Fullerene-doped hole transport NPB layer in organic light-emitting devices

The effect of C-60 doping in the hole transport NPB layer on the performance of the device was investigated by changing the C-60 content from 0 to 20 wt%. The OLEDs had a structure of ITO/NPB:C-60(0, 10, 15, and 20 wt%)/Alq(3)/LiF/Mg: Ag. The doping led to higher hole mobility in C-60-doped NPB layer from 5 wt% to 15wt%. Moreover, the hole mobility decreased when the doping content is 20%. The higher hole mobility resulted in efficient hole injection and low driving voltage at high luminance. For example, the driving voltage is about 4V and the maximum luminance of the OLEDs is 11000 cd/m(2) when the doping content is 15%. However, the driving voltage is about 6V and the maximum luminance of the OLEDs is 6500 cd/m(2) when the doping content is 15%. The hole mobility change resulted in the different performance in the devices.