Stabilization of hole transport layer by alternate deposition for organic light-emitting diodes

N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl) 4,4'-diamine (NPD) is often used as a hole-transport material in organic light-emitting diodes (OLEDs). The polycrystallization of organic films in OLEDs is thought to be one of the degradation factors of the device performance. It is found that the generation and growth of polycrystalline region in NPD film can be efficiently suppressed by alternate deposition method. The generation of polycrystalline region in the alternate structure specimen depends on the top layer material. The OLEDs having alternate hole-transport layer have the similar initial EL properties as the OLEDs having organic alloy and only NPD hole-transport layer.