**4.2 Polymer used in OLED**

The intrinsic versatility of the displays is determined by the material used. Polymers, thin metal tubes, and glass are examples of ductile materials that have been used. Conducting polymers, on the other hand, are more compact, lighter, and less costly. Polymers are widely used material forms for OLEDs because of these advantages [18].

In 1989, a team from Cambridge University discovered electroluminescence (EL) in a conjugated polymer [19]. The system had a very short lifespan of a few minutes and only had a weak emission of 0.1 percent outward quantum efficiency (EQE). After this discovery, the vigorous development of polymer OLED materials and the optimization of system design has started with Sumitomo Chemical Co. Ltd., Covion in Germany, Dow Chemicals in the United States, and Cam-bridge Display Technology (CDT) in the UK. Strong EQE of 51% and a long operational lifespan of many tens of thousands of hours have been reached as the culmination of over 20 years of R&D [20].

Polymer OLEDs, like small-molecule OLEDs, have the following characteristics [15, 21]: (a) high contrast ratio (luminance-on/−off), (b) large viewing angle, (c) bright colors, (d) slim devices, (e) high-speed image switching, and (f) low power consumption. The applicability of a cost-effective fabrication method in mass manufacturing is a noteworthy characteristic of polymer OLEDs.

The color of light emitted by polymers is highly influenced by the form of polymer, its chemical structure, and the existence of the side groups. Thus, a series of soluble luminous polymers that emit from 400 nm to 800 nm across the entire VIS spectrum could be made available by chemical modifications to the polymer structure. A fascinating aspect that affects the colors of light-emitting polymers is the use of emissions additive, also known as dyes. If a small volume of an appropriate dye is applied to a polymer, the energy can be transmitted by light absorption from the dye from the polymer to the dye. Different dyes may be used to adjust the device's color. A blue polymer containing a green dye will emit green light, while a blue polymer containing a red dye will emit red light. When choosing a material for a device, the glass transition temperature (Tg) of the polymer materials is critical. The study of several organic materials as active components is motivated by the need to refine the device's characteristics. Burroughes et al. published a high-quality green light-emitting polymer-based system using poly(p-phenylene vinylene) in 1990, bringing polymer electroluminescence research to a close.

### *4.2.1 Conducting polymer*

In 1976, conductive polymers were discovered. Shirakawa inadvertently produced the first conducting polymer polyacetylene capable of conducting electricity in the mid-1970s. While it was not stable in air, the fact that it could become conductive due to doping has prompted further studies into other recognized conjugated polymers. Many experiments have been conducted on conductive polymers such as Polypyrrole, polythiophene, and polyaniline since 1976 [22].

#### *Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

#### **Figure 5.**

*Chemical structure of conducting polymer [27].*


**11**

**Table 2.**

Glass transition temperature

Viscosity Linear relation with a

molecular weight

*Properties of small molecules, polymer, and dendrimers [8, 41, 42].*

Nonpolar solubility

*Conducting Polymer-Based Emissive Layer on Efficiency of OLEDs*

The Nobel chemistry prize was awarded to MacDiarmid, Shirakawa, and Heeger in 2000 for the discovery and advancement of conducting polymers, demonstrat-

**morphology**

Conductive polymers have a backbone that is π-conjugated (alternating single and double bonds), allowing overlapping of bound electrons in the polymeric chain [24]. Through incorporating various electron releasing/withdrawing functional groups into the polymeric backbone and managing the electron–hole injecting/ transporting ability of the synthesized conducting polymer, and the conductivity of the polymers can be effectively tuned to achieve emission in the desired luminance range [25]. The creation of charge carriers induces an increase in conductivity. The mechanism of conduction in these polymers is described by a quasi-one-dimensional system and bandgap model. The basic self-localized nonlinear excitation, i.e. a quasione-dimensional structure is defined by solitons, polarons, and bipolarons [19, 26]. There are different types of conducting polymer used in OLED shown in **Figure 5**.

