The effect of NPB morphology on OLEDs optoelectronic characteristics

N,N'-bis(Inaphthyl)N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) surface morphologies deposited on different temperature substrates were investigated using atomic force microscopy(AFM). NPB is an organic materials often used as hole transport layer(HTL) in organic light-emitting devices (OLEDs). It has been found that the NPB morphology turned from island morphology at high temperature(100°C) to grain morphology at room temperature. At the same time, the growth mode of NPB film also changed from the Volmer-Weber type to the Stranski-Krastanov type. The results show the wettability between NPB and ITO becomes worse when the substrate temperature increases, which prevents NPB from forming continuous thin film. To characterize the effect of NPB surface morphology, the devices with the structure of Glass/ITO/NPB/Alq₃/Al were fabricated using NPB films deposited at different substrate temperature and their performances were compared. The results show that the NPB morphology has significant effects on the performance of OLEDs.