Tandem organic light-emitting diodes with an effective nondoped charge-generation unit

摘要

An undoped charge-generation unit (CGU) with the structure of lithium fluoride (LiF)/aluminum/molybdenum trioxide (MoO_3) 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 (Alq_3) 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 MoO_3 plays a critical role of charge generation in this undoped CGU. The LiF/Al layers were used to enhance the electron injection from MoO_3 layer to the adjacent Alq_3 layer. Furthermore, fabrication of this CGU involves no sputtering or doping process, which can render tandem OLEDs processing more feasible.