Four cyclometalated heteroleptic Ir(III) complexes [(Et-Cz-BTz)₂Ir(pic)], [(Et-Cz-BTz)₂Ir(pic-N-O)], [(Et-Cz-BTz)₂Ir(EO₂-pic)], and [(Et-Cz-BTz)₂Ir(EO₂-pic-N-O)] containing benzothiazole linked ethylcarbazole (Et-Cz-BTz) as a main ligand have been successfully synthesized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). All the Ir(III) complexes emit bright yellow (541–582 nm) phosphorescence at room temperature. Among the four Ir(III) complexes, the [(Et-Cz-BTz)₂Ir(EO₂-pic)] showed the best luminous efficiency of 60 cd A⁻¹ and external quantum efficiency (EQE) of 19% with CIE coordinates (0.467, 0.524), which is one of the best efficiencies for solution-processed yellow PhOLEDs using heteroleptic Ir(III) complexes as an emitting layer. Interestingly, upon replacing the thiazole linking group in the main ligand by benzothiazole, the EQE increased from 6.08 to 19%.