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Two bis-cyclometalated heteroleptic iridium complexes incorporating 1-phenylisoquinoline (piq) as the main cyclometalating ligand and 3-hydroxy-2-methyl-4-pyrone (ma) or 2-ethyl-3-hydroxy-4H-pyran-4-one (ema) as the auxiliary ligand, namely Ir(piq)₂(ma) (Ir-1) and Ir(piq)₂(ema) (Ir-2), were developed and applied as deep-red phosphors in organic light-emitting diodes (OLEDs). The two auxiliary ligands had similar influences on the photophysical, electrochemical, and electroluminescent properties of the iridium complexes. Ir(piq)₂(ma) (Ir-1) showed better luminescence performance in a simple phosphorescent OLED compared to the traditional red iridium complex Ir(piq)₂(acac) and exhibited a current efficiency of 9.39 cd A⁻¹ (EQE of 12.09%). In contrast, Ir(piq)₂(ema) exhibited an efficiency of 8.6 cd A⁻¹ (EQE of 10.19%).