**4. Emissive materials**

As seen in **Figure 4**, OLED emissive materials are classified into two groups: small molecules and polymers [8]. Subgroups of the polymer group are known as non-conjugated or conjugated. Small molecule and polymer groups may use dendritic compounds as intermediate materials. Organic material-based electroluminescent (EL) systems have excellent low-power driving voltage and bright emission properties [14]. Tiny molecular organic compounds, conjugated oligomers, and polymers with precise chain lengths are examples of organic light-emitting materials that have distinct electrical and optical properties. The interpretation of the operating process in organic EL devices is dependent on the carrier mobility of organic materials. Conducting polymers must have high conductivity, strong solubility, and mechanical properties. They must also be resistant to acids and bases. Dendrimers are a new kind of polymer that can be used in OLEDs. Bernanose et al. made the first discoveries of electroluminescence of organic materials in the early 1950s [15].

**9**

research to a close.

*4.2.1 Conducting polymer*

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

These are materials that are crystalline or semi-crystalline and have high aqueous solubility. Tang et al. were the first to create efficient OLEDs using small molecules [17]. Small molecules have advantages like simple synthesis and purification, and the techniques of vapor deposition allow for the production of complex,

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

of hours have been reached as the culmination of over 20 years of R&D [20].

manufacturing is a noteworthy characteristic of polymer OLEDs.

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

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

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

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].

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

high-performance multi-layered layers.

**4.2 Polymer used in OLED**

advantages [18].

**4.1 Small molecules**

**Figure 4.** *Schematic classification of emissive materials [16].*
