  We report high luminance organic light-emitting diodes by use of acid
functionalized multi-walled carbon nanotube (o-MWCNTs) as efficient hole
injector electrodes with a simple and solution processable device structure. At
only 10 V, the luminance can reach nearly 50,000 cd/m2 with an external quantum
efficiency over 2% and a current efficiency greater than 21 cd/A. The
investigation of hole-only devices shows that the mechanism for hole injection
is changed from injection limited to bulk limited because of the higher
effective work function of the anode modified by the o-MWCNTs. We expect the
enhancement of the local electric field brought about by both the dielectric
inhomogeneities within the o-MWCNT containing anode and the high aspect ratio
carbon nanotubes, improves hole injection from the anode to organic active
layer at much lower applied voltage.
