The following device structure using mCP as the host provided the best maximum external quantum efficiency (EQEmax): indium-tin-oxide (ITO, 100 nm)/HAT-CN (10 nm)/ TAPC (30 nm)/mCP (10 nm)/mCP: 5 wt% of 1B-CzCrs, 2B-CzCrs or 1B-DTACrs (20 nm)/ PPT (10 nm)/ TmPyPB (30 nm)/ Liq (2 nm)/ Al (100 nm). All the device data are depicted in Fig. , Figs. S29, S30, Table and Table . The OLED with 1B-CzCrs exhibited a higher EQEmax of 12.8% than the device with 2B-CzCrs (EQEmax of 8.4%). The EL of 1B-CzCrs and 2B-CzCrs was observed at lEL of 451 and 450 nm, similar to the corresponding PL emission in mCP. Both devices demonstrated a relatively low turn-on voltage below 4.0 V. Furthermore, the device with 1B-CzCrs showed a much higher EQEmax of 12.8% than that with 1B-DTACrs (2.1%). This, again, confirms that TADF is operational in the former and not the latter. In the context of the high ΦPL of the two emitters, the modest EQEmax values can be explained by the dipole orientation of the emitters in the emissive layer. To investigate the molecular orientation of both emitters, angle-dependent PL measurements were performed on the 5 wt% doped films of these two emitters in mCP films (Fig. ).The horizontal-dipole ratios (Θ) is 0. ). Since the PL emissions of 1 wt% doped film of 1B-CzCrs and 2B-CzCrs in mCBP host have CIE coordinates closer to the BT.2020 standard for blue (Table ), the performance of devices, 1 wt% 1B-CzCrs and 2B-CzCrs in mCBP were investigated. Although the devices with both emitters gave CIEy < 0.05, the EQEmax values were inferior compared to the devices using mCP as the host (Fig. ,