TADF emitter acts as a sensitizer, which plays an important role in harvesting the singlet excitons of fluorescence emitters. Therefore, the TADF emitters should have an shift for efficient energy transfer. Furthermore, an efficient FRET (high kFRET) process requires the maximum overlap between the absorption spectrum of the fluorescence emitter and the emission spectrum of the TADF emitter. Finally, the first singlet excited state (S1) of the fluorescence emitter must be below that of the S1 state of the TADF emitter (Fig. ). The concentration of two emitters is also crucial for efficient HF behavior. Typically, a doping strategy is followed, in which a high amount of TADF emitter and a lesser amount of fluorescence emitter in the active layer is required.