There was a continuous success in designing HF-OLEDs after the first examples reported by the Adachi group. Using a suitable host and the right choice of TADFfluorescence emitter couple , such as T4:F3, T5:F4, T6:F5, and T7:F6 (Figs ), they succeeded to fabricating blue-, green-, yellow-, and red-emitting HF-OLEDs, respectively (Table ). With the limit of ~15-50 wt% doping concentrations of TADF emitters and 1 wt% of fluorescence emitter, the maximum EQE of the blue-, green-, yellow-, and red-emitting HF-OLEDs were 13.4%, 15.8%, 18.0%, and 17.5%, respectively. There was a huge increase in EQE compared to conventional fluorescent emitters in all devices, demonstrating the EQE-enhancing role of HF emitters. No doubt, this approach demonstrates a path for designing several new HF emitters using theTADF-fluorescence emitter couple and their fruitful applications in OLEDs. However, a low EQE (13.4%) was observed for the first blue HF-OLED based on the T4:F3 emitter couple (Table ). This might be attributed to the incomplete FRET from the TADF emitter to the fluorescent emitter, as the TADF emitter (T4) emits at longer wavelength than the fluorescence emitter (F3), decreasing the spectral overlap JF (eq. 4).