ITO/ TAPC (35 nm)/TCTA (10 nm)/CzSi (10 nm)/x wt% emiiter/mCP (20 nm)/ The electroluminescence peak of the OLED, lEL, of 493 nm and the FWHM of 46 nm match those of the PL spectrum of the 2 wt% films in mCP (lPL of 494 nm and FWHM of 49 nm). The EL is narrower compared to the previously reported device with DDiKTa (9 wt% in DPEPO), which emitted at lEL of 500 nm and had a FWHM of 59 nm. This small red-shifted emission compared to the SS PL in 2 wt% mCP film can be attributed to a combination of the use of the higher polarity DPEPO host and the higher doping concentration. By contrast, the EL is red-shifted compared to the device with DiKTa (3.5 wt% in mCP), which emitted at lEL of 465 nm and had a FWHM of 59 nm. The corresponding Commission Internationale de l'Éclairage (CIE) coordinates are (0.16, 0.50) for the device with DDiKTa-F, which are close to those of the device with DDiKTa (0.18, 0.53) yet are red-shifted compared to the device with DiKTa (0.14, 0.18). The device with DDiKTa-F showed an EQEmax of 15.3%, which is of similar magnitude to those of DDiKTa (19.0%) and DiKTa (14.7%). Gratifyingly, the efficiency roll-off was less severe, with an EQE at 100 cd/m 2 at 9.9% for the device with DDiKTa-F, which was higher than those of DDiKTa (EQE100 of 8.1%) and DiKTa (EQE100 of 8.3%). This modestly improved efficiency roll-off can be explained by a higher figure of merit (FOM) that describes productive exciton utilization, 𝐹𝑀𝑂 =