**4.1 Small molecules**

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

tion according to device manufacturing.

**4. Emissive materials**

film formulation to electrical transport, emission, and optical properties [12]. The availability of effective and reliable light emitters over the entire visible spectral spectrum is more important. In this regard, the distinction between fluorescent and phosphorescent materials must be made. The arrival and further creation of emitters based on heavy-metal oriented metal–organic complexes, as seen in **Figure 3**, was a watershed moment. These materials are used in different layers of OLED to enhance properties and efficiency. Powerful spin-orbit coupling in these compounds combines singlet and triplet states even more than in pure hydrocarbons, allowing phosphorescence to become a permitted transfer. Meanwhile, promising efficiency data for OLEDs based on these materials have been published; however, the bottleneck remains the limited supply and stability of deep-blue phosphorescent emitters. As there are different layers in OLEDs, we need different types of materials for enhancing the properties of OLEDs. The most important layer of the OLEDs is an emissive layer, where recombination takes place and light emits in a specific direc-

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].

**8**

**Figure 4.**

*Schematic classification of emissive materials [16].*

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, high-performance multi-layered layers.
