Driving voltage of organic light-emitting diodes (OLEDs) was lowered by applying (NPB/MoO3)(x)/NPB as a hole transport layer (HTL). (NPB/MoO3)(x) was multi-layer periodic (MLP) structure with x changed from 0 to 3. Compared with the conventional device with 0-periodic structure, the driving voltage of the device with 1-periodic structure was the lowest. This was due to charge transfer (CT) complex formation between NPB and MoO3. The driving voltage of tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting devices (OLEDs) could be lowered by 0. 8 V at 1 000 cd x m(-2) by using multiple structure of NPB/MoO3/NPB.