We had previously synthesised the deep-blue emitting Ir(NHC) complex mer-Ir(tfpmi)3 with moderate FPL of 46.6% in doped films (Figure ), which produced some of the deepest blue and highest efficiency phosphorescent OLEDs. In an effort to improve both FPL and the corresponding device performance, we have now developed a second generation of deep blue-emitting iridium complexes incorporating bulky groups attached to the imidazole ring of the NHC ligand. We cross-compared the use of the previously reported benzyl group (Bn) with the bulkier 2,4,6-trimethylbenzyl (tmBn) group. Optimisation of the device structure using the iridium complex as an exciton blocking layer and a stepwise doping scheme led to high-performance deep-blue OLEDs. Our best OLED was fabricated with mer-Ir(tfpi_tmBn)3 as the emitter in a device with stepwise emitter doping and using a neat film of the same emitter as an electron-blocking layer (EBL). This deep-blue emitting device [CIE(0.16, 0.08)] achieved an EQEmax of 14.9% and showed a moderate efficiency roll-off with an EQE100 of 11.7%. A comparison of device performance (Figure ) reveals that these are among the best performing deep-blue OLEDs.