**4.4. Carbon nanotubes field emission template in FEOLEDs**

30 Organic Light Emitting Devices

secondary electrons.

Fig. 8 (c) shows the operating principle of a dynode, which is a cross-sectional schematic view of the metal channel type dynode. The secondary electron emission generated from the dynode can be understood by the following processes: (a) the primary electrons penetrate into a certain depth of an insulating layer (the secondary electron material); (b) through collision the energy of primary electrons is transferred to bound electrons of the insulator to release them and (c) released electrons migrate to the surface and escape into the vacuum as

**Figure 8.** Schematic structure of a FEOLED with (a) strip electron multiplier and (b) strip Al.

In Fig. 8 (a) is shown a FEOLED with the strip electron multiplier formed on an OLED as a part of anode. The organic multilayer structure of FEOLED is the same as that of OLED,

One of the most important issues related with the characteristics of OLEDs is the number of injected electrons and holes should be balanced. It is well know that the direct electron-hole recombination in the light emitting layer occurs due to OLEDs. Therefore, an effective cathode structure for efficient electron injection is critical to optimal performances of OLEDs. A nanometer-size interfacial layer between the metal cathode and organic material in OLEDs plays the critical role in the carrier injection efficiency. In order to improve the injection efficiency of electrons, the low work function metal or alloys such as LiF are usually used to form low energy barriers for electron injection from the cathode to the

(c) the operating principle of a dynode and (d) A sheet of a dynode

**4.3. Organic light emitting diode in FEOLED** 

which works with the same mechanism.

In this section, FEOLEDs are discussed for their optoelectronic characteristics in terms of external electron supplement into the organic light emitting layer. Hence, FEOLEDs must have an excellent field emission cathode to emit electrons. The carbon nanotubes (CNTs) template is the best choice to be adopted in FEOLEDs as the field emission cathode. CNTs can work in less stringent vacuum conditions (<1x10**-5** torr) and have higher emission currents than metal and semiconductor micro-tip field-emission sources. Iijima discovered carbon nanotubes (CNTs) in 1991[36]. CNTs have a superior mechanical strength, good heat conductance, and ability to emit cold electrons at relatively low voltages because of their high aspect ratios and nanometer-scale tips.

The traditional method of fabricating field emitters is based on the use of multi-needle field emission cathodes and precision technological processes based on electron lithography techniques. Metal and semiconductors are usually used as cathode materials, which, unfortunately, have rather high work functions (4-5eV) [37]. The application of CNTs in field emission template is very extensive. For example, CNTs can be directly synthesized on a substrate by CVD on an anodic aluminum oxide (AAO) template, and by screen print. However, the fabrication of a CNTs template is time consuming. There is a new and more convenient method to fabricate the CNTs templates by the spray method [38-39].

CNTs thin films are usually fabricated by two methods, such as: drop-drying from solvent [41] and filtration and spin-coating [42], but these methods have severe limits in the film quality, like in uniformity, homogeneity, and production efficiency. CNTs thin films consisting of multi-walled CNTs (MWCNTs) are fabricated by the spray method, which is an easy and convenient method to deposit CNTs and can achieve large area deposition [43]. The procedures of the fabrication of CNTs template are described as below: First CNTs are suspended in 1, 2- dichloroethylene (DCE) and second, sonication of 30 mg CNTs in 50 ml DCE solvent for 2 hours. To obtain good adhesion between CNTs and ITO glass substrate, an indium (In) metal layer is deposited onto the ITO glass substrate by thermal evaporation. After annealing at 300 °C for 15 min in N2 atmosphere, CNTs are firmly adhered to the In layer and produce good field emission characteristics.
