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

Abstract 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 10 nm/CBP:MoO₃ 5 nm showed a much increased current density than the one using NPB:MoO₃ 5 nm/CBP:MoO₃ 5 nm at a given voltage larger than 4 V, 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.