Very high-efficiency and low voltage phosphorescent organic light-emitting diodes based on a p-i-n junction

Green phosphorescent organic light-emitting devices (OLEDs) employing tris(2-phenylpyridine) iridium doped into a wide energy gap hole transport host have been studied. N,N,N-',N-'-tetrakis(4-methoxyphenyl)-benzidine doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane is used as a hole injection and transport layer, 4,7-diphenyl-1,10-phenanthroline and cesium are coevaporated as a n-doped electron transport layer, and an intrinsic emission layer is sandwiched between these two doped layer. Such a p-i-n device features efficient carrier injection from both contacts into the doped transport layers and low ohmic losses in these highly conductive layers. Thus, low operating voltages are obtained compared to conventional undoped OLEDs. By modifying the device structure, we optimized the carrier balance in the emission layer and at its interfaces. For the optimized device, the maximum power efficiency is 53 lm/W, and a luminance of 1000 cd/m(2) is reached at 3.1 V with a power efficiency of 45 lm/W. (C) 2004 American Institute of Physics.