The combination of two p-doped layers for improving the hole current of organic light-emitting diodes

The combination of MoO3-doped 4,4-N,N-bis [N-1-naphthyl-N-phenyl-amino]biphenyl (NPB:MoO3) and 4,4-N,N-dicarbazole-biphenyl (CBP:MoO3) was used to enhance the hole conduction in organic light-emitting diodes (OLEDs). It is found that the OLED using NPB:MoO3 10nm/CBP:MoO3 5nm showed a much increased current density than the one using NPB:MoO3 5nm/CBP:MoO3 5nm at a given voltage larger than 4V, mainly because the hole transport barrier across the p-doped heterojunction in the former device became smaller than that in the latter device with the driving voltage increasing, despite the fact that the 10-nm NPB:MoO3 in the former device caused more Ohmic loss than the 5-nm one in the latter device. As a result of the higher conductivity of NPB:MoO3 than that of CBP:MoO3, the OLED using the combination of 15-nm NPB:MoO3 and 5-nm CBP:MoO3 showed significantly increased performance than the one using the single 20-nm CBP:MoO3. We provide a useful way of advancing the OLEDs toward the practical applications in general lighting and flat-panel displays.