Phosphorescent organic light emitting diodes (PHOLEDs) have been fabricated with structure of indium tin oxide/MoO3 doped 4,4′-N,N'-dicarbazole-biphenyl (CBP) 30 nm/tris(4-carbazoyl-9-ylphenyl)amine 10 nm/CBP doped with tris(2-phenylpyridine)iridium(III) (CBP:Ir(ppy)3) x/bathocuproine 50 nm/LiF 1 nm/Al, where x = 2.5, 5, 10, and 20 nm, respectively. The current efficiency (CE) of device with x = 10 nm is higher than those with x = 2.5 and 5 nm, mostly because the width of exciton formation zone (5.7 nm) with x = 10 nm is larger than those (2.5 and 5 nm) with x = 2.5 and 5 nm. However, the current density with x = 10 nm decreases than those with x = 2.5 and 5 nm at a certain driving voltage, since the ~ 4.3 nm CBP:Ir(ppy)3 accommodating no exciton formation with x = 10 nm plays a role of transporting holes, raising ohmic loss of hole and thereby increasing driving voltage. When x increases from 10 to 20 nm, the width of exciton formation zone rises from 5.7 to 6.8 nm with CE almost unchanged, and the current density decreases as a result of increased ohmic loss of hole. The current research is useful to develop high-efficiency and low-driving voltage PHOLEDs. [ABSTRACT FROM AUTHOR]