Where, 𝐼𝑄𝐸 and  𝑜𝑢𝑡 , denote the internal quantum efficiency and the light-out-coupling efficiency, respectively. Eq. 1 revealed that  𝐼𝑄𝐸 depends on  (charge balance factor of injected holes and electrons),   (efficiency of radiative exciton production), and  𝑃𝐿 (photoluminescence quantum yield). In case of fluorescence emitters,   is strongly limited, since the 75% of electrically generated dark triplet excitons. Therefore, there was a search for techniques/emitters to harvest them, that is, the development of second and third generation of emitters. The Second generation of OLEDs are based on phosphorescent emitters (e.g., Iridium(III) and Platinum (II) complexes) that harvest both singlet and triplet excitons, reaching 100% of maximum  𝐼𝑄𝐸 . Though it overwhelms the limitations of conventional fluorescent OLEDs, its broad emission renders the poor color purity of the devices. In addition, sustainability and cost concerns are always present when using rare-earth materials. The third generation of OLEDs based on TADF organic emitters with a very small singlet-triplet energy gap (ΔEST), which produces both singlet and triplet excitons through an efficient triplet to singlet upconversion process, enables them to reach 100% of  𝐼𝑄𝐸 . The TADF emitters significantly enhanced the OLED performance, promising low-cost and sustainable emitters. However, the wide emission spectra of TADF emitters still limit the low color purity of the devices, and the long-lived excitons are leading to degradation processes that reduce the  𝐸𝑄𝐸 and device stability.