HAT-CN/CuPc is adopted as hole injection layer (HIL) in organic light-emitting diodes (OLEDs). While this combination is inserted in the ADN blue device, the driven voltage is lower down efficitively without losing the current efficiency. This result is caused by two reasons. The one is the improved crystallinity of CuPc film that is induced by the HAT-CN thin film, which can decrease the electric resistance of HIL effectively. And the other one is the efficacious hole injection efficiency that is achieved by HAT-CN/CuPc. The turn-on voltage of HAT-CN/CuPc device can reduce to 3.4 V, which is 0.5 V lower than that with CuPc as HIL.