Based on the device structure shown in the inset of Figure , with a 10 nm thick single-layer EBL structure, devices with mCP showed higher current density and luminance than that with CzSi at the same voltage (Figure ). The EQEmax of devices with mCP reached over 10%, which was twice that with CzSi. However, the efficiency roll-off was significant with mCP EBL-based devices. The poor device performance of the single EBL devices was attributed to either triplet exciton leakage into the EBL or carrier imbalance induced by the low hole mobility of CzSi. In addition, both mCP and CzSi have LUMO energy level of around 2.4 eV, which is not sufficient to block electrons transported by either mer-Ir(tfpi_tmBn)3 or TSPO1 whose LUMO levels are 2.11 eV and 2.50 eV, respectively. To address these issues, a double-layer EBL structure was investigated, including a 5 nm thick mCP or CzSi and a 5 nm thick self-blocking layer consisting of mer-Ir(tfpi_tmBn)3. Consequently, the device performance was greatly improved as shown in Figure . Not only did EQEmax increase to 14.2% and 7.3% for mCP/mer-Ir(tfpi_tmBn)3 EBL and CzSi/mer-Ir(tfpi_tmBn)3 EBL devices, respectively, but their efficiency roll-off were reduced as well with EQE at 100 cd m - 2 (EQE100) of 11.0% and 6.5%, respectively. We contend that the self-blocking strategy effectively reduced the triplet exciton diffusion and electron leakage into the HTL, especially in the case when an efficient carrier blocking layer with sufficiently high triplet energy level is unavailable. Finally, the device performance was improved further by optimizing the doping profile to a stepwise doping strategy, which was expected to lead to more balanced bipolar carrier transport and wider light emission zone. To investigate the bipolar carrier transport, single carrier devices were fabricated firstly, in which cD was varied from 0, 10, 20, to 30 wt%. The single carrier devices showed dominant electron conduction. With increasing cD from 10 wt% to 30 wt%, there was a monotonic increase of the hole current in the HODs, which was attributed to the trapping state-assisted hole transport of dopants because of their shallower HOMO level (5.4 eV) than that of TSPO1 (6.7 eV). By contrast, a decrease of the electron current was observed with cD increasing from 0 to 20 wt% in the EODs, implying a large carrier scattering effect of dopants under the electron transport condition, which is consistent with their energy level alignment (i.e., a shallower LUMO of 2.09-2.11 eV for the emitters than that of TSPO1, 2.5 eV). With a further increase of the cD to 30 wt%, an increase of the electron current was observed. Considering the dependence of both bipolar carrier conduction and FPL (Figure ) on cD, a stepwise doping profile of EML was designed, which comprised four sublayers of the same thickness of 6 nm. The doping concentration of the sub-EML was highest at 30 wt% near the HTL, which then decreased gradually with distance to 20 wt%, 10 wt%, and finally 6 wt%. The total EML thickness and averaged doping concentration across the EML were the same in both the uniform doping and stepwise doping devices, the latter shown in the inset of Figure . showed improved performance compared with the uniformly doped device, with EQEmax of 14.9%, implying nearly 100% exciton utilization efficiency. The EQE at 100 cd m -2 and 1000 cd m -2 were 11.7%, and 3.0%, respectively. In addition, PEmax was increased to 6.3 lm W -1 , double that reported for the device with mer-Ir(tfpmi)3 which employed a uniform doping strategy. The device showed deep blue emission with lEL at 424 nm, FWHM of 66 nm, and CIE coordinates of (0.16, 0.08). The device with the mer-Ir(tfpi_Bn)3 emitter showed a slightly lower device performance with EQEmax of 12.2%, PEmax of 5.1 lm W -1 , at the same CIE coordinates of (0.16, 0.08). The cross-comparison of the performance of the mer-Ir(tfpi_Bn)3 based device with that of the mer-Ir(tfpi_tmBn)3 device is shown in Figure .