A light-blue phosphorescent dendrimer for efficient solution-processed light-emitting diodes

We describe the preparation of a dendrimer that is solution-processible and contains 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a fac-tris[2-(2,4-difluorophenyl)pyridyl]iridium(III) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III) complex, bis[2-(2,4-difluorophenyl)pyridyl]picolinate iridium(in) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light-emitting layer comprised of a 30 wt.-% blend of the dendrimer in 1,3-bis(N-carbazolyl)benzene (mCP) and a 1,3,5-tris(2-N- phenylbenzimidazolyl)benzene electron-transport layer have external quantum and power efficiencies, respectively, of 10.4% and 11 1m W^-1 at 100 cd m^-2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out-coupling of light from the device.