**3. OLED materials**

OLEDs are constantly developing and emphasizing the existence of customized functions of organic materials that can be added to well-preserved thin films. As a result, the materials' specifications are diverse, ranging from processability and

**Figure 3.** *Types of organic materials used for OLEDs [13].*

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 direction according to device manufacturing.
