High efficiency electrophosphorescence device using a thin cleaving layer in an Ir-complex doped emitter layer

The authors demonstrate a considerable increase in current efficiency of fac-tris(2-phenylpyridine) iridium doped phosphorescent organic green-light emitting diode in which a thin 4,7 dipheny-1,10-phenanthroline (Bphen) layer acts as a cleaving layer. As 4nm Bphen layer divides the emitting layer (EML) into two sub-EMLs, a maximum current efficiency of 53cd∕A (corresponding to external efficiency quantum of 15%) is obtained, which is higher for 2.3 folds than that of the device without it, especially the current efficiency increases 64% over the reference device at a luminance of 40000cd∕m2. The increases are demonstrated to the high electron mobility and special energy level alignment of Bphen with 4,4′-N,N′-dicarbazole-biphenyl host. The efficiency improvement attributes to a higher exciton formation probability in the recombination zone and better balance of the carrier injection. The detail enhancement mechanism of the efficiency is also discussed.