Improve the efficiency of organic light-emitting diodes using a novel hole injection layer with triple-layer gradually doped structure

In this study, we reported the fabrication of an organic light-emitting diodes (OLED) using the metal oxide Nb-doped ZnO (NZO) and the common hole transport layer of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) to form a novel hole injection layer with gradually-doped triple-layer structure (HITL). The comparison between the stepwise doped double-layer and uniformly doped device showed that the HITL structure could effectively improve the current efficiency of the OLED device. The results of both current density-voltage measurement by hole-only device (HOD) and the UV photoelectron spectroscopy (UPS) confirmed that the highest occupied molecular orbital (HOMO) energy level could help to improve the hole injection performance. In addition, the non-destructive analysis method of capacitance-voltage was used to observe the movement of charge carriers during the operation of the device. The comparison study indicated that by the stepwise doped double-layer injection layer, the influence of the doping gradient on the carrier injection was confirmed, and a smaller doping gradient was found to show the better performance of carrier injection. Based on the results obtained from this study, the maximum luminance and current efficiency of OLED device with HITL was found to be enhanced by approximately 2.48 times and 32.6% respectively when compared with the device without hole injection layer. Various measurements carried out in this study showed that the HITL structure could effectively improve the characteristics of hole carriers, and thereby the better charge balance effect and a more stable current efficiency performance could be achieved by reducing the current efficiency roll-off effect.