Improved performance of organic light emitting diodes by pentacene as hole transporting layer

Three kinds of organic light emitting diodes with tris(8-hydroxyquinoline) aluminum (Alq3) as emitting layers were fabricated. It was found that the current efficiency (8.2 cd/A) of devices with pentacene as hole transporting layer (HTL) was obtained at 10 V, which was increased by a factor of 3.2 in comparison with that (2.6 cd/A) of the devices with N,N'-bis-(1-naphthyl)-N,N'-1-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) as HTL. The improvement was attributed to the large hole mobility of pentacene and the increased interfacial surface area induced by its pyramid-like morphology, which result in the increase of the recombination region at the interfaces between pentacene and Alq3 layers. Investigation by atomic force microscopy (AFM) shows the surface morphology of pentacene and NPB thin films.