This paper elucidates the enhancement of the optoelectronic properties of organic light-emitting diodes (OLEDs) by the p-doping effect of molybdenum oxide (MoOx)-doped 4,4',4 ''-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA). The device performance strongly depends on both the thickness and the doping concentration of the MoOx-doped 2-TNATA. As the doping concentration is increased from 25 to 75%, the hole ohmic properties of the hole-only device with the glass/indium tin oxide (ITO)/MoOx-doped 2-TNATA (100 nm)/Al structure were improved, due to the increase in the p-type doping effect. Also, the photoemission spectra revealed that the p-type doping effects cause the lowering of the hole-injecting barrier height, as well as the improvement of the hole conductivity. This improvement is caused by the formation of the charge transfer complex (MoOx-/2-TNATA(+)) that is generated by doping MoOx into 2-TNATA, markedly increasing the number of hole carriers. With MoOx-doped 2-TNATA as a hole ohmic contact and fullerene (C-60)/lithium fluoride (LiF) as an electron ohmic contact, the OLED with the glass/ITO/MoOx-doped 2-TNATA (75%, 60 nm)/NPB (10 nm)/Alq(3) (35 nm)/C-60 (5 nm)/LiF (1 nm)/Al (150nm) structure showed both a maximum luminance of 127,600 cd/m(2) at 6.4 V and power efficiency of 4.7 lm/W at about 1000 cd/m(2). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3519846] All rights reserved.