The fundamental development of new emitters for organic light-emitting diodes (OLEDs) has always grabbed a lot of attention. Since they play an important role in developing efficient OLEDs in terms of color purity, efficiency, and stability; key aspects for applications in displays, biomedical, and solid-state lighting. Fig. displays the multilayered architecture of OLEDs and their mechanisms allowing direct charge injection, transport, and recombination to form emitting excitons. According to the principle of spin statistics, the excitation of the emitter leads to the formation of 25% singlet excitons and 75% triplet excitons. The emitters based on small molecules present in the active layers of OLEDs are broadly classified into three types, such as fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF) emitters (Fig. ). Their basic photophysical properties are well reported in many excellent reviews and literatures. The first generation of OLEDs are based on fluorescent emitters (emission comes only from the S1 state after excitation), which harvest only 25% of singlet excitons and therefore reach 25% of maximum internal quantum efficiency (IQE), since 75% of triplet excitons are dark. The external quantum efficiency (EQE) of an OLED device depends on IQE and relates to each other as follows: 9