The FPL of the two meridional isomers in the 30 wt% doped film (78 and 79%) are higher than the less bulky N-methyl analogue mer-Ir(tfpmi)3, which showed a FPL of 38%. The FPL of the two meridional isomers are slightly lower than other recently reported complexes containing bulky N-benzyl substituents, mer-Ir(pmp_Bn)3 and mer-Ir(CF3pbp)3, which have FPL of 95 and 89%, respectively, in doped (20 wt% and 50 wt%, respectively) TSPO1 films. Considering the higher FPL, reduced aggregation, and their greater availability, the meridional isomers were explored as emitters in OLEDs. Firstly, using the device structure with the stepwise doped EML, i.e., ITO/HAT-CN (5 nm)/NPB (45 nm)/TCTA (10 nm)/mCP (10 nm)/mer-Ir(tfpi_tmBn)3 (5 nm)/EML (24 nm)/TSPO1 (6 nm)/TmPyPB (20 nm)/LiF (0.5 nm)/Al (100 nm), as shown in Figure , the device performance as a function of host choice was compared. Although both TSPO1 and DPEPO could well confine the triplet exciton on emitters within the EML, resulting in similar EQEmax, the device with TSPO1 showed higher maximum luminance (Lmax) and lower efficiency roll-off than that with DPEPO, which we attributed to the better carrier transport ability of TSPO1. To improve the device performance further by suppressing the exciton/electron leakage into the HTL, a blocking layer with high triplet energy level and shallow LUMO level was needed.