Influence of MoO3’s blend in hole transporting layer on the performance of Alq3-based OLEDs

MoO 3 was introduced in a typical hole transporting material N,N’-diphenyl-N,N’-bis(1-naphthyl) (1,1’-biphenyl)-4,4’diamine (NPB) to improve the performance of tris-(8-hydroxyquinoline) aluminum (Alq 3 ) based organic light emitting diodes (OLEDs). It is found that MoO 3 in NPB layer has a significant quenching effect on the electroluminescence of the device, although the current density-voltage characteristics of the devices is improved. At a current density of 20 mA/cm 2 , the driving voltage of the device with MoO 3 -blended NPB (50 wt%) is 5.83 V, which is 0.77 V lower than that (6.6 V) of the device without MoO 3 , while the brightness (54.3 cd/m 2 ) or current efficiency (0.27 cd/A) of the former is one order of magnitude lower than that (735 cd/m 2 or 3.68 cd/A) of the latter. The formation of energy gap states by the charge transfer between MoO 3 and NPB or Alq 3 is used to explain the results. • MoO 3 as a quencher exhibits an electroluminescence quenching effect. • Inserting a pure separator layer can prevent MoO 3 ’s diffusion to emission layer. • Quenching effect originates from the gap states by charge transfer.