We have demonstrated organic light-emitting diodes (OLEDs) by incorporating copper iodide (CuI) in 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) as a hole injection layer (HIL) based on the emitting system of C545T–Alq₃. The device with CuI-doped m-MTDATA HIL shows a very low operating voltage of about 4.26 and 5.70 V for 1000 and 10 000 cd m⁻², respectively, which reveals a great improvement over the undoped device m-MTDATA HIL (4.85 V for 1000 cd m⁻² and 7.72 V for 10 000 cd m⁻²). Furthermore, the CuI-doped device exhibits a maximum power efficiency of 5.88 lm W⁻¹, about 58% higher than that of the corresponding undoped device (3.71 lm W⁻¹). The improved performance of the CuI-doped device could be attributed to the enhanced hole injection and transport due to the generation of free charge carriers in the CuI-doped layer by charge transfer from m-MTDATA to CuI molecules, leading to an increase in electrical conductivity and formation of ohmic contact at the ITO/HIL interface.