**3.1. Structure of organic light emitting diodes (OLEDs)**

A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and both anode and cathode terminals. The basic structure of a typical OLED is shown in Fig. 2. The first layer above the glass substrate is a transparent conducting anode, typically indium tin oxide (ITO). In Flexible OLEDs the anode is made of a transparent conductance plastic substrate. There are two different types of OLEDs. Traditional OLEDs use small organic molecules deposited on glass to produce light. The other type of OLEDs uses large plastic molecules called polymers. The single or multilayer small organic molecular or polymer film is deposited on the transparent anode. Appropriate multilayer structures are used to enhance the performance of the device by lowering the barrier for hole injection from the anode and by controlling the electron and hole recombination region. The injected holes move from the interface of the organic/electrode into the organic light emitting layer, where the defect density is high. Therefore, the organic layer deposited on the anode should generally be a good hole transport layer (HTL). Similarly, the organic layer in contact with the cathode should be an optimized electron transporting layer (ETL). Generally, the anode of OLEDs is an ITO film, the cathode is typically a low-to-medium work function (Φ) metal such as Ca (Φ= 2.87 eV), Al (Φ= 4.3 eV) or Mg**0.9**Ag**0.1** (Mg, Φ= 3.66 eV) deposited either by e-beam or thermal evaporation [23].

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

24 Organic Light Emitting Devices

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

Using organic material for light emitting diodes (LEDs) is fascinating due to their vast variety and relative ease of controlling their composition to tune their properties by chemical means. For example, by applying an electric field to an anthrancene single crystal, Pope *et al.* in 1965 observed blue electroluminescence (EL) [20]. Soon after alternating current EL was also achieved using an emissive polymer [21].The observation of efficient bright EL, defined as the number of photons emitted from the face of the device per injected electron or hole, the investigation on the organic optoelectronic device commenced to investigate and developed slowly until Tang and Vanslyke demonstrated efficient green electroluminescence [22] from a vapor deposited organic compound in 1987. Till now,

A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and both anode and cathode terminals. The basic structure of a typical OLED is shown in Fig. 2. The first layer above the glass substrate is a transparent conducting anode, typically indium tin oxide (ITO). In Flexible OLEDs the anode is made of a transparent conductance plastic substrate. There are two different types of OLEDs. Traditional OLEDs use small organic molecules deposited on glass to produce light. The other type of OLEDs uses large plastic molecules called polymers. The single or multilayer small organic molecular or polymer film is deposited on the transparent anode. Appropriate multilayer structures are used to enhance the performance of the device by lowering the barrier for hole injection from the anode and by controlling the electron and hole recombination region. The injected holes move from the interface of the organic/electrode into the organic light emitting layer, where the defect density is high. Therefore, the organic layer deposited on the anode should generally be a good hole transport layer (HTL). Similarly, the organic layer in contact with the cathode should be an optimized electron transporting layer (ETL). Generally, the anode

**3. Organic light emitting diodes (OLEDs)** 

OLEDs are the best flat light emitting source.

**3.1. Structure of organic light emitting diodes (OLEDs)** 

#### **3.2. Mechanism of the operation 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 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 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 of 10-100 [24].

Field Emission Organic Light Emitting Diode 27

*Method (4)* The emission from a white OLED is filtered by a microcavity, which is composed of a dielectric quarter wavelength stack as the bottom mirror, the metal contact as the top mirror and an inactive material as a filler layer to adjust the cavity thickness [26]. However, the microcavity resonance causes strong viewing angle dependence of emitted colors, limiting this method to applications which need small viewing angle. In this method about

*Method (5)* The color variation is achieved by voltage and/or polarity tuning. Only molecular OLEDs are capable of three color tuning. This method shows low efficiency and/ or requires high driving voltage. Hence, the color variable devices based on the polarity and/ or voltage-tuning are still far from applications. White light emission OLEDs can also serve as backlight panels of LCDs. White is the most important color in the lighting industry. A number of device structure concepts have been proposed to achieve white emission. These include the mixing of three primary colors from respective layers in a multilayer structure [28], the doping of appropriate amount of red, green, and blue dopants in the same host [29],

OLEDs have become viable now for flat panel displays after intensive research and progress in the past decade. Through proper material design/choice and device fabrication, various OLEDs with colors of high brightness have been developed for use in single- or full-color applications. As the operation of an OLED depends on the carrier transport in HTL and ETL, hole and electron confinement in EML and then their recombination to emission light. In most cases the number of injected holes in an OLED is more than electrons. Therefore,

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

the microcavity effect of one emission layer [30], use of exciplex formation, etc.

improving efficient electron injection is essential for efficient and stable OLEDs.

**4. The field emission organic light emitting diodes (FEOLEDs)** 

electron bombardment [32], but also not easy to control highly efficient emission.

±15**0** viewing angle can be achieved [27].

**4.1. Structure of FEOLEDs** 

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

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 recombination within a well-defined zone.
