**1. Introduction**

Several flat panel displays (FPDs) technologies, such as liquid crystal displays (LCDs), plasma display panels (PDPs), light-emitting diodes (LEDs), organic light-emitting devices (OLEDs) and field emission displays (FEDs), have been developed. They coexist because each technology has its own unique properties and applications.

In recent years, the developments in the OLEDs have gradually reached very advantageous of existence. These advantageous characteristics include self-luminous, wide viewing angle and low power consumption, etc. which make OLEDs very useful for numerous display applications and lighting devices. To effectively improve the characteristics of an OLEDs, there are many ways to be adopted. Such as: (a) structure: using the quantum well structure or multilayer structure to enhance efficiency by promoting the radiative recombination capability; (b) material: use of a low work function metal as the cathode or a high carrier mobility material to allow efficient carrier injection into OLEDs structure; (c) doping: by doping the guest material into the host material to increase the efficiency of recombination, such as phosphorescent sensitizer.

However, the performance of OLEDs using above methods will be limited. Therefore, in this chapter, we propose new FEOLEDs with electron multiplier.[1,2-5, 6]

An effective enhancement in the lighting efficiency is achieved by using the external electron source supplement into the OLEDs. The FEOLEDs can simply be divided into two types: FEOLED (original diode type) and triode type. The structure of FEOLEDs is similar to that of the field emission diodes (FEDs), but formers utilize an organic EL light emitting layer instead of an inorganic phosphor thin film used in FEDs. The mechanism of operation of FEOLEDs is the same as OLEDs.

In FEOLEDs also a hole blocking layer is used to confine the electron-hole pairs to enhance recombination in the organic light emitting layer. Besides, to avoid damage to the organic

© 2012 Yokoyama, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Yokoyama, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

material by electron beam bombardment, an aluminum (Al) thin film is coated on the organic light emitting layer facing to the carbon nanotubes (CNTs) template to protect the organic light emitting materials and thereby enhance the luminous efficiency of the FEOLEDs. In a triode FEOLED an electron multiplier layer is inserted between the anode and cathode. This layer amplifies the field emission electrons and then injects them into the organic light emitting layer.

Field Emission Organic Light Emitting Diode 23

set of electromagnetic deflection coils raster the electron beam across a phosphor screen, which is typically held at a potential of 15-30 kV [9]. In a FED the electron source consists of a matrix-addressed array of millions of cold emitters. This is field emission arrays (FEAs), which is placed in closed proximity (0.2mm) to a phosphor faceplate and is aligned such

The idea of a FED dates back to the 1960s, when Ken Shoulders of the Stanford Research Institute (SRI) proposed electron beam micro devices based on FEAs [11]. The first operating FEAs were demonstrated by Capp Spindt, also of SRI, in 1968 [12]. Despite many advantages of the spindt-type FEA fabrication technique, scaling this method to large area substrates (>400 mm on the side) is still a major challenge. Another difficulty associated with the scale up of spindt process is the large size of the evaporator required to deposit the spindt tips. Most phosphors have low luminous efficiency at voltages below 3 kV because of the low electron penetration depth and high, non-radiative recombination rates at the surface. While raising the emission current density increase the brightness, high current density leads to faster aging of the phosphor, thus further decreasing the brightness [13].

Among various kinds of emitters in field emission devices, carbon nanotubes (CNTs) have been attracting a considerable attention due to their excellent field emission characteristics of high field emission current density and low turn-on electric field [14]. In order to enhance the field emission electron ability and emission uniformity in large area CNT-FED panels, additional methods are required to improve uniformity by inserting the gate design for electron multiplier and focusing. A gate coated with the secondary electron emission (SEE) materials for obtaining electron amplification is called a dynode [15]. In general, any insulator with low work function is suitable for SEE application [16]. The mechanism of dynode can be simplified by the following processes: (i) the primary electrons penetrate into a certain depth of an insulating layer; (ii) through collision, the energy of the primary electrons is transferred to the bound electrons of the insulator, leading to a release of electrons; (iii) the released

electrons migrate to the surface and escape into the vacuum as secondary electrons.

electric field E at the emitting surface and metal's work function (

meet the requirement for low voltage flat panel display usage.

Therefore, the field emission involves the extraction of electrons from a solid by tunneling through the surface potential barrier. The emitted current depends directly on the local

The field-emission properties of wide band gap materials (WBGMs) is favorable for the emission, as it is considered a property unique to the surface of emitter [18]. The role of the WBGMs in CNTs field emission is to decrease the effective work function of emitters, which increases emissivity. We can assure that the carbon nanotubes are excellent electron sources, providing a stable current at very low fields and capable of operating in moderate vacuum. In summary, the light emitting principle of FEDs is that the electrons are excited and accelerated by the high electric field under vacuum, so as to become sufficiently energized to bombard the inorganic phosphor to emit light. Although CNT-FED was very successful in achieving the result in different low voltage phosphors such as ZnGa2O4+ In2O3, ZnO:Zn low voltage phosphors research, the applied voltage was about 300 V [19]. Thus, it does not yet

Φ

) as shown in Fig.1 [17].

that each phosphor pixel has a dedicated set of field emitters [10].

In this chapter, as stated above, we have proposed and discussed two kinds of electron multipliers: 1) a dynode and 2) a strip electron multiplier. The dynode is formed between the cathode and organic light emitting layer to provide electron amplification capability as well as it makes a FEOLED more stable. The electrons emitted from the cathode move towards the dynode as they are attracted by the applied electric field. The primary electrons impact the secondary electron material of dynode to produce the secondary electrons. Finally, both primary and secondary electrons are directly injected into the organic light emitting layer and increase the current density of FEOLEDs. Due to the presence of a dynode it is difficult to fabricate very thin FEOLEDs. Therefore, the strip electron multiplier has been proposed as it can easily be incorporated in FEOLEDs. The strip electron multiplier is formed on the organic light emitting layer facing to the CNTs cathode, and made by strip Al coated with secondary electron material (Cesium iodide, CsI). CsI does not completely cover the strip Al.

The mechanism of electron amplifier ability of the strip electron multiplier is the same that of a dynode, but the process of fabrication is easier. The organic light emitting layer of FEOLEDs integrated with the strip electron multiplier forms an OLEDs, which can operate independently. Accordingly, applying an electric field to the CNTs template and strip electron multiplier one can attract electrons to impact the strip electron multiplier to generate the secondary electrons. Therefore, the current density of OLEDs is increased by supplementing the electrons into the multilayer of the organic light emitting layer. In this way, the luminance efficiency of FEOLEDs with strip electron multiplier can further be enhanced by one and a half times more than of OLEDs.

The organization of this chapter is as follows. First the concept and mechanism of operation of FEDs and OLEDs are introduced. Then are illustrated the basic concepts and luminescent mechanisms of FEOLEDs and experimental procedure of fabricating them, FEOLEDs of diode structure and triode structure and their characteristics of electron multiplier are discussed next. Finally, we discuss the advantages and disadvantages of the conventional OLEDs and novel FEOLEDs, including some suggestions for future work..
