In a bid to improve the device performance and given the narrowband emission and high FPL values for tBuPh-BN and DPA-tBuPh-BN, HF OLEDs were then fabricated (Figure ). 5CzBN was used as the TADF assistant dopant because of its high FPL of 70%, fast kRISC of 1.13×10 5 s -1 , and a strong overlap between the absorption spectra of tBuPh-BN and DPA-tBuPh-BN and the PL spectrum of 5CzBN in toluene (Figure ). An optimized doping ratio of 2 wt% emitters: 12 wt% 5CzBN: 86 wt% SF3-TRZ was identified to be used as the EML as this formulation minimized the chance for triplet excitons from the host to transfer to the emitters and permitted an efficient Förster resonance energy transfer from 5CzBN to the emitters (Figure and) while conserving their narrowband emission. As shown in Figure , the HF devices with tBuPh-BN and DPA-tBuPh-BN showed narrowband emission at lPL of 492 and 480 nm and FWHMs of 34 nm (0.17 eV) and 38 nm (0.20 eV). The corresponding CIE coordinates were (0.11, 0.41) and (0.13, 0.29), respectively, which are similar to those of the 2 wt% emitter OLEDs (Figure , Table ). The turn-on voltages (Von) were reduced from 3.5 and 3.7 V to 3.1 and 3.3 V and the EQEmax values were significantly improved to 20.9 and 15.9% for the HF devices with tBuPh-BN and DPA-tBuPh-BN, respectively. The relatively higher EQEmax for the HF OLED with tBuPh-BN can be attributed to the higher FPL of the emitter. Despite their higher EQEmax, the efficiency roll-off of the HF devices remained severe (EQE100 of 9.8 and 10.1% and EQE1000 of 7.1 and 7.8% for the HF devices with tBuPh-BN and DPA-tBuPh-BN, respectively).