**2. Field emission light emitting diodes (FEDs)**

A FED is a vacuum electron device, sharing many common features with the vacuum fluorescent displays (VFDs) and cathode ray tubes (CRTs) [7]. Like in a VFDs or CRTs, the image in a FED is created by impacting electrons from a cathode onto a phosphor coated screen. In a CRTs the electron source is made up of up to three thermionic cathodes [8]. A 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 that each phosphor pixel has a dedicated set of field emitters [10].

22 Organic Light Emitting Devices

organic light emitting layer.

cover the strip Al.

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

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

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

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

A FED is a vacuum electron device, sharing many common features with the vacuum fluorescent displays (VFDs) and cathode ray tubes (CRTs) [7]. Like in a VFDs or CRTs, the image in a FED is created by impacting electrons from a cathode onto a phosphor coated screen. In a CRTs the electron source is made up of up to three thermionic cathodes [8]. A

OLEDs and novel FEOLEDs, including some suggestions for future work..

enhanced by one and a half times more than of OLEDs.

**2. Field emission light emitting diodes (FEDs)** 

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.

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 electric field E at the emitting surface and metal's work function (Φ) as shown in Fig.1 [17]. 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 meet the requirement for low voltage flat panel display usage.

Field Emission Organic Light Emitting Diode 25

= 3.66 eV) deposited either by

Φ) metal

of OLEDs is an ITO film, the cathode is typically a low-to-medium work function (

**3.2. Mechanism of the operation of organic light emitting diodes (OLEDs)** 

an OLED is current driven and hence called electroluminescence ( EL) .

**Figure 3.** Schematic sketch of the active energy levels of organic light emitting diodes (OLEDs)

The light emission from OLEDs is through electroluminescence (EL), which can be described in three steps (see Fig. 3) as follows: step 1: when a forward bias voltage is applied to an OLED, holes and electrons are injected. These injected charge carriers have to overcome their respective interface barriers and then holes occupy into the highest occupied molecular orbital (HOMO) energy level of the hole transport layer (HTL) and electrons into the lowest unoccupied molecular orbital (LUMO) energy level of the electron transporting layer (ETL). The HOMO of HTL is similar to the valence band in bulk semiconductors, and LUMO of ETL is similar to the conduction band. Step 2: The externally applied field on OLED drives the injected holes and electrons to the interface of HTL and ETL, where they are accumulated; holes in HOMO of HTL and electrons in LUMO of ETL. Step 3: Due to organic solids have low dielectric constant and strong binding energy both carriers ( holes and electrons) move toward the interface between the two transport layers (HTL and ETL) and recombine in the light emitting layer (EML) to form excitons. Then these excitons emit light through the transparent electrode (ITO coated on glass substrate). In general environment, the exctions exist in an unstable state and their radiative move to recombination releases energy in the form of the light and heat. According to the above three steps, illustrated in Fig. 3, the light emission from

= 4.3 eV) or Mg**0.9**Ag**0.1** (Mg,

Φ

such as Ca (

Φ

= 2.87 eV), Al (

e-beam or thermal evaporation [23].

Φ

**Figure 2.** Schematic structure of organic light emitting diodes.

**Figure 1.** Schematic diagram of the field emission barriers for a planar and a micro-tip emitter.
