A series of novel sublimable cationic iridium(iii) complexes with 2-phenylquinoline as the major cyclometalated ligand and imidazole-type ancillary ligands were designed and synthesized. It is found that complexes containing active hydrogen atoms in the ancillary ligands exhibited poor electroluminescence performance. By introducing a phenyl group into the ancillary ligands to protect the active hydrogen atoms and simultaneously increase the steric hindrance of complexes, the device performance was drastically improved, due to the reduced negative effects of the active hydrogen atoms and the decreased intermolecular interaction and aggregation. We obtained high efficiency devices with a superior external quantum efficiency of 14.7%, high brightness, a low turn-on voltage of only 2.4 V and low efficiency roll-off at high luminance, the best results for yellow OLEDs based on sublimable cationic iridium(iii) complexes.