In this manuscript, the effects of molybdenum trioxide (MoO3) thicknesses on the performance of organic light emitting diodes (OLEDs) based on indium tin oxide (ITO) as anode, MoO3 as hole injection layer (HIL), N-N-diphenyl-N,N-(bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine) (TPD) as hole transport layer(HTL), tris(8-hydroxyquinoline) aluminum (Alq3) as emission layer (EML) and electron transport layer(ETL), lithium fluoride (LiF) as electron injection layer (EIL), and Aluminum(Al) as cathode have been analyzed. The results show that the MoO3 leads to the improvement of turn on voltage, current injection and luminance of the OLED devices. Also this material leads to decreasing of the operating and driving voltages, so that the OLED devices with thickness of 5nm for HIL provide the best performance from the point of driving and operating voltages view among all OLEDs fabricated in this work. Thus, this thickness could be considered as the optimal layer thickness..
