission material with high PL quantum efficiency and nice color saturation. For improvement of carrier injection, for instance, it contains ITO pretreatment and adjunction of hole injection layer and electron injection layer. For improvement of carrier balance, it includes multilayer structure and the optimizing thickness of organic layers.
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