**Property Small molecules Polymers Dendrimers** Structure Compact Noncompact Compact,

Structural control high low Very high Synthesis Solution technique polymerization Stepwise growth Shape Fixed Random Spherical Architecture Regular Irregular Regular

Crystallinity Semi crystalline/crystalline Semi crystalline/crystalline Non crystalline/

high low high

Linear relation with a molecular weight

Aqueous solubility High low High

Reactivity Moderate low High Compressibility High low

Polydispersity Monodisperse plydisperse monodisperse

globular

**Application Refs.**

[40]

LEDs, laser, optocouples, triodes, photodiodes, photodetectors.

amorphous

A nonlinear relationship with a molecular weight

high low

*DOI: http://dx.doi.org/10.5772/intechopen.98652*

poly(*p*phenylene vinylene) (PPV)

**Table 1.**

**Polymer Synthesis process Polymer** 

1. via witting reaction

route

routes

2. soluble precursor

3. electrochemical

*Different types of polymer with synthesis process, their morphology, and application.*

ing the significance of driving polymers [23].


#### **Table 1.**

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

**Polymer Synthesis process Polymer** 

oxidation 2. Electrochemical

synthesis, 2. In-situ polymerization 3. Sonochemically

reaction

2. In-situ polymerization

Polyazulene 1. Oxidizing 1. spherical

PEDOT 1. Electrochemical 1. Nanotubes and

3. emulsion droplet electochemisty

4. template-free solution method

Polypyrrole 1. Electrochemical 1. Nanotubes and

Polyaniline 1. Chemical

*Chemical structure of conducting polymer [27].*

**Figure 5.**

Polycarbazole 1. Chemical

Polyfurane 1. Suzuki coupling

**morphology**

1. Nanofiber 2. Nanotubes and nanowires

nanowires

nanoparticle

nanowires

2. Nanoflowers

3.Nanoparticles

4.Nanofibers


Polyacetylene RH catalyst — Sensor, diode,

Polythiophene 1. Solvothermal — OLED,

**Application Refs.**

[24, 28]

[29]

[30]

[31–33]

[34]

[36]

[24, 37–39]

ETL in OLED, FED, OLED

FED, OLED, solar cell

catalyst.

PH sensor, OLEDs

Supercapacitor

OLED, electrochemical,

FED, HTL, LED, Diode

OLED, HTL, [35]

**10**

*Different types of polymer with synthesis process, their morphology, and application.*

The Nobel chemistry prize was awarded to MacDiarmid, Shirakawa, and Heeger in 2000 for the discovery and advancement of conducting polymers, demonstrating the significance of driving polymers [23].

Conductive polymers have a backbone that is π-conjugated (alternating single and double bonds), allowing overlapping of bound electrons in the polymeric chain [24]. Through incorporating various electron releasing/withdrawing functional groups into the polymeric backbone and managing the electron–hole injecting/ transporting ability of the synthesized conducting polymer, and the conductivity of the polymers can be effectively tuned to achieve emission in the desired luminance range [25]. The creation of charge carriers induces an increase in conductivity. The mechanism of conduction in these polymers is described by a quasi-one-dimensional system and bandgap model. The basic self-localized nonlinear excitation, i.e. a quasione-dimensional structure is defined by solitons, polarons, and bipolarons [19, 26]. There are different types of conducting polymer used in OLED shown in **Figure 5**.


#### **Table 2.**

*Properties of small molecules, polymer, and dendrimers [8, 41, 42].*

### *4.2.2 Synthesis of conducting polymer*

Many researchers have reported different ways for the synthesis of conducting polymer. The structure and properties of the polymer depend on the synthesis process. There are many processes used to increase efficiency, few of them are tabulated in **Table 1**.

#### **4.3 Dendrimers**

A light-emitting center is typically connected to one or more branched dendrons in light-emitting dendrimers. Surface groups are bound to the distal end of the dendrons to provide the solubilities needed for solution processing. The center (light emission), branching groups (charge transport), and surface groups can all be modified independently to the dendritic structure (processing properties) tabulated in **Table 2**.
