**4.1. Structure of FEOLEDs**

26 Organic Light Emitting Devices

of 10-100 [24].

recombination within a well-defined zone.

**3.3. Full Color of organic light emitting diodes (OLEDs)** 

OLED's, 4). microcavity filtered OLED's and 5). color-tunable OLED's.

deposit the R、G and B OLEDs with individual pixels of R、G and B emission.

filters on white light OLEDs are used to change the emission into R, G and B colors.

The carrier transmission in organic molecules is different from that in inorganic semiconductors or crystalline materials. There are no continuous energy bands in organic semiconductors. In organic semiconductors, consisting of organic molecules, there are delocalized π electrons, which are relatively free but confined on individual molecules due to weak intermolecular interactions. Therefore, the hopping theory is the most commonly used to describe the phenomenon of carrier transfer in organic solids. Driven by the electric field, electrons are excited or injected into the LUMO of one molecule in ETL and hop to the LUMO of neigh-boring molecule and thus electron transport occurs. Likewise, injected holes get transported by hopping from the HOMO of one molecule to another in HTL. In fact, as the charge carriers are injected externally and do not exist before the application of the electric field, the location of holes in HOMO and electrons in LUMO deforms the associated bond length and structure. Therefore, the movement of an injected electron or hole is coupled with the local deformation zone to form a unit, this unit is called polaron. Hence, in organic semiconductors the movement of the electrons or holes is often accompanied by the deformation of its structure, which is called self-trapped electron or hole. As such selftrapped charge carriers move slower the carrier mobility in organic semiconductors is in general lower than that in inorganic semiconductors or metals. The hole mobility in organic materials is typically 10**-7-**10**-3** cm**2**/Vs and the electron mobility is typically lower by a factor

The organic materials are usually insulators (such as plastic). Generally only a very small amount of current can be injected into the organic material by applying certain electric field, and EL occurs from the recombination of these injected electrons and holes. Therefore, if the current is less then injected carriers will be less and number of excitons to recombine will be limited. Therefore, to a large extent EL depends on improving the carrier injection efficiency from both electrodes, and on obtaining balanced and controlled electron–hole

There are five potential methods to make an OLED emit in red (R), green (G) and blue (B) color spectral regions [35]: 1). side by side patterning of red (R), blue (B) and green (G) OLED's, 2). absorptive filtering of white OLED, 3). fluorescent down-conversion of blue

*Method (1)* This method is employed a precisely positioned shadow mask to selectively

*Method (2)* A white OLED device can be made using materials with very broad emission spectra, or using two or more sequentially deposited light emitting layers and then color

*Method (3)* The full color pixel can be using a single blue OLED to pump wavelength downconverters, which efficiently absorb blue light and re-emit the energy as green or red light.

As shown in Fig. 4**,** the basic structure of a FEOLED is to utilize an organic EL light-emitting material instead of inorganic phosphor thin film in FEDs [31]. The anode of the FEOLEDs can be a multi-layered organic solid or an OLED . But both have the same structure, which includes a hole injection layer (HIL), a hole transport layer (HTL) and a light emitting layer (EML). The cathode of FEOLEDs is made of CNTs template as electron source. In such a structure, it is not only difficult to protect the light emitting layer (EML) from high-energy electron bombardment [32], but also not easy to control highly efficient emission.

**Figure 4.** The schematic diagram of a FEOLED.

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 Figs.5. and 6.

Field Emission Organic Light Emitting Diode 29

**Figure 6.** Two configurations of FEOLEDs with (a).Dynode and (b).strip electron multiplier

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

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

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

**4.2. Fabrication of FEOLEDs** 

electrons injected into the organic layer.

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.

OLED.

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 higher luminous efficiency in FEOLEDs than conventional OLEDs [33]

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 the luminance efficiency of OLEDs.

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

**Figure 6.** Two configurations of FEOLEDs with (a).Dynode and (b).strip electron multiplier
