Color Tuning of OLEDs Using the Ir Complexes of White Emission by Adjusting the Band Gap of Host Materials

We report on white organic light-emitting diodes (WOLEDs) based on single white dopants, $Ir(pq)_2$($F_2$-ppy) and $Ir(F_2-ppy)_2$(pq), where $F_2$-ppy and pq are 2-(2,4-difluorophenyl) pyridine and 2-phenylquinoline, respectively. The similar phosphorescent lifetime of two ligands lead to luminescence emission in two ligands simultaneously. However, the emission color of the devices was reddish, because the energy was not transferred efficiently from the 4,4,N,N'-dicarbazolebiphenyl (CBP) to the $F_2$-ppy ligand, due to the small band gap of the CBP. Accordingly, we used 1,4-phenylenesis(triphenylsilane) (UGH2) with a large band gap, instead of CBP as the host material. As a result, it was possible to adjust the emission color by the host material. The luminous efficiency of the device with $Ir(F_2-ppy)_2$(pq) doped in UGH2 was about 11 cd/A at 0.06 cd/$m^2$.