Electrical characteristics of organic light-emitting diodes with the thickness variation of buffer layer

In the ITO/buffer layer/N,N'-biphenyl-N,N'-bis-(1-naphenyl)-[1,1'-biphenyl] 4,4'-diamine (NPB)/tris (8-hydroxy-quinoline) aluminum (AIq/sub 3/)/Al device, we investigated the effect of hole-injection buffer layer which is the one of the electrical characteristics of organic light-emitting diodes. A thermal evaporation was performed to make a thickness of NPB layer by the rate of 0.5 /spl sim/ 1 [/spl Aring//s] in a base pressure of 5x10/sup -6/ [Torr]. We measured V-I characteristic and efficiency with the thickness variation of hole-injection buffer layer. The PTFE and PVK materials used the buffer layer helped to improve the performance of the device in several aspect, such as good mechanical junction, reducing the operating voltage, energy band adjustment. Compared with the devices without hole-injection buffer layer, we have obtained optimal thickness 20 [nm] from the device structure of ITO/buffer layer/Alq/sub 3//Al. And using the PTFE of PVK buffer layer, the external quantum efficiency of devices has improved about 24.5 [%], and 51.3 [%], respectively.