Influence of the thickness of N,N'-Bis(naphthalene-1-yl)-N,N'-bis(phenyl) benzidine layer on the performance of organic light-emitting diodes

We investigated the influence of the thickness of hole-transport layer, N,N'-biphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB), on the performance of the typical bi-layer organic light-emitting diodes (OLEDs). It was found that both the current efficiency and the power efficiency of bi-layer OLEDs were improved when the thickness of the hole-transport layer varied from 30 to 120 nm. By investigating the hole-injection efficiency of ITO/NPB contact with various thicknesses of NPB film, we found that the hole-injection efficiency was reduced with the thickness of NPB layer increasing from 60 to 180 nm, which improved the injected carriers balance in devices and increased the efficiency of the bi-layer OLEDs.