Efficiency Improvement of Organic Light-emitting Diodes depending on Thickness of Hole Injection Materials

Abstract

In the device structure of ITO/hole injection layer/N, N'-biphenyl-N, N'- bis-(1-naphenyl)-[1,1'-biphenyl]4,4'-diamine(NPB)/tris(8-hydroxyquinoline) aluminum(Alq3)/Al, we investigated an effect of hole-injection materials (PTFE, PVK) on the electrical characteristics and efficiency of organic light-emitting diodes. A thermal evaporation was performed to make a thickness of NPB layer with a evaporation rate of 0.5 ~ 1.0 Å/s in a base pressure of 5×10-6 Torr. We measured current-voltage characteristics and efficiency with a thickness variation of hole-injection layer. The PTFE and PVK hole-injection layer improve a performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage and energy band adjustment. Compared with the devices without a hole-injection layer, we have obtained that an optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq3/Al. And using the PTFE or PVK hole-injection layer, the external quantum efficiencies of the devices were improved by 24.5 % and 51.3 %, respectively.