**4.2. Fabrication of FEOLEDs**

28 Organic Light Emitting Devices

the luminance efficiency of OLEDs.

Figs.5. and 6.

Since the basic structure of a FEOLED uses a direct current (DC), a protection layer is needed. Additionally, it can be deposited on EML. Such a protection layer in FEOLEDs increases the operating lifetime since this layer protects against electron bombardment .To improve this, a few different structures of FEOLEDs are presented in

Fig. 5 (a) shows the device structure of a FEOLED with the protection layer, which is made of a secondary electron material used as an electron multiplier. By apply operating voltage to FEOLEDs, the electron and hole recombination occurs in the EML by mechanisms similar to OLEDs. As OLEDs are current injection devices, electron injection must be improved to ensure their efficiency and stability. Therefore, as shown in Fig. 5(b), a field emission electrons layer is introduced in OLEDs to increase the electron density, which exhibits a

Fig.6 shows the schematic diagrams of FEOLEDs with a dynode in (6a) and a strip electron multiplier in 6(b). According to the above described mechanism of FEOLEDs, the luminance intensity increases with the increase in electron injection. A common way to solve this problem is to introduce a dynode or an electron multiplier into the FEOLEDs . As shown in Fig.6 (a), dynode has holes whose whole inner surface is coated with a secondary electron material such as Be, Mg, or Ca oxide to increase the electron amplification factors. Fig.6 (b) shows a FEOLED device with strip electron multiplier. The strip electron multiplier was first proposed here by the author, which consisted of a strip of Al coated with another striped secondary electron material [3]. Both of the dynode and strip electron multiplier used in FEOLEDs are made for increasing the number of electrons and allow carriers to achieve a more balanced state in OLEDs and then enhance

**Figure 5.** Two configurations of FEOLEDs (a) with protection layer and (b).with Al metal cathode.

higher luminous efficiency in FEOLEDs than conventional OLEDs [33]

A FEOLED is that an organic emission layer (organic EL) is utilized instead of inorganic phosphor thin film in field emission display (FED). The organic EL in a FEOLED consists of a hole injection layer (HIL), a hole transport layer (HTL) and light emitting layer (EML). A FEOLED is able to attain higher luminance and low power consumption than conventional OLED.

Fig.7 (a) shows the structure of a basic FEOLED. The anode of FEOLED is ITO on which are coated the organic multi layers. The cathode consists of a field emission electron, which provides the electron injection into the organic material layer. Fig.7 (b) shows the FEOLED structure with a dynode, which acts as an electron multiplier to increase the number of electrons injected into the organic layer.

Fig. 8 (a) shows the structure of FEOLED with the strip electron multiplier. Due to the dynode is difficult to set in the narrow space of FEOLEDs. Therefore, the strip electron multiplier was to be proposed. Refer to Fig. 7(b), the dynode can be of the metal channel, box, line focus, or MPC type [1]. The secondary electron material in the dynode can be Cu-Be or Ag-Mg alloys. However, the strip electron multiplier was form with the secondary electron material (MgO or CsI) and Al, as shown in Fig. 8 (b).

**Figure 7.** The components of FEOLED with (a) the basic device and (b) dynode structure.

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 secondary electrons.

Field Emission Organic Light Emitting Diode 31

organic material [34]. It has been shown that LiF is very effective in terms of facilitating electron injection. However, recently alkali metal carbonates (Cs2CO3) have also been reported to be efficient electron injection material [35, 44-45]. We believe that if the electrons are efficiently injected from the cathode to organic layer, then this will improve the charge carrier balance in OLEDs and hence improve the device efficiency. This idea will work the

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

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

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

According to the above mentioned FEOLED structure, FEOLEDs can be divided into two

convenient method to fabricate the CNTs templates by the spray method [38-39].

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

same way in FEOLEDs.

high aspect ratios and nanometer-scale tips.

layer and produce good field emission characteristics.

models: 1) Original type FEOLEDs and 2) triode devices.

**4.5. Operation principle of FEOLEDs** 

**Figure 8.** Schematic structure of a FEOLED with (a) strip electron multiplier and (b) strip Al. (c) the operating principle of a dynode and (d) A sheet of a dynode
