Light emission in red phosphorescent organic light-emitting diodes with mixed ligand dopant materials was studied. 1-phenylisoquinoline (piq) and phenylpyridine (ppy) moieties were introduced as ligands for Ir based phosphorescent dopants and light emission mechanism was investigated by changing the doping concentration of devices. Charge trapping dominated light emission in Ir(piq)(3) but the charge trapping was not significant in mixed ligand dopant materials because of ppy ligand with less charge trapping character. High efficiency of 2.02 lm/W could be obtained in the heteroleptic red dopants compared with 1.58 lm/W in Ir(piq)(3) and power efficiency improvement by ppy ligand was significant at low doping concentration. (C) 2007 Elsevier B.V. All rights reserved.