The luminance and the charge transport mechanisms for phosphorescent organic light-emitting devices (PHOLEDs) fabricated utilizing a tris(2-phenylpyridine)iridium (Ir(ppy)(3))-doped N,N'-dicarbazolyl-3,5-benzene (mCP) emitting layer (EML) were investigated to improve the luminance efficiency of PHOLEDs. When Ir(ppy)(3) molecules existed on the electron transport layer (EFL) side of the EML, the current density of the PHOLED with an mCP EML containing an Ir(ppy)(3)-doped thin layer increased. The Ir(ppy)(3)-related electroluminescence (EL) intensity of the PHOLEDs with an Ir(ppy)(3)-doped EML increased with increasing operating voltage. The increase in the current density was due to the lowest unoccupied molecular orbital levels of the Ir(ppy)(3) and the 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline ETL being identical, and the increase in the EL intensity could be attributed to a movement of the recombination zone to the center of the EML due to an increase in the electron injection from the ETL into the EML at high voltages. (C) 2011 Elsevier B.V. All rights reserved.