Ultra-high efficiency electrophosphorescent p-i-n OLEDs with double emission layers

We present highly efficient, low-voltage multilayer organic light emitting diodes based on the phosphorescent emitter tris(2-phenylpyridine) iridium (Ir(ppy)(3)). The phosphor is doped into various wide gap electron-transport or hole-transport host materials embedded in between doped transport layers. We use 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as p-type hole-injection and transport layer, while cesium (Cs) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are co-evaporated for the n-type doped electron transport layer. Sandwiched between these two transport layers, we insert one or two emission layers. This p-i-n structure results in efficient carrier injection from both contacts into the doped transport layers and low ohmic losses. Thus, lower operating voltages are obtained compared to conventional undoped OLEDs. By doping Ir(ppy)(3) into a double layer structure of predominantly electron and hole transporting hosts, a power efficiency of 70 lm/W and an external quantum efficiency of 19.5% is achieved at 100 cd/m(2) (2.95V). Besides this, the efficiency decays only weakly with increasing current density (or brightness). A quantum efficiency of 13.5% is still obtained at a current density of 100 mA/m(2) with a luminance around 50,000 cd/m(2). This improvement can be attributed mainly to the fact that we prevent any charge accumulation at hole or electron blocking layers and spread the generation region to both sides of the interface between the two parts of the emission layer. Moreover, losses due to non-radiative decay of triplet excitons diffusing into regions without the phosphorescent dye are avoided.