An undoped charge-generation unit (CGU) with the structure of lithium fluoride (LiF)/aluminum/molybdenum trioxide (MoO3) was demonstrated in tandem organic light-emitting diodes (OLEDs). Tandem OLEDs with two identical emissive units consisting of 4,4′-bis-(1-naphthyl-N-phenylamino)-biphenyl (NPB)/tris(8-hydroxyquinoline) aluminum (Alq3) exhibited superior performance over a corresponding single-unit device. Under a driving current of 650 A m−2, the current efficiency of the tandem OLEDs was about 6.95 cd A−1, almost double that of the single-unit OLEDs. The operational stability of the tandem OLEDs was also enhanced compared with that of the single-unit OLEDs. It was demonstrated that the MoO3 plays a critical role of charge generation in this undoped CGU. The LiF/Al layers were used to enhance the electron injection from MoO3 layer to the adjacent Alq3 layer. Furthermore, fabrication of this CGU involves no sputtering or doping process, which can render tandem OLEDs processing more feasible.