Kwon and co-workers selected TADF emitters T1 and T2 (Fig. Concerning the low-energy part of the visible spectrum, the Kwon group reported a red HF-OLED combining F2 and a TADF emitter T3 (Figs. and). Owing to the sharp emission and high PL of BODIPYs, they selected a BODIPY-based red emitting fluorescence emitter F2 (emission maximum at 620 nm), featuring a high PL of 99% and a small FWHM of 31 nm in solution. However, the deep lowest unoccupied molecular orbital (LUMO) of 3.83 eV may induce electron trapping directly on BODIPY in the emissive layer. Therefore, a TADF emitter with deep LUMO would be best for these BODIPY-based fluorescence emitters, which help reduce the trapping of electrons in the emissive layer. Thus, they selected a TADF emitter T3 (Fig. ) that possesses a deep LUMO (3.9 eV) along with better spectral overlap with the absorption spectrum of F2 and the emission spectrum of T3. In addition, a high PL of 98% and high kFRET of 5.10 × 10 7 s -1 were obtained for 0.7% doped films. Therefore, the selection of the T3 and F2 combination resulted in an efficient HF-OLED (see below). 57