We have studied the effect of hole injection layers (HILs) on the performance of phosphorescent green organic light-emitting diodes (OLEDs). The PEDOT:PSS and other new PEDOT-based materials of HIL 1.3N and HIL 3.3N were used as HILs. The OLED with HIL 1.3N, having the work function of 6.08 eV, exhibits low driving voltage and high efficiency because of the improved hole injection. The position of emission zone, studied by analyzing the electroluminescence spectra, is found to be at the center of the emission layer (EML), while the emission zone is near the interface for the OLEDs with PEDOT:PSS and HIL 3.3N. This is due to the improved charge carrier balance within the EML caused by the efficient HIL of HIL 1.3N. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.002111jes] All rights reserved.